# Patent

## Title
Technique for quantifying logical space trapped in an extent store

## Abstract

A technique quantifies logical storage space trapped in an extent store due to overlapping write requests associated with volume metadata managed by the volume layer. The volume metadata is illustratively organized as a multi-level dense tree metadata structure, wherein each level of the dense tree metadata structure (dense tree) includes volume metadata entries for storing the volume metadata. When a level of the dense tree is full, the volume metadata entries of the level are merged with a next lower level of the dense tree in accordance with a merge operation. Illustratively, the technique may be invoked during the merge operation to examine the volume metadata entries at each level of the dense tree involved in the merge and determine the LBA range overlap of the entries. To that end, the technique may include an algorithm configured to calculate the overlapping space per level and then aggregate the overlapping space of all levels involved in the merge operation to arrive at a result that quantifies the logical storage space trapped in the extent store.

## Background
RELATED APPLICATION
The present application is a continuation of U.S. patent application Ser. No. 14/482,863, entitled TECHNIQUE FOR QUANTIFYING LOGICAL SPACE TRAPPED IN AN EXTENT STORE, filed on Sep. 10, 2014 by Sriranjani Babu et al., the contents of which are incorporated by reference herein in their entirety.


BACKGROUND
Technical Field
The present disclosure relates to storage systems and, more specifically, to quantifying logical space trapped in an extent store of a storage system.
Background Information
A plurality of storage systems may be interconnected as a cluster and configured to provide storage service relating to the organization of storage containers stored on storage devices coupled to the systems. The storage system cluster may be further configured to operate according to a client/server model of information delivery to thereby allow one or more clients (hosts) to access the storage containers. The storage devices may be embodied as solid-state drives (SSDs), such as flash storage devices, whereas the storage containers may be embodied as files or logical units (LUNs). Each storage container may be implemented as a set of data structures, such as data blocks that store data for the storage container and metadata blocks that describe the data of the storage container. For example, the metadata may describe, e.g., identify, locations of the data on storage devices in the cluster. The storage systems may maintain the metadata describing the locations of the storage container data throughout the cluster in a data structure.
Assume an input/output (I/O) request, such as a write request having new data, is issued by a host to change existing data of a storage container, such as a LUN. For example, the new data may overlap (and thus overwrite) a portion of the existing data on the storage devices. When processing the request, the storage system may create new metadata identifying the location of the new data and insert the new metadata into the data structure, e.g., as a new metadata entry. The new metadata entry may partially overlap with an existing metadata entry identifying the location of the existing data on the storage devices. The existing metadata entry may be adjusted to cover any non-overlapping portions of the existing data without performing any costly read-modify-write operations that, e.g., update the existing data with the new data after reading the portion of the existing data. However, the partially overlapping metadata entry may trap unused (unmapped) logical storage space of the storage devices.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and further advantages of the embodiments herein may be better understood by referring to the following description in conjunction with the accompanying drawings in which like reference numerals indicate identically or functionally similar elements, of which:
FIG. 1 is a block diagram of a plurality of nodes interconnected as a cluster;
FIG. 2 is a block diagram of a node;
FIG. 3 is a block diagram of a storage input/output (I/O) stack of the node;
FIG. 4 illustrates a write path of the storage I/O stack;
FIG. 5 illustrates a read path of the storage I/O stack;
FIG. 6 is a block diagram of various volume metadata entries;
FIG. 7 is a block diagram of a dense tree metadata structure;
FIG. 8 is a block diagram of a top level of the dense tree metadata structure;
FIG. 9 illustrates mapping between levels of the dense tree metadata structure;
FIG. 10 illustrates a workflow for inserting a volume metadata entry into the dense tree metadata structure in accordance with a write request;
FIG. 11 illustrates merging between levels of the dense tree metadata structure;
FIG. 12 illustrates batch updating between levels of the dense tree metadata structure;
FIG. 13 is an example simplified procedure for merging between levels of the dense tree metadata structure;
FIGS. 14a-c illustrate types of trapped logical space: three-way split, back overlap, and front overlap; and
FIG. 15 illustrates a technique for quantifying logical space trapped for an exemplary sequence of write requests that are aggregated into extents for storage in an extent store.



OVERVIEW
Embodiments described herein are directed to a technique for quantifying logical storage space trapped in an extent store due to overlapping write requests associated with metadata managed by a volume layer of a storage input/output (I/O) stack executing on one or more nodes of a cluster. The metadata is illustratively volume metadata embodied as mappings from logical block addresses (LBAs) of a logical unit (LUN) accessible by a host to durable extent keys maintained by an extent store layer of the storage I/O stack. Each extent key is a unique cluster-wide identifier associated with a storage location for an extent, which is a variable length block of data that may be aggregated from one or more write requests directed to a LBA range of the LUN. The volume metadata is illustratively organized as a multi-level dense tree metadata structure, wherein each level of the dense tree metadata structure (dense tree) includes volume metadata entries for storing the volume metadata. When a level of the dense tree is full, the volume metadata entries of the level are merged with a next lower level of the dense tree in accordance with a merge operation.
