Patent Publication Number: US-2022222000-A1

Title: Optimized deduplication based on backup frequency in a distributed data storage system

Description:
INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 17/153,667 filed on Jan. 20, 2021, which claims priority to U.S. Pat. App. No. 63/070,162 filed on Aug. 25, 2020. Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet of the present application are hereby incorporated by reference in their entireties under 37 CFR 1.57. 
    
    
     COPYRIGHT NOTICE 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document and/or the patent disclosure as it appears in the United States Patent and Trademark Office patent file and/or records, but otherwise reserves all copyrights whatsoever. 
     BACKGROUND 
     Deduplication in a distributed data storage platform requires tightly coupled communications among components to ensure that deduplication tracking is kept current at the various nodes that form the storage platform. However, communicating a lot of information frequently among many components places a substantial burden on network bandwidth and component cycle time that is needed for processing data storage and retrieval requests. Therefore, a solution is needed that scales well to large and very active data storage platforms while maintaining highly accurate distributed deduplication tracking with low communications overhead. 
     SUMMARY 
     The present inventors devised a technological solution that optimizes deduplicated storage of backup copies at a distributed data storage platform (hereinafter the “distributed data storage system”). The present solution is scalable and guarantees that space reclamation will not affect deduplicated data integrity even without perfect synchronization between components. The illustrative approach balances the need for aggressive space reclamation of stale data against the need to minimize re-writes of existing deduplicated data. Discarding data too aggressively will slow system performance as new data needs to be written that was already on the distributed data storage system. Conversely, failing to discard stale data reduces the usable storage of the distributed data storage system. 
     By understanding certain “behavioral” characteristics and schedule cadences of backup operations that generate backup copies received at the distributed data storage system, the present approach pro-actively ages data blocks that are not re-written by subsequent backup copies, while promoting continued retention of data blocks that are re-written. Backup copies are typically generated on a substantially regular schedule, e.g., weekly full backups followed by daily incremental backups. Thus, the contents of full backup copies effectively replace earlier full and incremental backup copies, e.g., on a weekly basis. Since the illustrative distributed data storage system is an append-only system, unique data blocks that are not supplied again by later backup copies become stale after several full backup cycles, because the source data being backed up has changed and generates different backup data blocks. Eventually, older backup copies will become stale and the backup system that generated them will prune them from the distributed data storage system. At this point, stale data blocks that are no longer referenced by any backup copies are pro-actively deleted (“garbage collected”) from the distributed data storage system. 
     The present inventors devised an expiry scheme that operates at block-level granularity on the distributed data storage system. This approach advantageously overcomes some prior-art deficiencies in which a single data block referenced by a single virtual disk would prevent discarding all other data blocks associated with the entire virtual disk, thus retaining a lot of stale data. This prior-art granularity scheme operated at the virtual disk level. The present approach also overcomes other prior-art deficiencies in which a new deduplication store was opened periodically, which automatically expired data blocks in the preceding deduplication store, but disadvantageously required re-writes of retained older data blocks already in the system. 
     Here, in contrast to these prior-art solutions, each unique deduplicated data block is given an expiry timeframe based on the block&#39;s arrival time at the distributed data storage system (i.e., when a backup copy supplies the data block) and further based on backup frequencies of the various virtual disks referencing the data block. The present solution includes a global (or system-wide) deduplication repository, which is configured as a virtual disk that is partitioned and replicated across storage nodes. The global system deduplication virtual disk (or “system-wide deduplication virtual disk) is not exposed as a storage target to the backup system generating the backup copies and is managed as a strictly internal resource of the distributed data storage system. By using the system-wide deduplication virtual disk, the distributed data storage system maximizes deduplication ratios across data sources, i.e., regardless of the user virtual disk addressed by an incoming backup copy. However, different user virtual disks may have different backup schedules. Therefore, the illustrative block-level expiry approach considers the various backup schedules in assigning expiry timeframes and deciding when and whether to discard individual data blocks. 
     A periodic “garbage collection” or “GC” or discard cycle evaluates each deduplicated data block tracked by the distributed data storage system. Blocks “written” (i.e., supplied by an incoming backup copy) in the preceding cycle are analyzed for reference counting and expiry timeframes. Illustratively, each backup copy received by the distributed data storage system comprises one or more files, and the distributed data storage system may address each file to a distinct user virtual disk. When the backup system prunes stale backup copies from the distributed data storage system, the pruning causes the distributed data storage system to logically delete the various user virtual disks comprising the backup files associated with the particular backup copy. 
     Because of deduplication, not every block that comes in with a backup copy is actually added to the distributed data storage system, but the write request is noted and tracked. A new data block with a new hash value receives a unique system-wide identifier (e.g., the “dedupe block identifier” or “DDblockID”) and the new data block is added to the global deduplication virtual disk. A block that is “written” one or more times within a preceding cycle will have its expiry extended at least one more cycle during the analysis performed by the discard cycle. Should subsequent “writes” continue, the block&#39;s expiry will be extended again, thus ensuring that the block stays in the global deduplication virtual disk because it is still current. But when the subsequent writes wane, the block will eventually expire. If there are no more backup copies (and corresponding user virtual disks) on the distributed data storage system that reference the particular data block, the discard cycle identifies the data block for deletion, and a compaction engine will delete the block from the distributed data storage system. Should the expired data block appear again in a later write request, it will be seen as a new block and written anew to the distributed data storage system. 
     One of the key aspects of the disclosed approach is that in each discard cycle, the expiry timeframe of a deduplicated data block (DDblockID) may be extended to accommodate the least frequent full-backup frequency of any user virtual disk that references the data block. In this way, all virtual disks referencing the DDblockID are guaranteed the data block&#39;s survival at least through the next full backup operation. This key aspect provides flexibility and scalability without compromising data integrity. Moreover, this key aspect maintains the block-level granularity of the disclosed expiry scheme, which improves the effectiveness of the discard process. 
     The illustrative distributed data storage system comprises storage proxies that intercept backup copies being written to the distributed data storage system. Rather than adding every incoming data block to the distributed data storage system, the storage proxy applies a deduplication routine to the data block. The storage proxy uses hashing (e.g., MD5 without limitation) to compute a hash value for the incoming data block and checks whether the hash value is present in a local index or data structure at the storage proxy (e.g., DDCache). 
     If the storage proxy does not find the hash value in DDCache, the storage proxy passes the hash value to a metadata subsystem on a storage service node and receives an update therefrom, comprising a DDblockID and expiry timeframe if the DDblockID is known, i.e., has been previously added to the system-wide deduplication virtual disk. If the hash value is new to the metadata subsystem or if the expiry timeframe for the DDblockID is in the past (i.e., expired DDblockID according to the metadata subsystem), the data block is treated as a new block, and the storage proxy will store the data block to the system-wide deduplication virtual disk. The system-wide deduplication virtual disk is partitioned and duplicated across a plurality of data storage subsystems that are distinct from the metadata subsystem. The data storage subsystem assigns a new DDblockID to the new data block. The metadata subsystem updates its own tracking data structures, assigns a future expiry timeframe (“epoch”) to the new DDblockID, and transmits the DDblockID and its expiry epoch to the storage proxy for updating its own local index (e.g., DDCache). If it turns out that the metadata subsystem reported to the storage proxy that the DDblockID corresponding to the hash value is not expired, the storage proxy updates its DDCache and the write request is noted in data structures at the metadata subsystem. 
     Otherwise, if the storage proxy found the hash value of the data block within its own index (e.g., DDCache), the storage proxy checks the expiry epoch of the corresponding DDblockID. If the DDblockID is not expired, the storage proxy reports the write request to the metadata subsystem so that it can update its local tracking data structures. On the other hand, if the storage proxy determines that the DDblockID has an expired epoch (according to DDCache), the storage proxy submits the hash value to the metadata subsystem as if not found in DDCache as described above. 
     Thus, during the I/O cycle of a given data block, its hash value is calculated, its presence in the global deduplication virtual disk is determined, and its pre-existing expiry (if any) determines whether the data block is added to the global deduplication virtual disk. The index at the storage proxy (e.g., DDCache) is updated only as needed to minimize updates sent by the metadata subsystem; no attempt is made to keep DDCache fully synchronized with the tracking data structures at the metadata subsystem. The metadata subsystem updates certain local tracking data structures for every incoming write request. One of these data structures is the illustrative DDTracker column family, which is updated for every write request from an incoming backup copy and enables proper reference counting. 
     Illustratively, a discard cycle runs weekly based on the typical cadence of full backup cycles, but the invention is not so limited. During a first phase of the discard cycle (the preparation stage), the metadata subsystem scans each DDTracker column family and adds the resulting processed information to a persistent column family used for tracking DDblockID reference counts (e.g., DDRefCount). For example, DDblockID_7 that was “written” or addressed to user virtual disk  1  (e.g., uservdisk_1) is added to DDRefCount showing that uservdisk_1 “wrote” DDblockID_7 to the system during an epoch as indicated by DDTracker. DDRefCount assigns an expiry to this DDblockID_7 instance based on the full backup frequency of uservdisk_1. Likewise, another data block having the same DDblockID (e.g., DDblockID_7), which was “written” or targeted to another user virtual disk (e.g., uservdisk_N) is added to DDRefCount showing that uservdisk_N “wrote” DDblockID_7 to the system during a write epoch as indicated by DDTracker. DDRefCount assigns an expiry to this other DDblockID_7 instance based on the full backup frequency of uservdisk_N, which may differ from that of uservdisk_1. Thus, every undeleted DDblockID in the global deduplication virtual disk has entries (columns) in DDRefCount corresponding to the various epochs when a new instance of the data block entered the system and columns for user virtual disks referencing the DDblockID. A maximum value of the various expiry columns, i.e., the longest expiry timeframe assigned to the DDblockID in the system (e.g., Eu) ensures that the DDblockID will survive between the sparsest virtual disk backups, and later helps to determine whether a certain DDblockID should be discarded. 
     Once the preparation stage has fully processed all DDTracker column families and persisted DDRefCount, a second phase of the discard cycle begins—the “garbage collection” or discard stage. The second phase checks whether the user virtual disks in DDRefCount are still present on the distributed data storage system. When a backup copy is pruned by the backup system that created it, the virtual disk(s) created to store the backup copy and/or its constituent files are deleted from the distributed data storage system configuration. Thus, backup copy deletions may result in DDRefCount decrementing the reference counts of certain DDblockIDs referenced by the corresponding virtual disks. The second phase further checks the maximum expiry epoch (e.g., Eu) of every DDblockID in DDRefCount. DDblockIDs with zero reference counts and expired Eu are placed by the metadata subsystem on discard lists. Because the global system deduplication virtual disk is distributed among a plurality of different storage containers on different storage service nodes, the discard lists are segregated by storage container and transmitted to the data storage subsystem nodes hosting the respective storage containers. There, the resident compaction engine ensures that the actual data blocks corresponding to the DDblockID are discarded, thus freeing up storage space. 
     On the other hand, a DDblockID that was “written” after the last discard cycle, is given an extension of time. Accordingly, the expiry epoch is incremented by the full backup frequency of the user virtual disk associated with the write request. If the maximum expiry epoch (Eu) increases at this point, the increase is reflected in the local index of the metadata subsystem (e.g., DDInfo). In this way, subsequent write requests of this DDblockID will be evaluated with respect to the updated (later) expiry epoch, thus extending the lifetime of the DDblockID on the distributed data storage system based on its being recently “re-written.” 
     One of the key advantages of the disclosed optimized deduplication scheme is that it continues to operate throughout the distributed data storage system even if storage proxies and/or metadata-hosting nodes are down. Thus, storage proxies and/or metadata nodes that are down cannot affect the expiry-based aging of deduplicated data blocks going on in the rest of the system. The present solution guarantees that stale references to DDblockIDs that may be lingering in these non-functional components cannot prevent a particular DDblockID from being discarded. When a storage proxy or metadata subsystem revives after one or more discard cycles, its local index (e.g., DDCache, DDInfo) indicates that many, if not all, incoming data blocks are expired, because expiry epochs have not been updated while down. This triggers a check-in with an operational metadata node as explained above. The check-in sets the record straight according to current expiry information at the working metadata node, handles the data block appropriately (e.g., new add vs. deduplicated), and provides an update to the newly revived component if needed. This scheme guarantees that stale data blocks will not be mistakenly referenced by components that revive with out-of-date tracking indexes. This aspect provides resiliency to the distributed data storage system without preventing space reclamation. 
     In sum, the present application discloses a technological improvement that enables system-wide deduplication with block-level expiry granularity. The useful life of each deduplicated data block is based on expiry parameters that relate to backup frequencies of the virtual disks referencing the data block, thus guaranteeing that data blocks are kept around between full backup cycles and are extended if still current. Blocks are retained as long as needed to bridge the gap between sparser backup operations. Tracking data structures are updated only as needed, thus saving processing cycles and network bandwidth. Moreover, the present solution guarantees that stale references to DDblockIDs lingering in non-functional components cannot dictate whether a particular DDblockID is discarded. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram depicting a distributed data storage system  100  according to an illustrative embodiment. 
         FIG. 1B  is a block diagram illustrating some details of distributed data storage system  100  comprising separately scalable storage service nodes according to an illustrative embodiment. 
         FIG. 1C  is a block diagram depicting certain subsystems of the storage service of distributed data storage system  100 , according to an illustrative embodiment. 
         FIG. 1D  is a block diagram depicting a virtual disk distributed across a plurality of storage service nodes and also depicting a plurality of storage resources available at each storage service node according to an illustrative embodiment. 
         FIG. 1E  is a block diagram depicting a typical I/O workflow for write operations originating with an application. 
         FIG. 1F  is a block diagram depicting a typical I/O workflow for read operations originating with an application. 
         FIG. 1G  is a block diagram illustrating a backup configuration that depicts an illustrative data storage management system using distributed data storage system  100  for storing secondary (backup) copies according to an illustrative embodiment. 
         FIG. 1H  is a block diagram illustrating a backup configuration in which a third-party backup system uses the illustrative distributed data storage system  100  for storing secondary (backup) copies according to an illustrative embodiment. 
         FIG. 2A  is a block diagram depicting components that play a role in the present deduplication solution according to an illustrative embodiment. 
         FIG. 2B  is a block diagram depicting components that play a role in the present deduplication solution according to an illustrative embodiment. 
         FIG. 2C  depicts an illustrative embodiment of a column family (e.g., DDCache, DDInfo) for associating hash values with corresponding DDblockID and associated expiry epoch. 
         FIG. 2D  depicts an illustrative column family for tracking write requests received by storage proxies, e.g., DDTracker. 
         FIG. 2E  depicts an illustrative column family that is updated during each discard preparation stage based at least in part on scanning and processing DDTracker information, e.g., DDRefCount. 
         FIG. 2F  depicts an illustrative column family that assigns the workload of scanning DDTracker column families for the purpose of updating DDRefCount. 
         FIG. 2G  depicts an illustrative column family that comprises DDblockID discard lists. 
         FIG. 3  depicts a fence diagram that illustrates some salient operations occurring during an I/O cycle of an incoming data block intercepted by a storage proxy. 
         FIG. 4  depicts some salient operations of a method  400  according to an illustrative embodiment. 
         FIG. 5  depicts some salient operations of block  408  in method  400  according to an illustrative embodiment. 
         FIG. 6  depicts some salient operations of a method  600  according to an illustrative embodiment. 
         FIG. 7  depicts some salient operations of block  606  in method  600  according to an illustrative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Detailed descriptions and examples of systems and methods according to one or more illustrative embodiments of the present invention may be found in the section entitled OPTIMIZED DEDUPLICATION BASED ON BACKUP FREQUENCY IN A DISTRIBUTED DATA STORAGE SYSTEM, as well as in the section entitled Example Embodiments, and also in  FIGS. 1G-7  herein. Furthermore, components and functionality for optimizing deduplication in a distributed data storage system may be configured and/or incorporated into the distributed data storage system described herein in  FIGS. 1A-1F . 
     Various embodiments described herein are intimately tied to, enabled by, and would not exist except for, computer technology. For example, hashing, analyzing, and transmitting data among the disclosed components described herein cannot reasonably be performed by humans alone, without the computer technology upon which they are implemented. 
     Generally, the systems and associated components described herein may be compatible with and/or provide at least some of the functionality of the systems and corresponding components described in one or more of the following U.S. patents and patent applications assigned to Commvault Systems, Inc., each of which is hereby incorporated by reference in its entirety herein. 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                 Title 
                 USPTO Ser. No. 
                 U.S. Pat. No. 
                 Filing Date 
               