Illustratively, the technique may be invoked during the merge operation to process the volume metadata entries at each level of the dense tree involved in the merge and determine the LBA (i.e., offset) range overlap of the entries. To that end, the technique may include an algorithm configured to calculate the overlapping space per level and then aggregate the overlapping space of all levels involved in the merge operation to arrive at a result that quantifies the logical storage space trapped in the extent store. As used herein, the extent store may be viewed as a global pool of extents stored on storage arrays of the cluster. Processing of the overlapping write requests during the merge operation may manifest as partial overwrites of one or more existing extents which, in turn, may result in logical storage space being trapped in the extent store. The algorithm may be employed to calculate an amount of trapped logical storage space of the existing extents including, e.g., de-duplicated extents, and, if the amount of trapped space exceeds a threshold, read-modify-write operations may be performed on the existing extents to recapture (i.e., free) that trapped space by, e.g., purging the extents consuming the trapped space and writing new extents having non-overlapped data.
In an embodiment, the technique may implement the algorithm according to a depth-first tree traversal of the volume metadata entries of the dense tree to compute trapped space using specific rules and data structures, such as a space lockup list embodied as a push-pop stack and a space lockup counter. As the algorithm progresses through the tree traversal, the specific rules are applied to add and/or remove items from the space lockup list, depending upon a previous extent key in the list, and accumulate amounts of trapped space to the space lockup counter. The algorithm is illustratively invoked during the merge operation, e.g., as a subroutine call to perform a space lockup calculation as each level of the tree is processed. The technique relies on the dense tree to perform appropriate traversal and merge operations of the list, e.g., the merge operation may provide sorted ordering of the volume metadata entries per level of the dense tree as the entries are compared and merged. Notably, the algorithm may apply to any system and associated data structure configured to process variable length, overlapping write requests in the form of (type, length, value and original length) fields to calculate overlapping logical storage space generated by a workload.
DESCRIPTION
Storage Cluster
FIG. 1 is a block diagram of a plurality of nodes 200 interconnected as a cluster 100 and configured to provide storage service relating to the organization of information on storage devices. The nodes 200 may be interconnected by a cluster interconnect fabric 110 and include functional components that cooperate to provide a distributed storage architecture of the cluster 100, which may be deployed in a storage area network (SAN). As described herein, the components of each node 200 include hardware and software functionality that enable the node to connect to one or more hosts 120 over a computer network 130, as well as to one or more storage arrays 150 of storage devices over a storage interconnect 140, to thereby render the storage service in accordance with the distributed storage architecture.
Each host 120 may be embodied as a general-purpose computer configured to interact with any node 200 in accordance with a client/server model of information delivery. That is, the client (host) may request the services of the node, and the node may return the results of the services requested by the host, by exchanging packets over the network 130. The host may issue packets including file-based access protocols, such as the Network File System (NFS) protocol over the Transmission Control Protocol/Internet Protocol (TCP/IP), when accessing information on the node in the form of storage containers such as files and directories. However, in an embodiment, the host 120 illustratively issues packets including block-based access protocols, such as the Small Computer Systems Interface (SCSI) protocol encapsulated over TCP (iSCSI) and SCSI encapsulated over FC (FCP), when accessing information in the form of storage containers such as logical units (LUNs). Notably, any of the nodes 200 may service a request directed to a storage container on the cluster 100.
FIG. 2 is a block diagram of a node 200 that is illustratively embodied as a storage system having one or more central processing units (CPUs) 210 coupled to a memory 220 via a memory bus 215. The CPU 210 is also coupled to a network adapter 230, one or more storage controllers 240, a cluster interconnect interface 250 and a non-volatile random access memory (NVRAM 280) via a system interconnect 270. The network adapter 230 may include one or more ports adapted to couple the node 200 to the host(s) 120 over computer network 130, which may include point-to-point links, wide area networks, virtual private networks implemented over a public network (Internet) or a local area network. The network adapter 230 thus includes the mechanical, electrical and signaling circuitry needed to connect the node to the network 130, which illustratively embodies an Ethernet or Fibre Channel (FC) network.
The memory 220 may include memory locations that are addressable by the CPU 210 for storing software programs and data structures associated with the embodiments described herein. The CPU 210 may, in turn, include processing elements and/or logic circuitry configured to execute the software programs, such as a storage input/output (I/O) stack 300, and manipulate the data structures. Illustratively, the storage I/O stack 300 may be implemented as a set of user mode processes that may be decomposed into a plurality of threads. An operating system kernel 224, portions of which are typically resident in memory 220 (in-core) and executed by the processing elements (i.e., CPU 210), functionally organizes the node by, inter alia, invoking operations in support of the storage service implemented by the node and, in particular, the storage I/O stack 300. A suitable operating system kernel 224 may include a general-purpose operating system, such as the UNIX® series or Microsoft Windows® series of operating systems, or an operating system with configurable functionality such as microkernels and embedded kernels. However, in an embodiment described herein, the operating system kernel is illustratively the Linux® operating system. It will be apparent to those skilled in the art that other processing and memory means, including various computer readable media, may be used to store and execute program instructions pertaining to the embodiments herein.
Each storage controller 240 cooperates with the storage I/O stack 300 executing on the node 200 to access information requested by the host 120. The information is preferably stored on storage devices such as solid state drives (SSDs) 260, illustratively embodied as flash storage devices, of storage array 150. In an embodiment, the flash storage devices may be based on NAND flash components, e.g., single-layer-cell (SLC) flash, multi-layer-cell (MLC) flash or triple-layer-cell (TLC) flash, although it will be understood to those skilled in the art that other block-oriented, non-volatile, solid-state electronic devices (e.g., drives based on storage class memory components) may be advantageously used with the embodiments described herein. Accordingly, the storage devices may or may not be block-oriented (i.e., accessed as blocks). The storage controller 240 includes one or more ports having I/O interface circuitry that couples to the SSDs 260 over the storage interconnect 140, illustratively embodied as a serial attached SCSI (SAS) topology. Alternatively, other point-to-point I/O interconnect arrangements such as a conventional serial ATA (SATA) topology or a PCI topology, may be used. The system interconnect 270 may also couple the node 200 to a local service storage device 248, such as an SSD, configured to locally store cluster-related configuration information, e.g., as cluster database (DB) 244, which may be replicated to the other nodes 200 in the cluster 100.
The cluster interconnect interface 250 may include one or more ports adapted to couple the node 200 to the other node(s) of the cluster 100. In an embodiment, Ethernet may be used as the clustering protocol and interconnect fabric media, although it will be apparent to those skilled in the art that other types of protocols and interconnects, such as Infiniband, may be utilized within the embodiments described herein. The NVRAM 280 may include a back-up battery or other built-in last-state retention capability (e.g., non-volatile semiconductor memory such as storage class memory) that is capable of maintaining data in light of a failure to the node and cluster environment. Illustratively, a portion of the NVRAM 280 may be configured as one or more non-volatile logs (NVLogs 285) configured to temporarily record (“log”) I/O requests, such as write requests, received from the host 120.
Storage I/O Stack
FIG. 3 is a block diagram of the storage I/O stack 300 that may be advantageously used with one or more embodiments described herein. The storage I/O stack 300 includes a plurality of software modules or layers that cooperate with other functional components of the nodes 200 to provide the distributed storage architecture of the cluster 100. In an embodiment, the distributed storage architecture presents an abstraction of a single storage container, i.e., all of the storage arrays 150 of the nodes 200 for the entire cluster 100 organized as one large pool of storage. In other words, the architecture consolidates storage, i.e., the SSDs 260 of the arrays 150, throughout the cluster (retrievable via cluster-wide keys) to enable storage of the LUNs. Both storage capacity and performance may then be subsequently scaled by adding nodes 200 to the cluster 100.
Illustratively, the storage I/O stack 300 includes an administration layer 310, a protocol layer 320, a persistence layer 330, a volume layer 340, an extent store layer 350, a Redundant Array of Independent Disks (RAID) layer 360, a storage layer 365 and a NVRAM (storing NVLogs) “layer” interconnected with a messaging kernel 370. The messaging kernel 370 may provide a message-based (or event-based) scheduling model (e.g., asynchronous scheduling) that employs messages as fundamental units of work exchanged (i.e., passed) among the layers. Suitable message-passing mechanisms provided by the messaging kernel to transfer information between the layers of the storage I/O stack 300 may include, e.g., for intra-node communication: i) messages that execute on a pool of threads, ii) messages that execute on a single thread progressing as an operation through the storage I/O stack, iii) messages using an Inter Process Communication (IPC) mechanism and, e.g., for inter-node communication: messages using a Remote Procedure Call (RPC) mechanism in accordance with a function shipping implementation. Alternatively, the I/O stack may be implemented using a thread-based or stack-based execution model. In one or more embodiments, the messaging kernel 370 allocates processing resources from the operating system kernel 224 to execute the messages. Each storage I/O stack layer may be implemented as one or more instances (i.e., processes) executing one or more threads (e.g., in kernel or user space) that process the messages passed between the layers such that the messages provide synchronization for blocking and non-blocking operation of the layers.
In an embodiment, the protocol layer 320 may communicate with the host 120 over the network 130 by exchanging discrete frames or packets configured as I/O requests according to pre-defined protocols, such as iSCSI and FCP. An I/O request, e.g., a read or write request, may be directed to a LUN and may include I/O parameters such as, inter alia, a LUN identifier (ID), a logical block address (LBA) of the LUN, a length (i.e., amount of data) and, in the case of a write request, write data. The protocol layer 320 receives the I/O request and forwards it to the persistence layer 330, which records the request into a persistent write-back cache 380, illustratively embodied as a log whose contents can be replaced randomly, e.g., under some random access replacement policy rather than only in serial fashion, and returns an acknowledgement to the host 120 via the protocol layer 320. In an embodiment only I/O requests that modify the LUN, e.g., write requests, are logged. Notably, the I/O request may be logged at the node receiving the I/O request, or in an alternative embodiment in accordance with the function shipping implementation, the I/O request may be logged at another node.