               
                   
               
             
            
               
                 STORAGE SYSTEM FOR 
                 14/322,813 
                 10,067,722 
                 Jul. 2, 2014 
               
               
                 PROVISIONING AND STORING DATA 
               
               
                 TO A VIRTUAL DISK 
               
               
                 METHOD FOR WRITING DATA TO A 
                 14/322,832 
                 9,875,063 
                 Jul. 2, 2014 
               
               
                 VIRTUAL DISK USING A 
               
               
                 CONTROLLER VIRTUAL MACHINE 
               
               
                 AND DIFFERENT STORAGE AND 
               
               
                 COMMUNICATION PROTOCOLS 
               
               
                 DISK FAILURE RECOVERY FOR 
                 14/322,850 
                 9,424,151 
                 Jul. 2, 2014 
               
               
                 VIRTUAL DISK WITH POLICIES 
               
               
                 CREATING AND REVERTING TO A 
                 14/322,855 
                 9,558,085 
                 Jul. 2, 2014 
               
               
                 SNAPSHOT OF A VIRTUAL DISK 
               
               
                 CLONING A VIRTUAL DISK IN A 
                 14/322,867 
                 9,798,489 
                 Jul. 2, 2014 
               
               
                 STORAGE PLATFORM 
               
               
                 WRITING TO A STORAGE 
                 14/322,868 
                 9,483,205 
                 Jul. 2, 2014 
               
               
                 PLATFORM INCLUDING A 
               
               
                 PLURALITY OF STORAGE 
               
               
                 CLUSTERS 
               
               
                 TIME STAMP GENERATION FOR 
                 14/322,871 
                 9,411,534 
                 Jul. 2, 2014 
               
               
                 VIRTUAL DISKS 
               
               
                 METHOD FOR WRITING DATA TO 
                 14/684,086 
                 9,864,530 
                 Apr. 10, 2015 
               
               
                 VIRTUAL DISK USING A 
               
               
                 CONTROLLER VIRTUAL MACHINE 
               
               
                 AND DIFFERENT STORAGE AND 
               
               
                 COMMUNICATION PROTOCOLS ON 
               
               
                 A SINGLE STORAGE PLATFORM 
               
               
                 DYNAMICALLY SPLITTING A RANGE 
                 14/723,380 
                 Abandoned 
                 May 27, 2015 
               
               
                 OF A NODE IN A DISTRIBUTED 
               
               
                 HASH TABLE 
               
               
                 STORAGE SYSTEM WITH VIRTUAL 
                 PCT/US2015/38687 
                 Expired 
                 Jun. 30, 2015 
               
               
                 DISKS 
               
               
                 GLOBAL DE-DUPLICATION OF 
                 15/155,838 
                 10,846,024 
                 May 16, 2016 
               
               
                 VIRTUAL DISKS IN A STORAGE 
               
               
                 PLATFORM 
               
               
                 DE-DUPLICATION OF CLIENT-SIDE 
                 15/156,015 
                 10,795,577 
                 May 16, 2016 
               
               
                 DATA CACHE FOR VIRTUAL DISKS 
               
               
                 PERSISTENT RESERVATIONS FOR 
                 15/163,446 
                 10,248,174 
                 May 24, 2016 
               
               
                 VIRTUAL DISK USING MULTIPLE 
               
               
                 TARGETS 
               
               
                 SYNCHRONIZATION OF METADATA 
                 15/834,921 
                 10,740,300 
                 Dec. 7, 2017 
               
               
                 IN A DISTRIBUTED STORAGE 
               
               
                 SYSTEM 
               
               
                 IN-FLIGHT DATA 
                 15/912,374 
                 10,848,468 
                 Mar. 5, 2018 
               
               
                 ENCRYPTION/DECRYPTION FOR A 
               
               
                 DISTRIBUTED STORAGE PLATFORM 
               
               
                 PERSISTENT RESERVATIONS FOR 
                 16/274,014 
                 10,691,187 
                 Feb. 12, 2019 
               
               
                 VIRTUAL DISK USING MULTIPLE 
               
               
                 TARGETS 
               
               
                 DISTRIBUTED DATA STORAGE 
                 63/053,414 
                   
                 Jul. 17, 2020 
               
               
                 SYSTEM USING ERASURE CODING 
               
               
                 ON STORAGE NODES FEWER THAN 
               
               
                 DATA PLUS PARITY FRAGMENTS 
               
               
                 DISTRIBUTED DATA STORAGE 
                 63/065,722 
                   
                 Aug. 14, 2020 
               
               
                 SYSTEM USING ERASURE CODING 
               
               
                 ON STORAGE NODES FEWER THAN 
               
               
                 DATA PLUS PARITY FRAGMENTS 
               
               
                 OPTIMIZED DEDUPLICATION BASED 
                 63/070,162 
                   
                 Aug. 25, 2020 
               
               
                 ON BACKUP FREQUENCY IN A 
               
               
                 DISTRIBUTED DATA STORAGE 
               
               
                 SYSTEM 
               
               
                 ANTI-ENTROPY-BASED METADATA 
                 63/081,503 
                   
                 Sep. 22, 2020 
               
               
                 RECOVERY IN A STRONGLY 
               
               
                 CONSISTENT DISTRIBUTED DATA 
               
               
                 STORAGE SYSTEM 
               
               
                 COMMISSIONING AND 
                 63/082,624 
                   
                 Sep. 24, 2020 
               
               
                 DECOMMISSIONING METADATA 
               
               
                 NODES IN A RUNNING 
               
               
                 DISTRIBUTED DATA STORAGE 
               
               
                 SYSTEM 
               
               
                 CONTAINER DATA MOVER FOR 
                 63/082,631 
                   
                 Sep. 24, 2020 
               
               
                 MIGRATING DATA BETWEEN 
               
               
                 DISTINCT DISTRIBUTED DATA 
               
               
                 STORAGE SYSTEMS INTEGRATED 
               
               
                 WITH APPLICATION 
               
               
                 ORCHESTRATORS 
               
               
                   
               
            
           
         
       
     
     Distributed Data Storage System 
     An example embodiment of the disclosed distributed data storage system is the Hedvig Distributed Storage Platform now available from Commvault Systems, Inc. of Tinton Falls, N.J., USA, and thus some of the terminology herein originated with the Hedvig product line. 
     The illustrative distributed data storage system comprises a plurality of storage service nodes that form a storage cluster. Data reads and writes originating from an application on an application host computing device are intercepted by a storage proxy, which is co-resident with the originating application. The storage proxy performs some pre-processing and analysis functions before making communicative contact with the storage cluster. The system ensures strong consistency of data and metadata written to the storage service nodes. 
     Terminology for the Distributed Data Storage System 
     Data and Metadata. To enhance the reader&#39;s understanding of the present disclosure, the term “metadata” is distinguished from the term “data” herein, even though both data and metadata comprise information stored on the illustrative distributed data storage system. Accordingly, “data” will refer to “payload” data, which is typically generated by an application or other data source that uses the distributed data storage system for data storage. Thus, the terms “data”, “payload”, and “payload data” will be used interchangeably herein. On the other hand, “metadata” will refer to other information in the distributed data storage system, e.g., information about the payload data, about the components hosting the payload data, about metadata-hosting components, about other components of the distributed data storage system, and also information about the metadata, i.e., “meta-metadata.” 
     Storage Service, e.g., Hedvig Storage Service. The storage service is a software component that installs on commodity x86 or ARM servers to transform existing server and storage assets into a fully-featured elastic storage cluster. The storage service may deploy to an on-premises infrastructure, to hosted clouds, and/or to public cloud computing environments to create a single storage cluster. 
     Storage Service Node (or storage node), e.g., Hedvig Storage Server (HSS), comprises both computing and storage resources that collectively provide storage service. The system&#39;s storage service nodes collectively form a storage cluster. One or more of the following storage service subsystems of the storage service may be instantiated at and may operate on a storage service node: (i) distributed fault-tolerant metadata subsystem providing metadata service, e.g., “Hedvig Pages”; (ii) distributed fault-tolerant data subsystem (or data storage subsystem) providing payload data storage, e.g., “Hedvig HBlock”; and (iii) distributed fault-tolerant pod subsystem for generating and maintaining certain system-level information, e.g., “Hedvig HPod.” The system stores payload data on certain dedicated storage resources managed by the data storage subsystem, and stores metadata on other dedicated storage resources managed by the metadata subsystem. Thus, another way to distinguish payload data from metadata in the illustrative system is that payload data is stored in and maintained by the data storage subsystem and metadata is stored in and maintained by the metadata subsystem. The pod subsystem, the metadata subsystem, and the data storage subsystem are all partitioned and replicated across various storage service nodes. These subsystems operate as independent services, they need not be co-located on the same storage service node, and they may communicate with a subsystem on another storage service node as needed. 
     Replica. The distributed data storage system replicates data and metadata across multiple storage service nodes. A “replica” or “replica node” is a storage service node that hosts a replicated copy of data and/or metadata that is also stored on other replica nodes. Illustratively, metadata uses a replication factor of 3, though the invention is not so limited. Thus, with a replication factor of 3 (“RF3”), each portion of metadata is replicated on three distinct metadata nodes across the storage cluster. 
     Virtual Disk (“vdisk”) and Storage Containers. The virtual disk is the unit of storage made visible by system  100  to applications and/or application nodes. Every virtual disk provisioned on the system is partitioned into fixed size chunks, each of which is called a storage container. Different replicas are assigned for each storage container. Since replica assignment occurs at the storage container level—not at a virtual disk level—the data for a virtual disk is distributed across a plurality of storage service nodes, thus allowing increased parallelism during I/Os and/or disk rebuilds. Thus, virtual disks are distributed and fault-tolerant. 
     Storage Pools. Storage pools are logical groupings of physical disks/drives in a storage service node and are configured as the protection unit for disk/drive failures and rebuilds. Within a replica, one or more storage containers are assigned to a storage pool. A typical storage service node will host two to four storage pools. 
     Metadata Node. An instance of the metadata subsystem executing on a storage service node is referred to as a metadata node that provides “metadata service.” The metadata subsystem executing on a storage service node stores metadata at the storage service node. The metadata node communicates with one or more other metadata nodes to provide a system-wide metadata service. The metadata subsystem also communicates with pod and/or data storage subsystems at the same or other storage service nodes. Some metadata nodes are designated owners of certain virtual disks whereas others are replicas but not owners. Owner nodes are invested with certain functionality for managing the owned virtual disk. 
     Metadata Node Identifier or Storage Identifier (SID) is a unique identifier of the metadata service instance on a storage service node, i.e., the unique system-wide identifier of a metadata node. 
     Storage Proxy. Each storage proxy is a lightweight software component that deploys at the application tier, i.e., on application servers or hosts. A storage proxy may be implemented as a virtual machine (VM) or as a software container (e.g., Docker), or may run on bare metal to provide storage access to any physical host or VM in the application tier. As noted, the storage proxy intercepts reads and writes issued by applications and directs input/output (I/O) requests to the relevant storage service nodes. 
     Erasure Coding (EC). In some embodiments, the illustrative distributed data storage system employs erasure coding rather than or in addition to replication. EC is one of the administrable attributes for a virtual disk. The default EC policy is (4,2), but (8,2) and (8,4) are also supported if a sufficient number of storage service nodes are available. The invention is not limited to a particular EC policy unless otherwise noted herein. 
       FIG. 1A  is a block diagram depicting a distributed data storage system  100  according to an illustrative embodiment. The figure depicts: a plurality of application nodes  102  that form an “application tier,” each application node comprising a storage proxy  106  and one of components  103 A,  104 A, and  105 A; and a storage cluster  110  comprising a plurality of separately scalable storage service nodes  120  and a plurality of specially-equipped compute hosts  121 . 
     Distributed data storage system  100  (or system  100 ) comprises storage proxies  106  and storage cluster  110 . System  100  flexibly leverages both hyperscale and hyperconverged deployment options, sometimes implemented in the same storage cluster  110  as depicted here. Hyperscale deployments scale storage resources independently from the application tier, as shown by storage service nodes  120  (e.g.,  120 - 1  . . .  120 -N). In such hyperscale deployments, storage capacity and performance scale out horizontally by adding commodity servers running the illustrative storage service; application nodes (or hosts)  102  scale separately along with storage proxy  106 . On the other hand, hyperconverged deployments scale compute and storage in lockstep, with workloads and applications residing on the same physical nodes as payload data, as shown by compute hosts  121 . In such hyperconverged deployments, storage proxy  106  and storage service software  122  are packaged and deployed as VMs on a compute host  121  with a hypervisor  103  installed. In some embodiments, system  100  provides plug-ins for hypervisor and virtualization tools, such as VMware vCenter, to provide a single management interface for a hyperconverged solution. 
     System  100  provides enterprise-grade storage services, including deduplication, compression, snapshots, clones, replication, auto-tiering, multitenancy, and self-healing of both silent corruption and/or disk/node failures to support production storage operations, enterprise service level agreements (SLAs), and/or robust storage for backed up data (secondary copies). Thus, system  100  eliminates the need for enterprises to deploy bolted-on or disparate solutions to deliver a complete set of data services. This simplifies infrastructure and further reduces overall Information Technology (IT) capital expenditures and operating expenses. Enterprise storage capabilities can be configured at the granularity of a virtual disk, providing each data originator, e.g., application, VM, and/or software container, with its own unique storage policy. Every storage feature can be switched on or off to fit the specific needs of any given workload. Thus, the granular provisioning of features empowers administrators to avoid the challenges and compromises of “one size fits all” storage and helps effectively support business SLAs, while decreasing operational costs. 
     System  100  inherently supports multi-site availability, which removes the need for additional costly disaster recovery solutions. The system provides native high availability storage for applications across geographically dispersed data centers by setting a unique replication policy and replication factor at the virtual disk level. 
     System  100  comprises a “shared-nothing” distributed computing architecture in which each storage service node is independent and self-sufficient. Thus, system  100  eliminates any single point of failure, allows for self-healing, provides non-disruptive upgrades, and scales indefinitely by adding more storage service nodes. Each storage service node stores and processes metadata and/or payload data, then communicates with other storage service nodes for data/metadata distribution according to the replication factor. 
     Storage efficiency in the storage cluster is characterized by a number of features, including: thin provisioning, deduplication, compression, compaction, and auto-tiering. Each virtual disk is thinly provisioned by default and does not consume capacity until data is written therein. This space-efficient dynamic storage allocation capability is especially useful in DevOps environments that use Docker, OpenStack, and other cloud platforms where volumes do not support thin provisioning inherently, but can support it using the virtual disks of system  100 . System  100  provides inline global deduplication that delivers space savings across the entire storage cluster. Deduplication is administrable at the virtual disk level to optimize I/O and lower the cost of storing data. As writes occur, the system  100  calculates the unique fingerprint of data blocks and replaces redundant data with a small pointer. The deduplication process can be configured to begin at storage proxy  106 , improving write performance and eliminating redundant data transfers over the network. System  100  provides inline compression administrable at the virtual disk level to optimize capacity usage. The system stores only compressed data on the storage service nodes. Illustratively, the Snappy compression library is used, but the invention is not limited to this implementation. To improve read performance and optimize storage space, the illustrative system periodically performs garbage collection to compact redundant blocks and generate large sequential chunks of data. The illustrative system balances performance and cost by supporting tiering of data among high-speed SSDs and lower-tier persistent storage technologies. 
     Application node (or host)  102  (e.g.,  102 - 1 ,  102 - 2 ,  102 - 3 ) is any computing device, comprising one or more hardware processors and computer memory for executing computer programs, that generates and/or accesses data stored in storage cluster  110 . Application(s) (not shown here but see, e.g., applications  132  in  FIG. 1B ) executing on an application node  102  use storage cluster  110  as a data storage resource. Application node  102  can take the form of: a bare metal host  105 A for applications with storage proxy  106 - 3 ; a virtual machine server with hypervisor  103 A and storage proxy  106 - 1 ; a container host hosting software container  104 A and storage proxy  106 - 2 ; and/or another computing device configuration equipped with a storage proxy  106 . 
     Hypervisor  103  (e.g.,  103 A,  103 B) is any hypervisor, virtual machine monitor, or virtualizer that creates and runs virtual machines on a virtual machine server or host. Software container  104 A is any operating system virtualization software that shares the kernel of the host computing device (e.g.,  102 ,  121 ) that it runs on and allows multiple isolated user space instances to co-exist. Docker is an example of software container  104 A. Bare metal  105 A refers to application node  102 - 3  running as a traditional computing device without virtualization features. Components  103 ,  104 A, and  105 A/B are well known in the art. 
     Storage proxy  106  (e.g.,  106 - 1 ,  106 - 2 ,  106 - 3 ,  106 -J . . .  106 -K) is a lightweight software component that deploys at the application tier, i.e., on application nodes  102  and/or compute hosts  121 . A storage proxy may be implemented as a virtual machine  106 - 1 , as a software container (e.g., Docker)  106 - 2 , and/or running on bare metal (e.g.,  106 - 3 ) to provide storage access to any physical host or VM in the application tier. The storage proxy acts as a gatekeeper for all I/O requests to virtual disks configured at storage cluster  110 . It acts as a storage protocol converter, load balances I/O requests to storage service nodes, caches data fingerprints, and performs certain deduplication functions. Storage protocols supported by storage proxy  106  include Internet Small Computer Systems Interface (iSCSI), Network File System (NFS), Server Message Block (SMB2) or Common Internet File System (CIFS), Amazon Simple Storage Service (S3), OpenStack Object Store (Swift), without limitation. The storage proxy runs in user space and can be managed by any virtualization management or orchestration tool. With storage proxies  106  that run in user space, the disclosed solution is compatible with any hypervisor, software container, operating system, or bare metal computing environment at the application node. In some virtualized embodiments where storage proxy  106  is deployed on a virtual machine, the storage proxy may be referred to as a “controller virtual machine” (CVM) in contrast to application-hosting virtual machines that generate data for and access data at the storage cluster. 
     Storage cluster  110  comprises the actual storage resources of system  100 , such as storage service nodes  120  and storage services  122  running on compute hosts  121 . In some embodiments, storage cluster  110  is said to comprise compute hosts  121  and/or storage service nodes  120 . 
     Storage service node  120  (e.g.,  120 - 1  . . .  120 -N) is any commodity server configured with one or more x86 or ARM hardware processors and with computer memory for executing the illustrative storage service, which is described in more detail in  FIG. 1C . Storage service node  120  also comprises storage resources as described in more detail in  FIG. 1D . By running the storage service, the commodity server is transformed into a full-featured component of storage cluster  110 . System  100  may comprise any number of storage service nodes  120 . 
     Compute host  121  (e.g.,  121 - 1  . . .  121 -M) is any computing device, comprising one or more hardware processors and computer memory for executing computer programs, that comprises the functional components of an application node  102  and of a storage service node  120  in a “hyperconverged” configuration. In some embodiments, compute hosts  121  are configured, sometimes in a group, within an appliance such as the Commvault Hyperscale™ X backup appliance from Commvault Systems Inc., of Tinton Falls, N.J., USA. 