Illustratively, dedicated logs may be maintained by the various layers of the storage I/O stack 300. For example, a dedicated log 335 may be maintained by the persistence layer 330 to record the I/O parameters of an I/O request as equivalent internal, i.e., storage I/O stack, parameters, e.g., volume ID, offset, and length. In the case of a write request, the persistence layer 330 may also cooperate with the NVRAM 280 to implement the write-back cache 380 configured to store the write data associated with the write request. In an embodiment, the write-back cache 380 may be structured as a log. Notably, the write data for the write request may be physically stored in the cache 380 such that the log 335 contains the reference to the associated write data. It will be understood to persons skilled in the art the other variations of data structures may be used to store or maintain the write data in NVRAM including data structures with no logs. In an embodiment, a copy of the write-back cache may also be maintained in the memory 220 to facilitate direct memory access to the storage controllers. In other embodiments, caching may be performed at the host 120 or at a receiving node in accordance with a protocol that maintains coherency between the data stored at the cache and the cluster.
In an embodiment, the administration layer 310 may apportion the LUN into multiple volumes, each of which may be partitioned into multiple regions (e.g., allotted as disjoint block address ranges), with each region having one or more segments stored as multiple stripes on the array 150. A plurality of volumes distributed among the nodes 200 may thus service a single LUN, i.e., each volume within the LUN services a different LBA range (i.e., offset range) or set of ranges within the LUN. Accordingly, the protocol layer 320 may implement a volume mapping technique to identify a volume to which the I/O request is directed (i.e., the volume servicing the offset range indicated by the parameters of the I/O request). Illustratively, the cluster database 244 may be configured to maintain one or more associations (e.g., key-value pairs) for each of the multiple volumes, e.g., an association between the LUN ID and a volume, as well as an association between the volume and a node ID for a node managing the volume. The administration layer 310 may also cooperate with the database 244 to create (or delete) one or more volumes associated with the LUN (e.g., creating a volume ID/LUN key-value pair in the database 244). Using the LUN ID and LBA (or LBA range), the volume mapping technique may provide a volume ID (e.g., using appropriate associations in the cluster database 244) that identifies the volume and node servicing the volume destined for the request, as well as translate the LBA (or LBA range) into an offset and length within the volume. Specifically, the volume ID is used to determine a volume layer instance that manages volume metadata associated with the LBA or LBA range. As noted, the protocol layer 320 may pass the I/O request (i.e., volume ID, offset and length) to the persistence layer 330, which may use the function shipping (e.g., inter-node) implementation to forward the I/O request to the appropriate volume layer instance executing on a node in the cluster based on the volume ID.
In an embodiment, the volume layer 340 may manage the volume metadata by, e.g., maintaining states of host-visible containers, such as ranges of LUNs, and performing data management functions, such as creation of snapshots and clones, for the LUNs in cooperation with the administration layer 310. The volume metadata is illustratively embodied as in-core mappings from LUN addresses (i.e., LBAs) to durable extent keys, which are unique cluster-wide IDs associated with SSD storage locations for extents within an extent key space of the cluster-wide storage container. That is, an extent key may be used to retrieve the data of the extent at an SSD storage location associated with the extent key. Alternatively, there may be multiple storage containers in the cluster wherein each container has its own extent key space, e.g., where the administration layer 310 provides distribution of extents among the storage containers. An extent is a variable length block of data that provides a unit of storage on the SSDs and that need not be aligned on any specific boundary, i.e., it may be byte aligned. Accordingly, an extent may be an aggregation of write data from a plurality of write requests to maintain such alignment. Illustratively, the volume layer 340 may record the forwarded request (e.g., information or parameters characterizing the request), as well as changes to the volume metadata, in dedicated log 345 maintained by the volume layer 340. Subsequently, the contents of the volume layer log 345 may be written to the storage array 150 in accordance with a checkpoint (e.g., synchronization) operation that stores in-core metadata on the array 150. That is, the checkpoint operation (checkpoint) ensures that a consistent state of metadata, as processed in-core, is committed to (i.e., stored on) the storage array 150; whereas the retirement of log entries ensures that the entries accumulated in the volume layer log 345 synchronize with the metadata checkpoints committed to the storage array 150 by, e.g., retiring those accumulated log entries that are prior to the checkpoint. In one or more embodiments, the checkpoint and retirement of log entries may be data driven, periodic or both.
In an embodiment, the extent store layer 350 is responsible for storing extents prior to storage on the SSDs 260 (i.e., on the storage array 150) and for providing the extent keys to the volume layer 340 (e.g., in response to a forwarded write request). The extent store layer 350 is also responsible for retrieving data (e.g., an existing extent) using an extent key (e.g., in response to a forwarded read request). The extent store layer 350 may be responsible for performing de-duplication and compression on the extents prior to storage. The extent store layer 350 may maintain in-core mappings (e.g., embodied as hash tables) of extent keys to SSD storage locations (e.g., offset on an SSD 260 of array 150). The extent store layer 350 may also maintain a dedicated log 355 of entries that accumulate requested “put” and “delete” operations (i.e., write requests and delete requests for extents issued from other layers to the extent store layer 350), where these operations change the in-core mappings (i.e., hash table entries). Subsequently, the in-core mappings and contents of the extent store layer log 355 may be written to the storage array 150 in accordance with a “fuzzy” checkpoint 390 (i.e., checkpoint with incremental changes recorded in one or more log files) in which selected in-core mappings, less than the total, are committed to the array 150 at various intervals (e.g., driven by an amount of change to the in-core mappings, size thresholds of log 355, or periodically). Notably, the accumulated entries in log 355 may be retired once all in-core mappings have been committed to include the changes recorded in those entries prior to the first interval.
In an embodiment, the RAID layer 360 may organize the SSDs 260 within the storage array 150 as one or more RAID groups (e.g., sets of SSDs) that enhance the reliability and integrity of extent storage on the array by writing data “stripes” having redundant information, i.e., appropriate parity information with respect to the striped data, across a given number of SSDs 260 of each RAID group. The RAID layer 360 may also store a number of stripes (e.g., stripes of sufficient depth) at once, e.g., in accordance with a plurality of contiguous write operations, so as to reduce data relocation (i.e., internal flash block management) that may occur within the SSDs as a result of the operations. In an embodiment, the storage layer 365 implements storage I/O drivers that may communicate directly with hardware (e.g., the storage controllers and cluster interface) cooperating with the operating system kernel 224, such as a Linux virtual function I/O (VFIO) driver.
Write Path
FIG. 4 illustrates an I/O (e.g., write) path 400 of the storage I/O stack 300 for processing an I/O request, e.g., a SCSI write request 410. The write request 410 may be issued by host 120 and directed to a LUN stored on the storage array 150 of the cluster 100. Illustratively, the protocol layer 320 receives and processes the write request by decoding 420 (e.g., parsing and extracting) fields of the request, e.g., LUN ID, LBA and length (shown at 413), as well as write data 414. The protocol layer may use the results 422 from decoding 420 for a volume mapping technique 430 (described above) that translates the LUN ID and LBA range (i.e., equivalent offset and length) of the write request to an appropriate volume layer instance, i.e., volume ID (volume 445), in the cluster 100 that is responsible for managing volume metadata for the LBA range. In an alternative embodiment, the persistence layer 330 may implement the above described volume mapping technique 430. The protocol layer then passes the results 432, e.g., volume ID, offset, length (as well as write data), to the persistence layer 330, which records the request in the persistent layer log 335 and returns an acknowledgement to the host 120 via the protocol layer 320. The persistence layer 330 may aggregate and organize write data 414 from one or more write requests into a new extent 470 and perform a hash computation, i.e., a hash function, on the new extent to generate a hash value 472 in accordance with an extent hashing technique 474.
The persistent layer 330 may then pass the write request with aggregated write date including, e.g., the volume ID, offset and length, as parameters 434 of a message to the appropriate volume layer instance. In an embodiment, message passing of the parameters 434 (received by the persistent layer) may be redirected to another node via the function shipping mechanism, e.g., RPC, for inter-node communication. Alternatively, message passing of parameters 434 may be via the IPC mechanism, e.g., message threads, for intra-node communication.
In one or more embodiments, a bucket mapping technique 476 is provided that translates the hash value 472 to an instance of an appropriate extent store layer (e.g., extent store instance 478) that is responsible for storing the new extent 470. Note that the bucket mapping technique may be implemented in any layer of the storage I/O stack above the extent store layer. In an embodiment, for example, the bucket mapping technique may be implemented in the persistence layer 330, the volume layer 340, or a layer that manages cluster-wide information, such as a cluster layer (not shown). Accordingly, the persistence layer 330, the volume layer 340, or the cluster layer may contain computer executable instructions executed by the CPU 210 to perform operations that implement the bucket mapping technique 476. The persistence layer 330 may then pass the hash value 472 and the new extent 470 to the appropriate volume layer instance and onto the appropriate extent store instance via an extent store put operation. The extent hashing technique 474 may embody an approximately uniform hash function to ensure that any random extent to be written may have an approximately equal chance of falling into any extent store instance 478, i.e., hash buckets are distributed across extent store instances of the cluster 100 based on available resources. As a result, the bucket mapping technique 476 provides load-balancing of write operations (and, by symmetry, read operations) across nodes 200 of the cluster, while also leveling flash wear in the SSDs 260 of the cluster.