       FIG. 1B  is a block diagram illustrating some details of distributed data storage system  100  comprising separately scalable storage service nodes  120  according to an illustrative embodiment. The figure depicts: application node  102 - 1  embodied as a VM host and hosting hypervisor  103 , storage proxy  106 - 1  embodied as a controller virtual machine, and client VM  131  hosting application  132 - 1 ; application node  102 - 2  hosting containerized storage proxy  106 - 2  and containerized application  132 - 2 ; and storage cluster  110  comprising nine (9) distinct physical storage service nodes  120  (e.g.,  120 - 1  . . .  120 - 9 ). Virtual machine hosts, virtual machines, and hypervisors are well known in the art. 
     Application  132  (e.g.,  132 - 1 ,  132 - 2 ) is any software that executes on its underlying host (e.g.,  102 - 1 ,  102 - 2 ) and performs a function as a result. The application  132  may generate data and/or need to access data which is stored in system  100 . Examples of application  132  include email applications, database management applications, office productivity software, backup software, etc., without limitation. 
     The bi-directional arrows between each storage proxy  106  and a storage service node  120  depict the fact that communications between applications  132  and storage cluster  110  pass through storage proxies  106 , each of which identifies a proper storage service node  120  to communicate with for the present transaction, e.g., storage service node  120 - 2  for storage proxy  106 - 1 , storage service node  120 - 4  for storage proxy  106 - 2 . 
       FIG. 1C  is a block diagram depicting certain subsystems of the storage service of distributed data storage system  100 , according to an illustrative embodiment. Depicted here are: storage proxy  106 ; application  132 ; and a storage service node  120  comprising a pod subsystem  130  (e.g., Hedvig “HPOD”), a metadata subsystem  140  (e.g., Hedvig “PAGES”), and a data storage subsystem  150  (e.g., Hedvig “HBLOCK”). Although storage service node  120  as depicted here comprises an instance of all three storage service subsystems, any given storage service node  120  need not comprise all three subsystems. Thus, a subsystem running on a given storage service node may communicate with one or more subsystems on another storage service node as needed to complete a task or workload. 
     Storage proxy  106  intercepts reads and writes issued by applications  132  that are targeted to particular virtual disks configured in storage cluster  110 . Storage proxy  106  provides native block, file, and object storage protocol support, as follows: 
     Block storage—system  100  presents a block-based virtual disk through a storage proxy  106  as a logical unit number (LUN). Access to the LUN, with the properties applied during virtual disk provisioning, such as compression, deduplication and replication, is given to a host as an iSCSI target. After the virtual disk is in use, the storage proxy translates and relays all LUN operations to the underlying storage cluster. 
     File storage—system  100  presents a file-based virtual disk to one or more storage proxies  106  as an NFS export, which is then consumed by the hypervisor as an NFS datastore. Administrators can then provision VMs on that NFS datastore. The storage proxy acts as an NFS server that traps NFS requests and translates them into the appropriate remote procedure call (RPC) calls to the backend storage service node. 
     Object storage—buckets created via the Amazon S3 API, or storage containers created via the OpenStack Swift API, are translated via the storage proxies  106  and internally mapped to virtual disks  170  (shown in  FIG. 1D ). The storage cluster  110  acts as the object (S3/Swift) target, which client applications  132  can utilize to store and access objects. 
     Storage Proxy  106  comprises one or more caches that enable distributed operations and the performing of storage system operations locally at the application node  102  to accelerate read/write performance and efficiency. An illustrative metacache stores metadata locally at the storage proxy, preferably on SSDs. This cache eliminates the need to traverse the network for metadata lookups, leading to substantial read acceleration. For virtual disks provisioned with client-side caching, an illustrative block cache stores data blocks to local SSD drives to accelerate reads. By returning blocks directly from the storage proxy, read operations avoid network hops when accessing recently used data. For virtual disks provisioned with deduplication, an illustrative dedupe cache resides on local SSD media and stores fingerprint information of certain data blocks written to storage cluster  110 . Based on this cache, the storage proxy determines whether data blocks have been previously written and if so, avoids re-writing these data blocks again. Storage proxy  106  first queries the dedupe cache and if the data block is a duplicate, storage proxy  106  updates metadata subsystem  140  to map the new data block(s) and acknowledges the write to originating application  132 . Otherwise, storage proxy  106  queries metadata subsystem  140  and if the data block was previously written to storage cluster  110 , the dedupe cache and metadata subsystem  140  are updated accordingly, with an acknowledgement to originating application  132 . Unique new data blocks are written to the storage cluster as new payload data. More details on reads and writes are given in  FIGS. 1E and 1F . 
     A simplified use case workflow comprises: 1. A virtual disk  170  is administered with storage policies via a web-based user interface, a command line interface, and/or a RESTful API (representational state transfer application programming interface). 2. Block and file virtual disks are attached to a storage proxy  106 , which presents the storage resource to application hosts, e.g.,  102 . For object storage, applications  132  directly interact with the virtual disk via Amazon S3 or OpenStack Swift protocols. 3. Storage proxy  106  intercepts application  132  I/O through the native storage protocol and communicates it to the underlying storage cluster  110  via remote procedure calls (RPCs). 4. The storage service distributes and replicates data throughout the storage cluster based on virtual disk policies. 5. The storage service conducts background processes to auto-tier and balance across racks, data centers, and/or public clouds based on virtual disk policies. 
     Pod subsystem  130  maintains certain system-wide information for synchronization purposes and comprises processing and tracking resources and locally stored information. A network of pods  130  throughout storage cluster  110 , where each pod comprises three nodes, is used for managing transactions for metadata updates, distributed-atomic-counters as a service, tracking system-wide timeframes such as generations and epochs, etc. More details on the pod subsystem may be found in U.S. Pat. No. 9,483,205 B2, which is incorporated by reference in its entirety herein. 
     Metadata subsystem  140  comprises metadata processing resources and partitioned replicated metadata stored locally at the storage service node. Metadata subsystem  140  receives, processes, and generates metadata. Metadata in system  100  is partitioned and replicated across a plurality of metadata nodes. Typically, metadata subsystem  140  is configured with a replication factor of 3 (RF3), and therefore many of the examples herein will include 3-way replication scenarios, but the invention is not so limited. Each metadata subsystem  140  tracks the state of data storage subsystems  150  and of other metadata subsystems  140  in storage cluster  110  to form a global view of the cluster. Metadata subsystem  140  is responsible for optimal replica assignment and tracks writes in storage cluster  110 . 
     Metadata synchronization logic (or “anti-entropy engine” (AE) not shown here) runs in metadata subsystem  140 . The metadata synchronization logic compares replicas of metadata across metadata nodes and ensures that the replicas agree on a superset of the metadata therein to avoid losing metadata. During storage and compaction of metadata-carrying string-sorted tables (SSTs), a consistent file identification scheme is used across all metadata nodes. When an application node writes to and reads from a virtual disk on distributed data storage system  100 , metadata is generated and stored in replicas on different metadata nodes. A modified log-structured merge tree is used to store and compact the metadata SST files. A fingerprint file is created for each metadata SST file that includes a start-length-hash value triple for each region of the metadata SST file. To synchronize, fingerprint files of two metadata SST files are compared, and if any hash values are missing from a fingerprint file, then key-value-timestamp triples corresponding to these missing hash values are sent to the metadata SST file that is missing them. An example of metadata synchronization logic is described in U.S. Pat. No. 10,740,300, which is incorporated by reference in its entirety herein. 
     Data storage subsystem  150  receives, processes, and stores payload data written to storage cluster  110 . Thus, data storage subsystem  150  is responsible for replicating data to other data storage subsystems  150  on other storage service nodes and striping data within and across storage pools. Data storage subsystem  150  comprises storage processing for payload data blocks (e.g., I/O, compaction, garbage collection, etc.) and stores partitioned replicated payload data at the storage service node. 
     The bold bi-directional arrows in the present figure show that metadata is communicated between storage proxy  106  and metadata subsystem  140 , whereas data blocks are transmitted to/from data storage subsystem  150 . Depending on the configuration, metadata subsystem  140  may operate on a first storage service node  120  or storage service  122  and data storage subsystem  150  may operate on another distinct storage service node  120  or storage service  122 . See also  FIGS. 1E and 1F . 
       FIG. 1D  is a block diagram depicting a virtual disk distributed across a plurality of storage service nodes and also depicting a plurality of storage resources available at each storage service node according to an illustrative embodiment. The present figure depicts: nine storage service nodes  120  ( 120 - 1  . . .  120 - 09 ); a virtual disk  170  that comprises data distributed over four of the storage service nodes— 120 - 1 ,  120 - 2 ,  120 - 4 , and  120 - 5 ; and storage resources  160  configured within storage service node  120 - 9 . 
     Each storage service node  120  (or compute host  121 ) is typically configured with computing resources (e.g., hardware processors and computer memory) for providing storage services and with a number of storage resources  160 , e.g., hard disk drives (HDD) shown here as storage disk shapes, solid state storage drives (SSD) (e.g., flash memory technology) shown here as square shapes, etc. The illustrative system uses commit logs, which are preferably stored on SSD before they are flushed to another disk/drive for persistent storage. Metadata commit logs are stored on dedicated metadata-commit-log drives “MCL”, whereas payload-data commit logs are stored on distinct dedicated data-commit-log drives “DCL.” As an example depicted in the present figure, pod system information is stored in storage resource “P” which is preferably SSD technology for faster read/write performance; the metadata commit log is stored in storage resource “MCL” which is preferably SSD technology; metadata is then flushed from the commit log to persistent storage “M” (SSD and/or HDD); the data commit log is stored in storage resource “DCL” which is preferably SSD technology; payload data is then flushed from the data commit log to persistent storage “D” (typically HDD). The storage resources  160  depicted in the present figures are shown here as non-limiting examples to ease the reader&#39;s understanding; the numbers and types of storage technologies among storage resources  160  will vary according to different implementations. 
     To accelerate read operations, client-side caching of data is used on SSDs accessible by storage proxy  106 . Data is also cached on SSDs at storage service nodes. For caching, the system supports the use of Peripheral Component Interconnect Express (PCIe) and Non-Volatile Memory Express (NVMe) SSDs. All writes are executed in memory and flash (SSD/NVMe) and flushed sequentially to persistent storage. Persistent storage uses flash technology (e.g., multi-level cell (MLC) and/or  3 D NAND SSD) and/or spinning disk technology (e.g., HDD)). Options are administrable at the virtual disk level. 
     Virtual disk (“vdisk”)  170  is the data storage representation of system  100  that is visible to and accessible by applications  132  as data storage resources. Virtual disk  170  is also referred to herein as “user virtual disk”  170  to reflect that it is visible to applications  132  as a data storage resource. In other words, each application  132  will use one or more virtual disks  170  for data storage without having knowledge of how system  100  as a whole is organized and configured. Every virtual disk  170  provisioned on the system is partitioned into fixed size chunks, each of which is called a storage container. Different replicas are assigned for each storage container. Since replica assignment occurs at the storage container level—not at a virtual disk level—the data for a virtual disk is distributed across a plurality of storage service nodes, thus allowing increased parallelism during I/Os and/or disk rebuilds. Thus, the virtual disks are distributed and fault-tolerant. Notably, the replication factor alone (e.g., RF3) does not limit how many storage service nodes  120  may comprise payload data of a given virtual disk  170 . Thus, different containers of the virtual disk may be stored and replicated on different storage service nodes, adding up to more total storage service nodes associated with the virtual disk than the replication factor of the virtual disk. 
     Any number of virtual disks  170  may be spun up, each one thinly provisioned and instantly available. Illustrative user-configurable attributes for virtual disk  170  include without limitation: Name—a unique name to identify the virtual disk. Size—to set the desired virtual disk size. System  100  supports single block and NFS virtual disks of unlimited size. Disk Type—to specify the type of storage protocol to use for the virtual disk: block or file (NFS). Object containers/buckets are provisioned directly from OpenStack via Swift, via the Amazon S3 API, etc. Workload Type—for NFS disk type, options include default, proprietary, or object storage target (OST) workload types. For proprietary and OST, if Enable Deduplication is selected, a Retention Policy can be added as well. For block disk type, the only option is default. Retention Policy—specifies a duration for proprietary and OST workloads, e.g., two weeks, one month, etc. Encryption—to encrypt both data at rest and data in flight for the virtual disk. Enable Deduplication—to enable inline global deduplication. Clustered File System—to indicate that the virtual disk will be used with a clustered file system. When selected, system  100  enables concurrent read/write operations from multiple VMs or hosts. Description—to provide an optional brief description of the virtual disk. Compressed—to enable virtual disk compression to reduce data size. Client-Side Caching—to cache data to local SSD or PCIe devices at the application tier to accelerate read performance. CSV—to enable Cluster Shared Volumes for failover (or high availability) clustering. A CSV is a shared disk containing a Windows NT File System (NTFS) or Resilient File System (ReFS) volume that is made accessible for read and write operations by all nodes within a Windows Server failover cluster. Replication Policy—to set the policy for how data will replicate across the cluster: Agnostic, Rack Aware, or Data Center Aware. Replication Factor (RF)—to designate the number of replicas for each virtual disk. Replication factor is tunable, typically ranging from one to six, without limitation. Block Size—to set a block virtual disk size to 512 bytes, 4k or 64k. File (NFS)-based virtual disks have a standard 512 size, and object-based virtual disks have a standard 64K size. Residence—to select the type of media on which the data is to reside: HDD, SSD. The present figure depicts only one virtual disk  170  for illustrative purposes, but system  100  has no limits on how many virtual disks it may support. 
       FIG. 1E  is a block diagram depicting a typical I/O workflow for write operations originating with an application. This figure depicts an application  132  writing to storage cluster  110 , illustratively writing to a virtual disk  170  configured with Replication Factor=3 (RF3). 