In response to the put operation, the extent store instance may process the hash value 472 to perform an extent metadata selection technique 480 that (i) selects an appropriate hash table 482 (e.g., hash table 482a) from a set of hash tables (illustratively in-core) within the extent store instance 478, and (ii) extracts a hash table index 484 from the hash value 472 to index into the selected hash table and lookup a table entry having an extent key 618 identifying a storage location 490 on SSD 260 for the extent. Accordingly, the extent store layer 350 contains computer executable instructions executed by the CPU 210 to perform operations that implement the extent metadata selection technique 480 described herein. If a table entry with a matching extent key is found, then the SSD location 490 mapped from the extent key 618 is used to retrieve an existing extent (not shown) from SSD. The existing extent is then compared with the new extent 470 to determine whether their data is identical. If the data is identical, the new extent 470 is already stored on SSD 260 and a de-duplication opportunity (denoted de-duplication 452) exists such that there is no need to write another copy of the data. Accordingly, a reference count (not shown) in the table entry for the existing extent is incremented and the extent key 618 of the existing extent is passed to the appropriate volume layer instance for storage within an entry (denoted as volume metadata entry 600) of a dense tree metadata structure (e.g., dense tree 700a), such that the extent key 618 is associated an offset range 440 (e.g., offset range 440a) of the volume 445.
However, if the data of the existing extent is not identical to the data of the new extent 470, a collision occurs and a deterministic algorithm is invoked to sequentially generate as many new candidate extent keys (not shown) mapping to the same bucket as needed to either provide de-duplication 452 or produce an extent key that is not already stored within the extent store instance. Notably, another hash table (e.g. hash table 482n) may be selected by a new candidate extent key in accordance with the extent metadata selection technique 480. In the event that no de-duplication opportunity exists (i.e., the extent is not already stored) the new extent 470 is compressed in accordance with compression technique 454 and passed to the RAID layer 360, which processes the new extent 470 for storage on SSD 260 within one or more stripes 464 of RAID group 466. The extent store instance may cooperate with the RAID layer 360 to identify a storage segment 460 (i.e., a portion of the storage array 150) and a location on SSD 260 within the segment 460 in which to store the new extent 470. Illustratively, the identified storage segment is a segment with a large contiguous free space having, e.g., location 490 on SSD 260b for storing the extent 470.
In an embodiment, the RAID layer 360 then writes the stripe 464 across the RAID group 466, illustratively as one or more full stripe writes 462. The RAID layer 360 may write a series of stripes 464 of sufficient depth to reduce data relocation that may occur within the flash-based SSDs 260 (i.e., flash block management). The extent store instance then (i) loads the SSD location 490 of the new extent 470 into the selected hash table 482n (i.e., as selected by the new candidate extent key), (ii) passes a new extent key (denoted as extent key 618) to the appropriate volume layer instance for storage within an entry (also denoted as volume metadata entry 600) of a dense tree 700 managed by that volume layer instance, and (iii) records a change to extent metadata of the selected hash table in the extent store layer log 355. Illustratively, the volume layer instance selects dense tree 700a spanning an offset range 440a of the volume 445 that encompasses the LBA range of the write request. As noted, the volume 445 (e.g., an offset space of the volume) is partitioned into multiple regions (e.g., allotted as disjoint offset ranges); in an embodiment, each region is represented by a dense tree 700. The volume layer instance then inserts the volume metadata entry 600 into the dense tree 700a and records a change corresponding to the volume metadata entry in the volume layer log 345. Accordingly, the I/O (write) request is sufficiently stored on SSD 260 of the cluster.
Read Path
FIG. 5 illustrates an I/O (e.g., read) path 500 of the storage I/O stack 300 for processing an I/O request, e.g., a SCSI read request 510. The read request 510 may be issued by host 120 and received at the protocol layer 320 of a node 200 in the cluster 100. Illustratively, the protocol layer 320 processes the read request by decoding 420 (e.g., parsing and extracting) fields of the request, e.g., LUN ID, LBA, and length (shown at 513), and uses the results 522, e.g., LUN ID, offset, and length, for the volume mapping technique 430. That is, the protocol layer 320 may implement the volume mapping technique 430 (described above) to translate the LUN ID and LBA range (i.e., equivalent offset and length) of the read request to an appropriate volume layer instance, i.e., volume ID (volume 445), in the cluster 100 that is responsible for managing volume metadata for the LBA (i.e., offset) range. The protocol layer then passes the results 532 to the persistence layer 330, which may search the write cache 380 to determine whether some or all of the read request can be serviced from its cached data. If the entire request cannot be serviced from the cached data, the persistence layer 330 may then pass the remaining portion of the request including, e.g., the volume ID, offset and length, as parameters 534 to the appropriate volume layer instance in accordance with the function shipping mechanism (e.g., RPC for inter-node communication) or the IPC mechanism (e.g., message threads, for intra-node communication).
The volume layer instance may process the read request to access a dense tree metadata structure (e.g., dense tree 700a) associated with a region (e.g., offset range 440a) of a volume 445 that encompasses the requested offset range (specified by parameters 534). The volume layer instance may further process the read request to search for (lookup) one or more volume metadata entries 600 of the dense tree 700a to obtain one or more extent keys 618 associated with one or more extents 470 within the requested offset range. As described further herein, each dense tree 700 may be embodied as a multiple levels of a search structure with possibly overlapping offset range entries at each level. The entries, i.e., volume metadata entries 600, provide mappings from host-accessible LUN addresses, i.e., LBAs, to durable extent keys. The various levels of the dense tree may have volume metadata entries 600 for the same offset, in which case the higher level has the newer entry and is used to service the read request. A top level of the dense tree 700 is illustratively resident in-core and a page cache 448 may be used to access lower levels of the tree. If the requested range or portion thereof is not present in the top level, a metadata page associated with an index entry at the next lower tree level is accessed. The metadata page (i.e., in the page cache 448) at the next level is then searched (e.g., a binary search) to find any overlapping entries. This process is then iterated until one or more volume metadata entries 600 of a level are found to ensure that the extent key(s) 618 for the entire requested read range are found. If no metadata entries exist for the entire or portions of the requested read range, then the missing portion(s) are zero filled.
Once found, each extent key 618 is processed by the volume layer 340 to, e.g., implement the bucket mapping technique 476 that translates the extent key to an appropriate extent store instance 478 responsible for storing the requested extent 470. Note that, in an embodiment, each extent key 618 is substantially identical to hash value 472 associated with the extent 470, i.e., the hash value as calculated during the write request for the extent, such that the bucket mapping 476 and extent metadata selection 480 techniques may be used for both write and read path operations. Note also that the extent key 618 may be derived from the hash value 472. The volume layer 340 may then pass the extent key 618 (i.e., the hash value 472 from a previous write request for the extent) to the appropriate extent store instance 478 (via an extent store get operation), which performs an extent key-to-SSD mapping to determine the location on SSD 260 for the extent.
In response to the get operation, the extent store instance may process the extent key 618 (i.e., hash value 472) to perform the extent metadata selection technique 480 that (i) selects an appropriate hash table (e.g., hash table 482a) from a set of hash tables within the extent store instance 478, and (ii) extracts a hash table index 484 from the extent key 618 (i.e., hash value 472) to index into the selected hash table and lookup a table entry having a matching extent key 618 that identifies a storage location 490 on SSD 260 for the extent 470. That is, the SSD location 490 mapped to the extent key 618 may be used to retrieve the existing extent (denoted as extent 470) from SSD 260 (e.g., SSD 260b). The extent store instance then cooperates with the RAID storage layer 360 to access the extent on SSD 260b and retrieve the data contents in accordance with the read request. Illustratively, the RAID layer 360 may read the extent in accordance with an extent read operation 468 and pass the extent 470 to the extent store instance. The extent store instance may then decompress the extent 470 in accordance with a decompression technique 456, although it will be understood to those skilled in the art that decompression can be performed at any layer of the storage I/O stack 300. The extent 470 may be stored in a buffer (not shown) in memory 220 and a reference to that buffer may be passed back through the layers of the storage I/O stack. The persistence layer may then load the extent into a read cache 580 (or other staging mechanism) and may extract appropriate read data 512 from the read cache 580 for the LBA range of the read request 510. Thereafter, the protocol layer 320 may create a SCSI read response 514, including the read data 512, and return the read response to the host 120.
Dense Tree Volume Metadata
As noted, a host-accessible LUN may be apportioned into multiple volumes, each of which may be partitioned into one or more regions, wherein each region is associated with a disjoint offset range, i.e., a LBA range, owned by an instance of the volume layer 340 executing on a node 200. For example, assuming a maximum volume size of 64 terabytes (TB) and a region size of 16 gigabytes (GB), a volume may have up to 4096 regions (i.e., 16 GB×4096=64 TB). In an embodiment, region 1 may be associated with an offset range of, e.g., 0-16 GB, region 2 may be associated with an offset range of 16 GB-32 GB, and so forth. Ownership of a region denotes that the volume layer instance manages metadata, i.e., volume metadata, for the region, such that I/O requests destined to an offset range within the region are directed to the owning volume layer instance. Thus, each volume layer instance manages volume metadata for, and handles I/O requests to, one or more regions. A basis for metadata scale-out in the distributed storage architecture of the cluster 100 includes partitioning of a volume into regions and distributing of region ownership across volume layer instances of the cluster.
Volume metadata, as well as data storage, in the distributed storage architecture is illustratively extent based. The volume metadata of a region that is managed by the volume layer instance is illustratively embodied as in memory (in-core) and on SSD (on-flash) volume metadata configured to provide mappings from host-accessible LUN addresses, i.e., LBAs, of the region to durable extent keys. In other words, the volume metadata maps LBA (i.e., offset) ranges of the LUN to data of the LUN (via extent keys) within the respective LBA range. In an embodiment, the volume layer organizes the volume metadata (embodied as volume metadata entries 600) as a data structure, i.e., a dense tree metadata structure (dense tree 700), which maps an offset range within the region to one or more extent keys. That is, LUN data (user data) stored as extents (accessible via extent keys) is associated with LUN offset (i.e., LBA) ranges represented as volume metadata (also stored as extents). Accordingly, the volume layer 340 contains computer executable instructions executed by the CPU 210 to perform operations that organize and manage the volume metadata entries of the dense tree metadata structure described herein.