     At step W, storage proxy  106  intercepts a write command issued by application  132 , comprising one or more payload data blocks to be written to a virtual disk  170  in storage cluster  110 . At step  1 W, storage proxy  106  determines the replica nodes  120  for the data blocks to be written and transmits the data blocks to one of the replica nodes  120 , e.g.,  120 - 4 . If the virtual disk is enabled for deduplication, storage proxy  106  calculates a data block fingerprint, queries the dedupe cache and, if necessary, further queries metadata subsystem  140  (at the virtual disk&#39;s metadata owner node, e.g.,  120 - 7 ), and either makes a metadata update or proceeds with a new write. At step  2 W, data storage subsystem  150  on replica node  120 - 4  receives and writes the data blocks locally and forwards them to other designated replica nodes, e.g.,  120 - 1  and  120 - 8 . At step  3 W, storage proxy  106  sends a write acknowledgment back to the originating application  132  after a quorum of data storage subsystem  150  replicas have completed step  2 W. For RF3, two acknowledged successful writes are needed from the three (RF3) replicas to satisfy the quorum (RF/2+1=3/2+1=2). Two of the three replicas are written synchronously, and one may be written asynchronously. At step  4 W, storage proxy  106  causes an atomic write to be made into metadata subsystem  140  at metadata owner node  120 - 7 , after which the write is deemed successful. At step  5 W, metadata subsystem  140  replicates the metadata from node  120 - 7  to designated metadata replica nodes, e.g.,  120 - 8  and  120 - 9 . 
       FIG. 1F  is a block diagram depicting a typical I/O workflow for read operations originating with an application. This figure depicts an application  132  reading from storage cluster  110 , illustratively reading from a virtual disk  170  configured with RF3. 
     At step R, storage proxy  106  intercepts a read request issued by application  132  for one or more data blocks from a virtual disk  170  in storage cluster  110 . At step  1 R, storage proxy  106  queries the local metacache for a particular data block to be read and if the information is not found in the local metacache, at step  1 R′ storage proxy  106  consults metadata subsystem  140  (e.g., at the vdisk&#39;s designated metadata owner node  120 - 7 ). At step  2 R, storage proxy  106  sends the data block details to one of the closest data storage subsystems  150 , based on observed latency, e.g., storage service node  120 - 4 . At step  3 R, data storage subsystem  150  reads the data block(s) and transmits the block(s) back, if found, to storage proxy  106 . If the read operation fails due to any error, the read is attempted from another replica. At step  4 R, storage proxy  106  serves the requested data block(s) to application  132 . If client-side caching is enabled for the targeted virtual disk  170  during provisioning, storage proxy  106  queries the local block cache at step  1 R to fetch the data block(s), and if found therein serves the data block(s) to application  132  at step  4 R, thereby bypassing data storage subsystem  150  at the storage service nodes(s) and eliminating the need to traverse the network to reach storage cluster  110 . 
     System Resiliency. System  100  is designed to survive disk, node, rack, and data center outages without application downtime and with minimal performance impact. These resiliency features include: high availability, non-disruptive upgrades (NDU), disk failures, replication, and snapshots and clones. 
     High availability (HA). A preferable minimum of three storage service node should be provisioned for an implementation of the illustrative system. Redundancy can be set as agnostic, at the rack level, or at data center level. The system initiates transparent failover in case of failure. During node, rack, or site failures, reads and writes continue as usual from/to remaining operational replicas. To protect against a single point of failure, storage proxies  106  install as a high availability active/passive pair (“HA pair,” not shown). A virtual IP address (VIP) assigned to the HA pair redirects traffic automatically to the active storage proxy  106  at any given time. If one storage proxy  106  instance is lost or interrupted, operations fail over seamlessly to the passive instance to maintain availability. This happens without requiring intervention by applications, administrators, or users. During provisioning, administrators can indicate that an application host  102 / 121  will use a clustered file system. This automatically sets internal configuration parameters to ensure seamless failover when using VM migration to a secondary physical host running its own storage proxy  106 . During live VM migration, such as VMware vMotion or Microsoft Hyper-V, any necessary block and file storage “follows” guest VMs to another host. 
     Non-disruptive upgrades (NDUs). The illustrative system supports non-disruptive software upgrades by staging and rolling the upgrade across individual components using the highly available nature of distributed data storage system  100  to eliminate any downtime or data unavailability. Storage service nodes  120  and storage services  122  undergo upgrades first one node at a time. Meanwhile, any I/O continues to be serviced from alternate available nodes, e.g., replicas. Storage proxies  106  are upgraded next, starting with the passive storage proxy in HA pairs. After the passive storage proxy upgrade is complete, it is made active, and the formerly active storage proxy  106  is upgraded and resumes service as the passive of the HA pair. This process eliminates any interruption to reads or writes during the upgrade procedure. 
     Disk Failures. The illustrative system supports efficient data and metadata rebuilds that are initiated automatically when there is a disk failure. Payload data is rebuilt from other data replicas and using information in the metadata subsystem. The metadata rebuild self-heals within the metadata service. 
     Replication. The illustrative system uses a combination of synchronous and asynchronous replication processes to distribute and protect data across the cluster and provide near-zero recovery point objectives (RPO) and recovery time objectives (RTO). For example, two of three replicas are written synchronously, and one is written asynchronously. The system supports any number of active data centers in a single storage cluster  110 , using a tunable replication factor and replication policy options. The replication factor designates the number of replicas to create for each virtual disk, and the replication policy defines the destination for the replicas across the cluster. Replicas occur at the storage container level of a virtual disk  170 . For example, if a 100 GB virtual disk with RF3 is created, the entire 100 GBs are not stored as contiguous chunks on three storage service nodes. Instead, the 100 GBs are divided among several storage containers, and replicas of each storage container are spread across different storage pools on different storage service nodes within the storage cluster. For additional disaster recovery protection against rack and data center failures, the illustrative system supports replication policies that span multiple racks or data centers using structured IP addressing, DNS naming/suffix, and/or customer-defined snitch endpoints. For “agnostic” replication policies, data is spread across the storage cluster using a best-effort to improve availability. For “rack aware” replication policies, data is spread across as many physically distinct racks as possible within in a single data center. For “data center aware” replication policies, data replicates to additional physical sites, which can include private and/or hosted data centers and public clouds. In a disaster recovery example, where the Replication Policy=Data Center Aware and the Replication Factor=3, the illustrative system divides the data into storage containers and ensures that three copies (RF3) of each storage container are spread to geographically dispersed physical sites, e.g., Data Centers A, B, and C. At any time, if a data copy fails, re-replication is automatically initiated from replicas across the data centers. 
     Snapshots And Clones. In addition to replication policies, data management tasks include taking snapshots and making “zero-copy” clones of virtual disks. There is no limit to the number of snapshots or clones that can be created. Snapshots and clones are space-efficient, requiring capacity only for changed blocks. 
     Encryption. The illustrative system provides software-based encryption with the Encrypt360 feature. This enables encryption of data at the point of ingestion (at storage proxy  106 ). Data encrypted in this way remains protected in flight between storage proxy  106  and storage service nodes  120 /storage service  122 , in flight among storage service nodes as part of replication, in-use at storage proxy  106 , and at rest while in storage. Any encryption scheme may be implemented, preferably 256-bit AES. Additionally, any third-party key management system can be attached. 
     Ecosystem Integration. The illustrative system works with and provides a secure distributed data storage system for a variety of data-generating platforms, including systems that generate primary (production) data and systems that generate backup data from primary sources. VMware. The illustrative system features a vCenter plug-in that enables provisioning, management, snapshotting, and cloning of virtual disks  170  directly from the vSphere Web Client. Additionally, the system incorporates support for the VMware vSphere Storage APIs Array Integration (VAAI). Docker. The illustrative system provides persistent storage for Docker software containers through a volume plugin. The volume plugin enables a user to create a persistent Docker volume backed by a virtual disk  170 . Different options, such as deduplication, compression, replication factor, and/or block size, may be set for each Docker volume, using “volume options” in the Docker Universal Control Plane (UCP) or using the “docker volume” command line. The virtual disk can then be attached to any host. The volume plugin also creates a file system on this virtual disk and mounts it using the path provided by the user. The file system type can also be configured by the user. All I/O to the Docker volume goes to virtual disk  170 . As the software container moves in the environment, virtual disk  170  will automatically be made available to any host, and data will be persisted using the policies chosen during volume creation. For container orchestration platforms, such as Kubernetes and OpenShift, the illustrative system  100  provides persistent storage for software containers through a proprietary dynamic provisioner and via other technologies that interoperate with the orchestration platform(s). OpenStack. The illustrative system delivers block, file, and object storage for OpenStack all from a single platform via native Cinder and Swift integration. The system supports granular administration, per-volume (Cinder) or per-container (Swift), for capabilities such as compression, deduplication, snapshots, and/or clones. OpenStack administrators can provision the full set of storage capabilities of system  100  in OpenStack Horizon via OpenStack&#39;s QoS functionality. As with VMware, administrators need not use system  100 &#39;s native web user interfaces and/or RESTful API, and storage can be managed from within the OpenStack interface. 
     Multitenancy. The illustrative system supports the use of rack-aware and data center-aware replication policies for customers who must satisfy regulatory compliance and restrict certain data by region or site. These capabilities provide the backbone of a multitenant architecture, which is supported with three forms of architectural isolation: LUN masking, dedicated storage proxies, and complete physical isolation. Using the LUN masking option, different tenants are hosted on a shared infrastructure with logical separation. Logical separation is achieved by presenting virtual disks only to a certain VM and/or physical application host (IP range). Quality of Service (QoS) is delivered at the VM level. Using the dedicated storage proxies option, storage access is provided with a dedicated storage proxy  106  per tenant. Storage proxies can be deployed on a dedicated physical host or a shared host. This provides storage as a shared infrastructure, while compute is dedicated to each tenant. Quality of Service (QoS) is at the VM level. Using the complete physical isolation option, different tenants are hosted on dedicated storage clusters (each running their own storage service and storage proxies) to provide complete logical and physical separation between tenants. For all of these multitenant architectures, each tenant can have unique virtual disks with tenant-specific storage policies, because the illustrative system configures policies at the virtual disk level. Policies can be grouped to create classes of service (CoS). 
     Thus, the illustrative distributed data storage system  100  scales seamlessly and linearly from a few nodes to thousands of nodes using virtual disks as the user-visible storage resource provided by the system. Enterprise storage capabilities are configurable at the virtual disk level. The storage service nodes can be configured in a plurality of physical computing environments, e.g., data centers, private clouds, and/or public clouds without limitation. Likewise, the storage proxies may execute in the same or different computing environment from the storage service nodes, e.g., within the same cloud computing environment, different cloud computing environments, different cloud availability zones, and/or in a non-cloud data center, thus enabling cloud-to-cloud and/or multi-cloud services, as well as non-cloud and/or hybrid service environments. 
     Optimized Deduplication Based on Backup Frequency in a Distributed Data Storage System 
       FIG. 1G  is a block diagram illustrating a backup configuration that depicts an illustrative data storage management system using distributed data storage system  100  for storing secondary (backup) copies according to an illustrative embodiment. The figure depicts: storage service nodes  120  configured with a global system deduplication virtual disk  1700 ; client computing device  1020  hosting application(s)  132  and data agent(s)  1420 ; backup computing device  1060  hosting media agent(s)  1440  and storage proxy  106 ; and storage manager  1400 . An example of the illustrative data storage management system is the Commvault Complete™ Backup and Recovery software solution from Commvault Systems, Inc. of Tinton Falls, N.J., USA. Hereinafter, secondary copies, which are distinguishable from primary (application-native) data, will be referred to as “backup copies” for simplicity and to ease the reader&#39;s understanding of the present disclosure. 
     Client computing device  1020  and backup computing device  1060  each comprise one or more hardware processors and computer memory for executing computer programs. Likewise, storage manager  1400  is hosted by and/or comprises one or more hardware processors and computer memory for executing computer programs. These components may operate in any computing environment, e.g., non-cloud data center, hybrid cloud, private cloud, and/or public cloud without limitation. 
     Components of the data storage management system include storage manager  1400 , one or more data agents  1420 , and one or more media agents  1440 . Primary data  1120  generated and used by client applications  132  is captured by a data agent  1420 , transmitted to a media agent  1440 , and converted into one or more backup copies that are sent to distributed data storage system  100  (via storage proxy  106 ) for storage. Control and management of the backup process is performed by storage manager  1400 . 
     Storage manager  1400  is a centralized storage and/or information manager that is configured to perform certain control functions and also to store certain critical information about the data storage management system—hence storage manager  1400  is said to manage the data storage management system. Storage manager  1400  communicates with, instructs, and/or controls data agents  1420  and media agents  1440 . According to certain embodiments, storage manager  1400  provides one or more of the following functions:
         communicating with data agents  1420  and media agents  1440 , including transmitting instructions, messages, and/or queries, as well as receiving status reports, index information, messages, and/or queries, and responding to same;   initiating execution of storage and/or information management operations;   initiating restore and recovery operations;   allocating secondary storage resources for secondary copy operations, e.g., distributed data storage system  100 ;   reporting, searching, and/or classification of data;   monitoring completion of and status reporting related to storage operations, information management operations, and jobs;   tracking age information relating to backup copies and initiating data pruning when appropriate;   protecting metadata of the data storage management system;   implementing job management, schedule management, event management, alert management, reporting, job history maintenance, user security management, disaster recovery management, and/or user interfacing for system administrators and/or end users of the data storage management system; etc.       

     Data agent  1420  is a component of the data storage management system and is generally directed by storage manager  1400  to participate in creating or restoring backup copies. A variety of different applications  132  can operate on a given client computing device  1020 , including operating systems, file systems, database applications, e-mail applications, and virtual machines, just to name a few. And, as part of the process of creating and restoring backup copies, the client computing device  1020  may be tasked with processing and preparing the primary data  1120  generated by these various applications  132 . Moreover, the nature of the processing/preparation can differ among application types, e.g., due to inherent structural, state, and formatting differences among applications  132  and/or the operating system of client computing device  1020 . Each data agent  1420  is therefore advantageously configured to participate in storage operations and/or information management operations based on the type of primary data  1120  that is being protected at a client-specific and/or application-specific level. Data agent  1420  may be a computer software program (e.g., in the form of a set of executable binary files) that executes on the same client computing device  1020  as the associated application  132  that data agent  1420  is configured to protect or on an associated computing device. For instance, data agent  1420  may take part in copying, archiving, migrating, and/or replicating of certain primary data  1120 . Data agent  1420  may receive control information from storage manager  1400 , such as commands to transfer copies of data objects and/or metadata to one or more media agents  1440 . Data agent  1420  also may format, compress, deduplicate, and/or encrypt certain primary data  1120 , as well as capture application-related metadata before transmitting the processed data to media agent  1440 . Data agent  1420  also may receive instructions from storage manager  1400  to restore (or assist in restoring) a backup copy such that the restored data may be properly accessed by application  132  as primary data  1120  in an application-native format. Each data agent  1420  may be specialized for a particular application  132 . 
     Media agent  1440  is a component of the data storage management system and is generally directed by storage manager  1400  in creating and restoring backup copies such as backup copies stored at distributed data storage system  100 . Whereas storage manager  1400  generally manages the data storage management system as a whole, media agent  1440  provides a portal to certain secondary storage resources, such as distributed data storage system  100  by having specialized features for communicating therewith, e.g., via storage proxy  106 . Media agent  1440  may be a software program (e.g., in the form of a set of executable binary files) that executes on a backup computing device  1060 . Media agent  1440  generally manages, coordinates, and facilitates the transmission of data between a data agent  1420  and secondary storage resources (e.g., system  100 ) associated with media agent  1440 . For instance, other components in the system may interact with media agent  1440  to gain access to data stored on distributed data storage system  100 , (e.g., to browse, read, write, modify, delete, or restore data). 
     The configuration depicted in the present figure uses distributed data storage system  100  as the storage target for backup copies. Backup copies generated by the data storage management system are transmitted by each media agent  1440  to a user virtual disk  170  (not shown here, but see virtual disk  170  in  FIG. 1D ), which is defined on the distributed storage system as the storage target for the backup copy. Hereinafter, virtual disks  170  will be referred to as “user virtual disks”  170  to distinguish them more clearly from the global deduplication virtual disk  1700 . Storage proxy  106  intercepts the write requests issued by the media agent  1440  and applies deduplication to the incoming data blocks as described herein. 
     The storage service nodes  120  comprise physical data storage resources as shown in another figure. The illustrative global system deduplication virtual disk  1700  is configured as a virtual disk that is partitioned and replicated across a plurality of storage service nodes  120 / 122 . Thus, the global system deduplication virtual disk  1700  is treated as a single logically-centralized repository of deduplicated data blocks across the distributed storage system, but physical storage is both partitioned and replicated across a plurality of storage service nodes  120 / 122 . 
     The global system deduplication virtual disk (or “system-wide deduplication virtual disk”)  1700  is partitioned into fixed size virtual chunks, each of which is called a storage container (illustratively embodied as a Hedvig Container). Different replicas are assigned for each storage container. Since replica assignment occurs at the storage container level, the data for a virtual disk such as the global system deduplication virtual disk  1700  is spread across the storage cluster. Replicas are chosen by metadata subsystem  140  according to replication factor and replication policy settings to support the application&#39;s data protection needs. See also  FIG. 2B . 
       FIG. 1H  is a block diagram illustrating a backup configuration in which a third-party backup system uses the illustrative distributed data storage system  100  for storing secondary (backup) copies according to an illustrative embodiment. This figure is analogous to  FIG. 1G , except that backup copies originate with a third-party backup system  1500  rather than from the illustrative data storage management system of  FIG. 1G . Accordingly, backup data is generated by backup system  1500  and addressed to a user virtual disk  170 , which is defined on the distributed storage system as the storage target for the backup copy. Storage proxy  106  intercepts the write requests and applies deduplication to the incoming data blocks as described herein. 
       FIG. 2A  is a block diagram depicting components that play a role in the present deduplication solution according to an illustrative embodiment. The present figure depicts: storage proxy  106  comprising deduplication tracking logic  206 ; pod subsystem  130  comprising epoch calculator  230 ; metadata subsystem  140  comprising deduplication tracking and garbage collection logic  240 ; and data storage subsystem comprising compaction logic  250  and write logic  252 . Notably, the pod, metadata, and data storage subsystems components need not reside on the same storage service node, and may be deployed on various different storage service nodes. More details are given in  FIG. 2B . 
       FIG. 2B  is a block diagram depicting certain components that play a role in the present deduplication solution, providing additional details about the components depicted in  FIG. 2A . 
     Storage proxy  106  comprises deduplication management and tracking logic (e.g., “DD tracking logic”)  206  and an index (e.g., “DDCache”)  207  for tracking hash values and DDblockIDs with their associated expiry epochs. See also  FIG. 2C . The DDCache  207  is where the DD tracking logic  206  checks whether a hash value computed for an incoming data block can be found, and if so, identifies a unique DDblockID and an expiry epoch for it. DD tracking logic  206  at storage proxy  106  illustratively performs the functionality of the storage proxy as depicted in  FIG. 3 . 
     Pod subsystem  130  uses an illustrative epoch calculator function  230  to provide a cluster-level epoch value  231  used for the illustrative discard or GC cycle. The current epoch value  231  is referred to herein as Enow. In every discard cycle, Enow is incremented during the discard preparation stage (+2) and incremented again during every discard stage (+1). Accordingly, the numerical values of Enow use the following pattern: Enow=0, 2(+2 GC Prep), 3(+1 GC), 5(+2 GC Prep), 6(+1 GC), 8, 9, etc. The scheme for setting the current epoch, Enow, is also depicted in block  401  of  FIGS. 4 and 6 . 
     Metadata subsystem  140  comprises deduplication management, tracking, and GC logic (e.g., the “DD tracking and garbage collection logic”)  240 . Metadata subsystem  140  also maintains several data structures, illustratively organized as column families, e.g., VdiskBlockInfo  241 , DDInfo  242 , DDTracker  243 , DDRefCount  244 , GCCycleInfo  245 , and Container-Specific Discard Lists  246 . See also  FIGS. 2C-2G . DD tracking and garbage collection logic  240  illustratively performs the functionality of metadata subsystem  140  as depicted in  FIG. 3 , as well as significant portions of methods  400  and  600  as depicted in  FIGS. 4 and 6 , respectively. 
     Data storage subsystem  150  comprises write logic  252  for adding new DDblockIDs to the global system deduplication virtual disk  1700 , or more specifically, for writing new data blocks to the storage container(s)  260  hosted by the particular storage service node, e.g., C1, C2, C3, which are numbered  260 - 1 ,  260 - 2 , and  260 - 3 , respectively. See also  FIG. 3 . Data storage subsystem  150  receives container-specific discard lists  246  from metadata subsystem  140 . Data storage subsystem  150  also comprises compaction logic  250  that removes the DDblockIDs received in the discard lists from the storage containers  260  at the storage service node  120 / 122 . See also  FIG. 6 . Thus, each storage service node  120 / 122  that hosts one or more storage containers  260  belonging to the global system deduplication virtual disk  1700  is responsible for adding new blocks to and deleting blocks from those storage containers. 
       FIG. 2C  depicts an illustrative column family for associating hash values with corresponding DDblockID and an associated expiry epoch, e.g., DDCache  207 , DDInfo  242 . This functions as an index of hash values. The DDCache  207  configured at a storage proxy  106  is configured according to this illustrative column family. Likewise, the DDInfo  242  configured at metadata subsystem  140  is also configured according to this column family. Information from DDInfo  242  is sometimes updated into DDCache  207  as needed. See also  FIG. 3 . Expiry extensions for certain DDblockIDs are updated into DDInfo  242  as needed during the GC phase of the discard cycle. See also  FIG. 7 . 
       FIG. 2D  depicts an illustrative column family for tracking write requests received by storage proxies, e.g., DDTracker  243 . In each discard cycle, a new DDTracker  243  column family tracks, for each user virtual disk  170 , every new write request intercepted by storage proxies  106 . The data is organized by storage container  260  belonging to the global system deduplication virtual disk  1700 . For each DDblockID, its corresponding hash value is included, and the DDblockID receives a timestamp of Enow+1. Notably, the DDTracker  243  tables are discarded after their information is scanned and used for updating the DDRefCount  244  table family. See also  FIG. 4 . Thus, DDTracker  243  provides information on write requests that came in during a certain discard cycle and DDTracker  243  is re-populated in the next discard cycle. 
       FIG. 2E  depicts an illustrative column family that is updated during each discard preparation stage based on scanning and processing DDTracker  243  information, e.g., DDRefCount  244 . The information is organized according to storage containers  260  of the global system deduplication virtual disk  1700 , and then by DDblockID. Columns for each user virtual disk  170  referencing the DDblockID are added. For every undeleted DDblockID, the preparation stage processes information in DDTracker  243 , updates DDRefCount  244  entries, and updates the DDblockID expiry column. The reference write epoch (Ref W Epoch) columns are populated from the timestamp column in DDTracker  243 . The reference expiry value (Ref Expiry) columns add to the write epoch a frequency for executing full backups of the particular user virtual disk  170  that references the DDblockID. 
     The expiry value assigned to the DDblockID (Eu) is the maximum value of the various Ref Expiry columns. Thus, the various backup frequencies of the various user virtual disks  170  are taken into consideration here. The Eu value is considered when deciding whether to discard a DDblockID. See also block  706  in  FIG. 7 . The Eu value is updated into DDInfo  242 , if necessary, to extend the life of a DDblockID. See also block  709  in  FIG. 7 . The DDRefCount  244  column families are persisted across discard cycles, unlike DDTracker  243 . 
       FIG. 2F  depicts an illustrative column family that assigns the workload of scanning DDTracker  243  column families (each DDTracker  243  associated with a specific user virtual disk  170 ) for the purpose of updating DDRefCount  244 . Various storage service nodes  120 / 122  are assigned the task. This table is generated on a first-time execution of the discard preparation stage and is persisted for future and repeated use in other discard cycles. See also block  406  in  FIG. 4 . 
       FIG. 2G  depicts an illustrative column family that comprises DDblockID discard lists  246 . These discard lists  246  are generated from the DDRefCount  244  column families during the discard (GC) stage, which follows the discard preparation stage. For each storage container  260  belonging to the global system deduplication virtual disk  1700 , discard lists  246  are added, one discard list  246  per epoch when the discard list  246  was generated. The reason for multiple epochs showing up here is that the compaction logic or process  250  that actually discards DDblockIDs occurs asynchronously from any particular discard cycle, so it is possible for multiple lists  246  to accumulate before the compaction process  250  is triggered on any given storage service node. 
     In regard to the column families depicted in  FIGS. 2C-2G , the depictions are illustrative and the invention is not so limited. In other embodiments, the data may be differently organized and the various stages responsible for generating and processing the data may also differ from what is depicted and described herein. 
       FIG. 3  depicts a fence diagram that illustrates some salient operations occurring during an I/O cycle of an incoming data block intercepted by a storage proxy  106 . The fence diagram depicts operations at storage proxy  106  in the left-hand column (e.g., using DD tracking logic  206  and DDCache  207 ), operations at a metadata subsystem  140  in the center column (e.g., using DD tracking and garbage collection logic  240  and a number of data structures, e.g.,  241 ,  242 ,  243 ), and operations at a data storage subsystem  150  in the right-hand column (e.g., using write logic  252 ) adding to the global system deduplication virtual disk  1700 , which is a replicated and partitioned virtual disk. 
     At block A, storage proxy  106  receives a data block (usrblock) targeting a user virtual disk  170  (e.g., usrvdisk_1). At block B, storage proxy  106  calculates a hash value for the received data block. At block C, storage proxy  106  determines whether the calculated hash value is in the DDCache index  207 . If yes, control passes to block L; otherwise, control passes to block D. 
     At block D, metadata subsystem  140  receives the hash value from storage proxy  106  and checks whether the hash value is in the DDInfo index  242 . If yes, in the event that the metadata subsystem finds the hash value received from the storage proxy in DDInfo  242 , metadata subsystem  140  responds to storage proxy  106  in the affirmative, providing the DDblockID and its associated expiry Ej according to DDInfo  242  and then control passes to block J. In the event metadata subsystem  140  has no record of the hash value in DDInfo  242 , it responds in the negative to storage proxy  106 ; in this case, storage proxy  106  causes the data block to be added to the global system deduplication virtual disk  1700  at block E. 
     At block E, data storage subsystem  150  writes the new data block (usrblock) to the global system deduplication virtual disk  1700  and a new and unique deduplication block identifier (DDblockID) is assigned to the new data block at this point. Illustratively, 4 KB is the data block size stored at distributed data storage system  100  and hence each DDblockID refers to a 4 KB data block. Henceforth, for simplicity, we shall refer to data blocks in distributed data storage system  100  using the term “DDblockID,” though it will be clear from context that the data block is stored in its entirety in the global system deduplication virtual disk  1700  and is tracked or referenced elsewhere by its DDblockID. Control then passes to blocks F and G. At block F, metadata subsystem  140  updates the VdiskBlockInfo  241  column family and the DDTracker  243  column family to reflect the write request and the DDblockID being added to the distributed data storage system. At block G, metadata subsystem  140  adds the new DDblockID to DDInfo  242  and assigns an expiry epoch (Ej) to DDblockID by adding 7 (as an illustrative example) to the value of the current epoch Enow. The increment of 7 is illustrative and stems from the fact that, because every discard cycle increments the epoch by a total of 3, it is desirable to give the data block at least two full discard cycles of initial lifetime (2×3=6). Since the timestamp epoch in DDTracker  243  is set to Enow+1, when 6 is added the increment becomes 7. Hence, at block G, Ej=Enow+7. Metadata subsystem  140  transmits this information to storage proxy  106 . At block H, storage proxy  106  updates its DDCache  207  by associating the hash value with the DDblockID and expiry epoch Ej. At this point, the I/O cycle for this data block is complete. 
     Block J is reached when storage proxy  106  receives a DDblockID and associated expiry Ej from metadata subsystem  140  (from DDInfo  242 ) at block D. At block J, storage proxy  106  determines whether the DDblockID is expired according to the information received from metadata subsystem  140 , i.e., whether Ej is earlier than Enow. If the DDblockID is expired, storage proxy  106  treats the incoming data block as a new data block to be added to the distributed data storage system and control passes to block E, which is described in more detail above. On the other hand, if at block J storage proxy  106  determines that the information received from DDInfo  242  indicates an unexpired DDblockID, the usrblock qualifies for deduplication and is not added to the global deduplication virtual disk  1700 . Instead, at block H, storage proxy  106  updates its DDCache  207  by associating the hash value with the DDblockID and expiry epoch Ej received from DDInfo  242 . Furthermore, at block F, metadata subsystem  140  updates the VdiskBlockInfo  241  column family and the DDTracker  243  column family to reflect the write request of the DDblockID. At this point, the I/O cycle for this data block is complete. 
     Block L is reached in the event that, at block C, storage proxy  106  finds the calculated hash value of usrblock in its DDCache  207 . In DDCache  207 , the hash value is associated with a DDblockID having an expiry epoch Ej and control passes to block L. At block L, storage proxy  106  determines, based on DDCache  207 , whether the DDblockID is expired, i.e., whether its expiry epoch Ej is earlier than Enow. If DDCache  207  indicates that the DDblockID is expired, storage proxy  106  treats the incoming data block as if it weren&#39;t found in DDCache  207  and control passes to block D. Otherwise, if DDCache  207  indicates that the DDblockID is not expired, control passes to block M. Block M is the same as block F, i.e., metadata subsystem  140  updates the VdiskBlockInfo  241  column family and the DDTracker  243  column family to reflect the fact that a write request was received in the current epoch for this DDblockID. At this point, the I/O cycle for usrblock ends with block M. 
       FIG. 4  depicts some salient operations of a method  400  according to an illustrative embodiment. Method  400  illustrates the first phase of a discard cycle, the discard preparation (“GC prep”) stage. Method  400  is performed by one or more components of the illustrative distributed data storage system  100 . The operations of metadata subsystem  140  in method  400  are illustratively performed by DD tracking and garbage collection logic  240 . 
     Block  401  depicts the scheme for setting the current epoch, Enow, to aid in the reader&#39;s understanding of the depicted method. In every discard cycle, Enow is incremented during the discard preparation stage (+2) and incremented again during every discard stage (+1). Accordingly, the numerical values of Enow use the following pattern: Enow=0, 2(+2 GC Prep), 3(+1 GC), 5(+2 GC Prep), 6(+1 GC), 8, 9, etc. This pattern is shown at the top of  FIG. 4  in block  401  and results from the operation at block  402 . Illustratively, epoch calculator  230  tracks and calculates Enow. 
     At block  402 , at the beginning of a discard preparation stage, the pod subsystem (e.g., using epoch calculator  230 ) increments the current epoch value by 2. The general formula is GC Prep Epoch=(3*N)−1, where N≥1 and N is a discard cycle that includes the preparation stage and the discard stage. 
     At block  404 , a loop is initiated for each user (user-defined) virtual disk configured on the distributed data storage system. The loop includes blocks  406 - 410 . 
     At block  406 , on a first-time execution of the discard preparation stage, metadata subsystem  140  generates the data structure in  FIG. 2F , e.g., column family  286 , which comprises certain workload assignments. Accordingly, the workload of scanning DDTracker  243  column families (see  FIG. 2D ) in each GC prep stage is assigned to a particular storage service node  120 / 122 . Thanks to consistent hashing, the primary metadata node associated with each user virtual disk  170  is a deterministic hostname that receives the workload assignment and will carry out this workload going forward. The present data structure  286  is persisted, so that it can be used in the event that its host storage service node fails. 
     At block  407 , the metadata node assigns the scan workload to one or more storage service nodes  120 / 122  according to data structure  286  in  FIG. 2F . 
     At block  408 , the assigned storage service node(s)  120 / 122 , using a respective metadata subsystem  140 , scan the DDTracker  243  column families (generated in epoch Enow-2 or Enow-3, if any). Since a write request can come in at any time, such as during a preparation stage, this scheme ensures that all such write requests are scanned during the next preparation stage. Thus, no write requests are left out of DDTracker  243  and therefore no write requests are left unscanned. Each DDTracker  243  column family is associated with a particular user virtual disk  170  (see  FIG. 2D ). The scan results are used to update DDRefCount  244  column families. See also  FIG. 2E . More details on block  408  are given in a subsequent figure. 
     At block  410 , after the scanning task is completed, the DDTracker  243  column families are discarded and control returns to block  404 . 
       FIG. 5  depicts some salient operations of block  408  in method  400  according to an illustrative embodiment. This block is performed by the primary metadata node associated with each user virtual disk  170  according to data structure  286  in  FIG. 2F . 
     At block  502 , a loop is initiated for each DDblockID in the DDTracker  243  being scanned. The loop includes block  504 . 
     At block  504 , columns are added to DDRefCount  244  to: (i) populate the user virtual disk  170  (uservdisk) column; (ii) from the timestamp entry in DDTracker  243 , populate the epoch value associated with the timeframe when the write request was received for the DDblockID, e.g., Ew; and (iii) assign an expiry epoch to the present reference based on the full backup frequency of the user virtual disk  170 , e.g., Ew plus the full backup frequency of the user virtual disk  170 . See also  FIG. 2E . Control passes back to block  502 . 
     At block  506 , after DDRefCount  244  is fully updated from the DDTracker  243  scans, DDRefCount  244  is persisted. DDRefCount  244  will be used later during the second phase of the discard cycle. 
       FIG. 6  depicts some salient operations of a method  600  according to an illustrative embodiment. Method  600  illustrates the second phase of the discard cycle, the discard (“GC”) stage. Method  600  is performed by one or more components of the illustrative distributed data storage system  100  unless otherwise noted. The operations of metadata subsystem  140  in method  600  are illustratively performed by the DD tracking and garbage collection logic  240 . 
     Block  401  depicts the scheme for setting the current epoch, Enow, to aid in the reader&#39;s understanding of the depicted method. See also  FIG. 4 . 
     At block  602 , at the beginning of a discard (GC) stage, pod subsystem  130  (e.g., using epoch calculator  230 ) increments the current epoch Enow value by 1. The general formula is GC Epoch=(3*N), where N≥1 and N is a discard cycle that includes the preparation stage and the discard stage. 
     At block  604 , a loop is initiated for each storage container  260  belonging to the global system deduplication virtual disk  1700 . The loop includes blocks  606 - 608 , which are executed by metadata subsystem  140  in the primary metadata node associated with the respective storage container  260 . 
     At block  606 , metadata subsystem  140  scans DDRefCount  244  to determine whether each DDblockID is both expired and has a zero reference count, in order to build the discard list  246  for the storage container  260 . More details are given in a subsequent figure. 
     At block  608 , the discard list  246  for the storage container  260  is pushed by metadata subsystem  140  to data storage subsystem  150  that hosts the storage container  260 , possibly on another storage service node  120 / 122  distinct from the one hosting metadata subsystem  140 . At data storage subsystem  150 , a compaction process  250  will discard the DDblockIDs when it executes. In some embodiments, data storage subsystem  150  pulls the container discard list(s)  246  from metadata subsystem  140  asynchronously, when it has processing cycle time available. This alternative approach ensures that storage service nodes  120 / 122  do not get overwhelmed by the garbage cleanup task and instead can pull the discard lists  246  whenever they deem fit. This approach also ensures that if a storage service node  120 / 122  is down when a discard list  246  is pushed thereto, the storage service node can still obtain the discard list  246  and purge the blocks later. See also  FIG. 2B . Control passes back to block  604 . 
       FIG. 7  depicts some salient operations of block  606  in method  600  according to an illustrative embodiment. This block is performed by the metadata subsystem in the primary metadata node associated with the respective storage container. 
     At block  702 , a loop is initiated for each DDblockID in the DDRefCount  244  column family. The loop includes blocks  703 - 712 . 
     At block  703 , the reference count and/or entries in DDRefCount  244  are decremented to account for the deletion of a user virtual disk  170  from distributed data storage system  100 . Illustratively, each backup copy received by distributed data storage system  100  comprises one or more files, and the distributed data storage system addresses each file to a file-specific user virtual disk  170 . When the backup system that generated the backup copies prunes a stale backup copy from the distributed data storage system, the result is that the distributed data storage system deletes the various user virtual disks  170  associated with the stale backup copy. Thus, if user virtual disks  170  are deleted, they no longer reference certain DDblockIDs and this change is reflected in DDRefCount  244  at this point. Therefore, it is possible that a DDblockID may reach a point when no user virtual disks  170  make reference to it anymore. 
     At block  704 , metadata subsystem  140  determines whether there are any references to the DDblockID in DDRefCount  244 . As long there are, the DDblockID is not discarded. 
     At block  706 , a maximum expiry epoch (e.g., Eu) is calculated for the DDblockID based on the reference expiry columns associated with the user virtual disks  170  that reference the DDblockID. Alternatively, the Eu value is extracted from DDRefCount  244 , where Eu=Max(Ref Expiry columns). See also  FIG. 2E . At this point, metadata subsystem  140  determines whether Eu is earlier than Enow-1. If so, the DDblockID is marked expired. 
     At block  708 , which is a decision point, if metadata subsystem  140  determines that a given DDblockID is both expired (block  706 ) and carries a zero reference count (block  704 ), control passes to block  710 ; otherwise control passes to block  709 . 
     Block  709  is reached when a DDblockID does not meet the requirements for being discarded, i.e., it has a non-zero reference count and/or is not expired. Here, the discard cycle considers whether an extension should be added to the expiry of the present DDblockID. If a new write request for the present DDblockID came in after the preceding analysis cycle (i.e., after the last time this evaluation was made), the DDblockID should be extended, because it is still current and actively being written by incoming backup copies. Accordingly, the Eu value from DDRefCount  244  is now populated into the DDInfo index  242 , replacing the existing Ej expiry with the later Eu value (i.e., Eu&gt;Ej). No replacement is needed or made if Ej≥Eu. Thus, here, a recently “written” data block has an opportunity for an extension to its expiry epoch long enough to span the sparsest full backups of all the user virtual disks  170  still referencing the DDblockID. Control passes back to block  702 . 
     Block  710  is reached when a DDblockID that is both expired (block  706 ) and has no valid reference counts from any user virtual disks  170  (block  704 ). This DDblockID is now guaranteed to be suitable to discard from distributed data storage system  100 . 
     At block  712 , the DDblockID is added to the discard list  246  for the storage container  260  being analyzed. See also  FIG. 2G . Control passes back to block  702 . 
     In regard to the figures described herein, other embodiments are possible within the scope of the present invention, such that the above-recited components, steps, blocks, operations, messages, requests, queries, and/or instructions are differently arranged, sequenced, sub-divided, organized, and/or combined. In some embodiments, a different component may initiate or execute a given operation. 
     Example Embodiments 
     Some example enumerated embodiments of the present invention are recited in this section in the form of methods, systems, and non-transitory computer-readable media, without limitation. 
     According to an example embodiment, a distributed data storage system comprises: a storage proxy that executes on a first computing device; a first storage service node that hosts a metadata subsystem; a second storage service node that stores a plurality of deduplicated data blocks, wherein a system-wide deduplication virtual disk comprises the plurality of deduplicated data blocks, and wherein the system-wide deduplication virtual disk is distributed across a plurality of storage service nodes of the distributed data storage system, including the second storage service node; wherein the storage proxy is configured to: intercept write requests addressed to one or more user virtual disks configured on the distributed data storage system, which are distinct from the system-wide deduplication virtual disk, wherein a first one of the write requests comprises a first data block addressed to a first user virtual disk, and cause the first data block to be stored in the system-wide deduplication virtual disk, at least at the second storage service node; and wherein the metadata subsystem is configured to: assign an expiry timeframe to a first unique system-wide identifier (the first DDblockID) that is based on a hash value of and is associated with the first data block, wherein the expiry timeframe is based at least in part on an arrival timeframe of the first one of the write requests at the storage proxy and is further based on a frequency of full backup operations configured for the first user virtual disk, and cause the second storage service node to delete the first data block from the system-wide deduplication virtual disk, based on determining that (i) a current timeframe is later than the expiry timeframe of the first DDblockID and (ii) no user virtual disk in the distributed data storage system makes reference to the first DDblockID. 
     The above-recited embodiment wherein the metadata subsystem is further configured to: cause the first data block to be deleted from the system-wide deduplication virtual disk, including from the second storage service node, even when second data blocks referenced by the first user virtual disk and associated with a second DDblockID, which is distinct from the first DDblockID, are retained after the expiry timeframe of the first DDblockID. The above-recited embodiment configured with system-wide block-level deduplication and block-level expiry granularity. The above-recited embodiment wherein the expiry timeframe for the first DDblockID is further based on a maximum value of (i) the frequency of full backup operations configured for the first user virtual disk and (ii) one or more frequencies of full backup operations corresponding to one or more other user virtual disks referencing the first DDblockID. The above-recited embodiment wherein the expiry timeframe is further based on a maximum value of (i) the frequency of full backup operations configured for the first user virtual disk and (ii) one or more frequencies of full backup operations corresponding to one or more other user virtual disks referencing second data blocks having a same hash value as the first data block. The above-recited embodiment wherein the metadata subsystem is further configured to: update a data structure that tracks write requests addressed to the first user virtual disk, including the first one of the write requests that comprises the first data block. The above-recited embodiment wherein the metadata subsystem is further configured to: if a second data block received in a second one of the write requests is determined to have a same hash value as the first data block, is associated with the first DDblockID, and arrived at the storage proxy after a preceding discard cycle executed by the metadata subsystem, extend the expiry timeframe of the first DDblockID to span a sparsest full backup frequency of all user virtual disks referencing the first DDblockID. The above-recited embodiment wherein the metadata subsystem is further configured to: receive the hash value of the first data block from the storage proxy, determine that the hash value is associated with the first DDblockID, and update a data structure that tracks write requests addressed to the first user virtual disk, including the first one of the write requests that comprises the first data block. The above-recited embodiment wherein the first data block is associated with the first DDblockID based on a hash value of the first data block; wherein the storage proxy is further configured to check whether the expiry timeframe is less than a current timeframe; and wherein the metadata subsystem is further configured to: update a data structure that tracks write requests addressed to the first user virtual disk, including the first one of the write requests that comprises the first data block. The above-recited embodiment wherein the first data block is part of a backup copy, which is addressed to at least the first user virtual disk. The above-recited embodiment wherein the first data block is part of a backup copy addressed to the distributed data storage system; wherein when the backup copy is pruned, each distinct user virtual disk configured for the backup copy is logically removed from the distributed data storage system, which causes references to data blocks of the backup copy to be removed from the distributed data storage system; and wherein the metadata subsystem is further configured to: retain a second data block supplied by the backup copy if at least one other user virtual disk, not associated with the backup copy, references a DDblockID associated with the second data block. 
     According to another example embodiment, a distributed data storage system for storing backup copies with deduplication comprises: a first storage service node that hosts a metadata subsystem; a second storage service node that stores deduplicated data blocks, wherein a system-wide deduplication virtual disk is distributed across a plurality of storage service nodes of the distributed data storage system, including the second storage service node, wherein the system-wide deduplication virtual disk comprises the deduplicated data blocks, and wherein each deduplicated data block in the system-wide deduplication virtual disk is associated with a corresponding unique system-wide identifier; wherein the metadata subsystem is configured to: track data blocks addressed to user virtual disks configured in the distributed data storage system, including a first data block in a first write request addressed to a first user virtual disk, which is distinct from the system-wide deduplication virtual disk; associate the first data block with a first unique system-wide identifier (the first DDblockID) based on a hash value of the first data block, assign an expiry timeframe to the first DDblockID, wherein the expiry timeframe is based at least in part on an arrival timeframe of the first write request at a storage proxy and is further based on a frequency of full backup operations configured for the first user virtual disk, for a second data block, which arrived in a second write request after a preceding discard cycle executed by the metadata subsystem, wherein the second data block has a same hash value as the first data block which is associated with the first DDblockID, extend the expiry timeframe of the first DDblockID to span a sparsest full backup frequency of all user virtual disks referencing the first DDblockID. 
     The above-recited embodiment wherein the metadata subsystem is further configured to: cause a data block having the hash value of the first data block and associated with the first DDblockID to be retained in the system-wide deduplication virtual disk, based on determining that at least one of: (i) a current timeframe is earlier than the expiry timeframe of the first DDblockID and (ii) at least one user virtual disk in the distributed data storage system makes reference to the first DDblockID. The above-recited embodiment wherein the metadata subsystem is further configured to: cause a data block having the hash value of the first data block and associated with the first DDblockID to be deleted from the system-wide deduplication virtual disk, including from the second storage service node, based on determining that: (a) a current timeframe is later than the expiry timeframe of the first DDblockID and (b) no user virtual disk on the distributed data storage system makes reference to the first DDblockID. The above-recited embodiment wherein the first user virtual disk is configured for deduplication using block-level expiry granularity. The above-recited embodiment wherein the expiry timeframe is further based on a maximum value of (i) the frequency of full backup operations configured for the first user virtual disk and (ii) one or more frequencies of full backup operations corresponding to one or more other user virtual disks referencing the first DDblockID. The above-recited embodiment wherein the expiry timeframe is further based on a maximum value of (i) the frequency of full backup operations configured for the first user virtual disk and (ii) one or more frequencies of full backup operations corresponding to one or more other user virtual disks referencing second data blocks having a same hash value as the first data block. The above-recited embodiment wherein the storage proxy is further configured to check whether the expiry timeframe is less than the current timeframe. The above-recited embodiment wherein the first data block is part of a backup copy, which is addressed to at least the first user virtual disk. The above-recited embodiment wherein the first data block is part of a backup copy addressed to the distributed data storage system, wherein pruning the backup copy causes references to data blocks of the backup copy to be removed from the distributed data storage system; and wherein the metadata subsystem is further configured to: retain a second data block supplied by the backup copy if at least one other user virtual disk, not associated with the backup copy, references a DDblockID associated with the second data block. 
     According to an illustrative embodiment, a method for providing deduplication with block-level expiry granularity in a cloud-based distributed data storage system comprises: in a first cloud computing environment, configuring a first storage service node that hosts a metadata subsystem of the distributed data storage system, and further configuring a second storage service node that hosts a data storage subsystem of the distributed data storage subsystem, wherein the distributed data storage subsystem comprises a system-wide deduplication virtual disk that is distributed across a plurality of storage service nodes of the distributed data storage system, including the second storage service node, and wherein the system-wide deduplication virtual disk stores deduplicated data blocks; by the first storage service node hosting the metadata subsystem, assigning an expiry timeframe to a first unique system-wide identifier (the first DDblockID) that is based on a hash value of, and is associated with, a first data block stored in the system-wide deduplication virtual disk, wherein the expiry timeframe is based at least in part on: an arrival timeframe of the first data block at the distributed data storage system, and is further based on a full backup frequency configured for a first user virtual disk addressed by a write request comprising the first data block; and by the first storage service node hosting the metadata subsystem, if a second data block, which is received in a second write request addressing a second user virtual disk after a preceding discard cycle executed by the metadata subsystem, is determined to have a same hash value as the first data block and is associated with the first DDblockID, extending the expiry timeframe of the first DDblockID to span a sparsest full backup frequency of all user virtual disks referencing the first DDblockID, including the first user virtual disk and the second user virtual disk. 
     The above-recited embodiment further comprising: by the first storage service node hosting the metadata subsystem, causing the second storage service node to delete the first data block from the system-wide deduplication virtual disk, based on determining that (i) a current timeframe is later than the expiry timeframe of the first DDblockID and (ii) no user virtual disk in the distributed data storage system makes reference to the first DDblockID. The above-recited embodiment further comprising: by the second storage service node hosting the data storage subsystem, deleting the first data block from the system-wide deduplication virtual disk, based on a determination by the metadata subsystem that (i) a current timeframe is later than the expiry timeframe of the first DDblockID and (ii) no user virtual disk in the distributed data storage system makes reference to the first DDblockID. The above-recited embodiment further comprising: by a compaction logic of the data storage subsystem, deleting the first data block from the system-wide deduplication virtual disk, based on a determination by the metadata subsystem that (i) a current timeframe is later than the expiry timeframe of the first DDblockID and (ii) no user virtual disk in the distributed data storage system makes reference to the first DDblockID. The above-recited embodiment wherein the first data block is part of a backup copy addressed to the distributed data storage system, wherein pruning the backup copy causes references to data blocks of the backup copy to be removed from the distributed data storage system; and retaining a third data block supplied by the backup copy if at least one other user virtual disk, not associated with the backup copy, references a DDblockID associated with the third data block. The above-recited embodiment wherein the first data block is part of a backup copy addressed to the distributed data storage system; wherein when the backup copy is pruned, each distinct user virtual disk configured for the backup copy is logically removed from the distributed data storage system, which causes references to data blocks of the backup copy to be removed from the distributed data storage system; and by the metadata subsystem causing a third data block supplied by the backup copy to be retained in the system-wide deduplication virtual disk if at least one other user virtual disk, not associated with the backup copy, references a DDblockID associated with the third data block. The above-recited embodiment wherein the first data block is part of a backup copy addressed to the distributed data storage system, and wherein the backup copy is generated in one of: within the first cloud computing environment, and outside the first cloud computing environment. The above-recited embodiment wherein the expiry timeframe for the first DDblockID is further based on a maximum value of (i) the full backup frequency configured for the first user virtual disk and (ii) one or more full backup frequencies corresponding to one or more other user virtual disks referencing the first DDblockID. The above-recited embodiment wherein the expiry timeframe is further based on a maximum value of (i) the full backup frequency configured for the first user virtual disk and (ii) one or more full backup frequencies corresponding to one or more other user virtual disks referencing second data blocks having a same hash value as the first data block. The above-recited embodiment further comprising: by a storage proxy that executes on a first computing device that is operational outside the first cloud computing environment: intercepting write requests addressed to one or more user virtual disks configured on the distributed data storage system, which are distinct from the system-wide deduplication virtual disk, including intercepting the write request comprising the first data block, and causing the first data block to be stored in the system-wide deduplication virtual disk, at least at the second storage service node. The above-recited embodiment further comprising: by a storage proxy that executes on a first computing device that is operational within the first cloud computing environment: intercepting write requests addressed to one or more user virtual disks configured on the distributed data storage system, which are distinct from the system-wide deduplication virtual disk, including intercepting the write request comprising the first data block, and causing the first data block to be stored in the system-wide deduplication virtual disk, at least at the second storage service node. The above-recited embodiment further comprising: by a storage proxy that executes on a first computing device, intercepting write requests addressed to one or more user virtual disks configured on the distributed data storage system, which are distinct from the system-wide deduplication virtual disk, including intercepting the write request comprising the first data block; and by the metadata subsystem: receiving the hash value of the first data block from the storage proxy, determining that the hash value is associated with the first DDblockID, and updating a data structure that tracks write requests addressed to the first user virtual disk, including the write request that comprises the first data block. The above-recited embodiment further comprising: by a storage proxy that executes on a first computing device, intercepting write requests addressed to one or more user virtual disks configured on the distributed data storage system, which are distinct from the system-wide deduplication virtual disk, including intercepting the write request comprising the first data block; and by the metadata subsystem: receiving the hash value of the first data block from the storage proxy, determining that the hash value is associated with the first DDblockID, and updating a data structure that tracks write requests addressed to the first user virtual disk, including the write request that comprises the first data block. 
     According to another illustrative embodiment, a method for providing deduplication with block-level expiry granularity in a distributed data storage system comprises: in a first cloud computing environment comprising a first storage service node that hosts a metadata subsystem of the distributed data storage system, and further comprising a second storage service node that hosts a data storage subsystem of the distributed data storage subsystem, wherein the distributed data storage subsystem is configured with a system-wide deduplication virtual disk that is distributed across a plurality of storage service nodes of the distributed data storage system, including the second storage service node, and wherein the system-wide deduplication virtual disk stores deduplicated data blocks: by the metadata subsystem at first storage service node, assigning an expiry timeframe to a first unique system-wide identifier (the first DDblockID), wherein the first DDblockID is based on a hash value of, and is associated with, a first data block stored in the system-wide deduplication virtual disk, wherein the expiry timeframe is based at least in part on: an arrival timeframe of the first data block at the distributed data storage system, and spans a full backup frequency configured for a first user virtual disk addressed by a write request comprising the first data block, wherein the first user virtual disk is distinct from the system-wide deduplication virtual disk; and by the metadata subsystem executing a discard cycle: determining that a second data block, which was received in a second write request addressing a second user virtual disk and arrived at the distributed data storage system after a preceding discard cycle has a same hash value as the first data block, associating the second data block with the first DDblockID based on the same hash value, and extending the expiry timeframe of the first DDblockID to span a sparsest full backup frequency of all user virtual disks referencing the first DDblockID, including the first user virtual disk and the second user virtual disk. 
     The above-recited embodiment wherein the assigning of the expiry timeframe occurs during an input-output cycle of the first data block, which is distinct from the discard cycle. The above-recited embodiment further comprising: by the metadata subsystem executing a discard cycle, causing the second storage service node to delete the first data block from the system-wide deduplication virtual disk, based on the metadata subsystem determining that (i) a current timeframe is later than the expiry timeframe of the first DDblockID and (ii) no user virtual disk in the distributed data storage system makes reference to the first DDblockID. The above-recited embodiment further comprising: by the second storage service node hosting the data storage subsystem, deleting the first data block from the system-wide deduplication virtual disk, based on a determination by the metadata subsystem that (i) a current timeframe is later than the expiry timeframe of the first DDblockID and (ii) no user virtual disk in the distributed data storage system makes reference to the first DDblockID. The above-recited embodiment wherein the first data block is part of a backup copy addressed to the distributed data storage system, wherein pruning the backup copy causes references to data blocks of the backup copy to be removed from the distributed data storage system; and retaining a third data block supplied by the backup copy if at least one other user virtual disk, not associated with the backup copy, references a DDblockID associated with the third data block. The above-recited embodiment wherein the first data block is part of a backup copy addressed to the distributed data storage system, and wherein the backup copy is generated in one of: within the first cloud computing environment, and outside the first cloud computing environment. The above-recited embodiment further comprising: by a storage proxy that executes on a first computing device that is one of: operational outside the first cloud computing environment, and operational within the first cloud computing environment: intercepting write requests addressed to one or more user virtual disks configured on the distributed data storage system, which are distinct from the system-wide deduplication virtual disk, including intercepting the write request comprising the first data block, and causing the first data block to be stored in the system-wide deduplication virtual disk, at least at the second storage service node. 
     According to another illustrative embodiment, a system comprises a distributed data storage platform having system-wide deduplication with block-level expiry granularity. The above-recited embodiment wherein the useful life of each deduplicated data block is based on expiry parameters that relate to backup frequencies of the virtual disks referencing the data block, thus guaranteeing that data blocks are kept around between full backup cycles and are extended if still current. The above-recited embodiment wherein data blocks are retained as long as needed to bridge the gap between sparser backup operations. The above-recited embodiment wherein tracking data structures are updated only as needed, thus saving processing cycles and network bandwidth. The above-recited embodiment wherein the distributed data storage platform guarantees that stale references to DDblockIDs lingering in non-functional components cannot dictate whether a particular DDblockID is discarded. 
     In other embodiments according to the present invention, a system or systems operates according to one or more of the methods and/or computer-readable media recited in the preceding paragraphs. In yet other embodiments, a method or methods operates according to one or more of the systems and/or computer-readable media recited in the preceding paragraphs. In yet more embodiments, a non-transitory computer-readable medium or media causes one or more computing devices having one or more processors and computer-readable memory to operate according to one or more of the systems and/or methods recited in the preceding paragraphs. 
     Terminology 
     Conditional language, such as, among others, “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment. 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense, i.e., in the sense of “including, but not limited to.” As used herein, the terms “connected,” “coupled,” or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words using the singular or plural number may also include the plural or singular number respectively. The word “or” in reference to a list of two or more items, covers all of the following interpretations of the word: any one of the items in the list, all of the items in the list, and any combination of the items in the list. Likewise the term “and/or” in reference to a list of two or more items, covers all of the following interpretations of the word: any one of the items in the list, all of the items in the list, and any combination of the items in the list. 
     In some embodiments, certain operations, acts, events, or functions of any of the algorithms described herein can be performed in a different sequence, can be added, merged, or left out altogether (e.g., not all are necessary for the practice of the algorithms). In certain embodiments, operations, acts, functions, or events can be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors or processor cores or on other parallel architectures, rather than sequentially. 
     Systems and modules described herein may comprise software, firmware, hardware, or any combination(s) of software, firmware, or hardware suitable for the purposes described. Software and other modules may reside and execute on servers, workstations, personal computers, computerized tablets, PDAs, and other computing devices suitable for the purposes described herein. Software and other modules may be accessible via local computer memory, via a network, via a browser, or via other means suitable for the purposes described herein. Data structures described herein may comprise computer files, variables, programming arrays, programming structures, or any electronic information storage schemes or methods, or any combinations thereof, suitable for the purposes described herein. User interface elements described herein may comprise elements from graphical user interfaces, interactive voice response, command line interfaces, and other suitable interfaces. 
     Further, processing of the various components of the illustrated systems can be distributed across multiple machines, networks, and other computing resources. Two or more components of a system can be combined into fewer components. Various components of the illustrated systems can be implemented in one or more virtual machines, rather than in dedicated computer hardware systems and/or computing devices. Likewise, the data repositories shown can represent physical and/or logical data storage, including, e.g., storage area networks or other distributed storage systems. Moreover, in some embodiments the connections between the components shown represent possible paths of data flow, rather than actual connections between hardware. While some examples of possible connections are shown, any of the subset of the components shown can communicate with any other subset of components in various implementations. 
     Embodiments are also described above with reference to flow chart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products. Each block of the flow chart illustrations and/or block diagrams, and combinations of blocks in the flow chart illustrations and/or block diagrams, may be implemented by computer program instructions. Such instructions may be provided to a processor of a general purpose computer, special purpose computer, specially-equipped computer (e.g., comprising a high-performance database server, a graphics subsystem, etc.) or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor(s) of the computer or other programmable data processing apparatus, create means for implementing the acts specified in the flow chart and/or block diagram block or blocks. These computer program instructions may also be stored in a non-transitory computer-readable memory that can direct a computer or other programmable data processing apparatus to operate in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the acts specified in the flow chart and/or block diagram block or blocks. The computer program instructions may also be loaded to a computing device or other programmable data processing apparatus to cause operations to be performed on the computing device or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computing device or other programmable apparatus provide steps for implementing the acts specified in the flow chart and/or block diagram block or blocks. 
     Any patents and applications and other references noted above, including any that may be listed in accompanying filing papers, are incorporated herein by reference. Aspects of the invention can be modified, if necessary, to employ the systems, functions, and concepts of the various references described above to provide yet further implementations of the invention. These and other changes can be made to the invention in light of the above Detailed Description. While the above description describes certain examples of the invention, and describes the best mode contemplated, no matter how detailed the above appears in text, the invention can be practiced in many ways. Details of the system may vary considerably in its specific implementation, while still being encompassed by the invention disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the invention should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the invention with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the invention to the specific examples disclosed in the specification, unless the above Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the invention encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the invention under the claims. 
     To reduce the number of claims, certain aspects of the invention are presented below in certain claim forms, but the applicant contemplates other aspects of the invention in any number of claim forms. For example, while only one aspect of the invention is recited as a means-plus-function claim under 35 U.S.C sec. 112(f) (AIA), other aspects may likewise be embodied as a means-plus-function claim, or in other forms, such as being embodied in a computer-readable medium. Any claims intended to be treated under 35 U.S.C. § 112(f) will begin with the words “means for,” but use of the term “for” in any other context is not intended to invoke treatment under 35 U.S.C. § 112(f). Accordingly, the applicant reserves the right to pursue additional claims after filing this application, in either this application or in a continuing application.