FIG. 6 is a block diagram of various volume metadata entries 600 of the dense tree metadata structure. Each volume metadata entry 600 of the dense tree 700 may be a descriptor that embodies one of a plurality of types, including a data entry (D) 610, an index entry (I) 620, and a hole entry (H) 630. The data entry (D) 610 is configured to map (offset, length) to an extent key for an extent (user data) and includes the following content: type 612, offset 614, length 616 and extent key 618. The index entry (I) 620 is configured to map (offset, length) to a page key (e.g., an extent key) of a metadata page (stored as an extent), i.e., a page containing one or more volume metadata entries, at a next lower level of the dense tree; accordingly, the index entry 620 includes the following content: type 622, offset 624, length 626 and page key 628. Illustratively, the index entry 620 manifests as a pointer from a higher level to a lower level, i.e., the index entry 620 essentially serves as linkage between the different levels of the dense tree. The hole entry (H) 630 represents absent data as a result of a hole punching operation at (offset, length) and includes the following content: type 632, offset 634, and length 636.
In an embodiment, the volume metadata entry types are of a fixed size (e.g., 12 bytes including a type field of 1 byte, an offset of 4 bytes, a length of 1 byte, and a key of 6 bytes) to facilitate search of the dense tree metadata structure as well as storage on metadata pages. Thus, some types may have unused portions, e.g., the hole entry 630 includes less information than the data entry 610 and so may have one or more unused bytes. In an alternative embodiment, the entries may be variable in size to avoid unused bytes. Advantageously, the volume metadata entries may be sized for in-core space efficiency (as well as alignment on metadata pages), which improves both read and write amplification for operations. For example, the length field (616, 626, 636) of the various volume metadata entry types may represent a unit of sector size, such as 512 bytes or 520 bytes, such that a 1 byte length may represent a range of 255×512 bytes=128K bytes.
FIG. 7 is a block diagram of the dense tree metadata structure that may be advantageously used with one or more embodiments described herein. The dense tree metadata structure 700 is configured to provide mappings of logical offsets within a LUN (or volume) to extent keys managed by one or more extent store instances. Illustratively, the dense tree metadata structure is organized as a multi-level dense tree 700, where a top level 800 represents recent volume metadata changes and subsequent descending levels represent older changes. Specifically, a higher level of the dense tree 700 is updated first and, when that level fills, an adjacent lower level is updated, e.g., via a merge operation. A latest version of the changes may be searched starting at the top level of the dense tree and working down to the descending levels. Each level of the dense tree 700 includes fixed size records or entries, i.e., volume metadata entries 600, for storing the volume metadata. A volume metadata process 710 illustratively maintains the top level 800 of the dense tree in memory (in-core) as a balanced tree that enables indexing by offsets. The volume metadata process 710 also maintains a fixed sized (e.g., 4 KB) in-core buffer as a staging area (i.e., an in-core staging buffer 715) for volume metadata entries 600 inserted into the balanced tree (i.e., top level 800). Each level of the dense tree is further maintained on-flash as a packed array of volume metadata entries, wherein the entries are stored as extents illustratively organized as fixed sized (e.g., 4 KB) metadata pages 720. Notably, the staging buffer 715 is de-staged to SSD upon a trigger, e.g., the staging buffer is full. Each metadata page 720 has a unique identifier (ID), which guarantees that no two metadata pages can have the same content. Illustratively, metadata may not be de-duplicated by the extent store layer 350.
In an embodiment, the multi-level dense tree 700 includes three (3) levels, although it will be apparent to those skilled in the art that additional levels N of the dense tree may be included depending on parameters (e.g., size) of the dense tree configuration. Illustratively, the top level 800 of the tree is maintained in-core as level 0 and the lower levels are maintained on-flash as levels 1 and 2. In addition, copies of the volume metadata entries 600 stored in staging buffer 715 may also be maintained on-flash as, e.g., a level 0 linked list. A leaf level, e.g., level 2, of the dense tree contains data entries 610, whereas a non-leaf level, e.g., level 0 or 1, may contain both data entries 610 and index entries 620. Each index entry (I) 620 at level N of the tree is configured to point to (reference) a metadata page 720 at level N+1 of the tree. Each level of the dense tree 600 also includes a header (e.g., level 0 header 730, level 1 header 740 and level 2 header 750) that contains per level information, such as reference counts associated with the extents. Each upper level header contains a header key (an extent key for the header, e.g., header key 732 of level 0 header 730) to a corresponding lower level header. A region key 762 to a root, e.g., level 0 header 730 (and top level 800), of the dense tree 700 is illustratively stored on-flash and maintained in a volume root extent, e.g., a volume superblock 760. Notably, the volume superblock 760 contains region keys to the roots of the dense tree metadata structures for all regions in a volume.
FIG. 8 is a block diagram of the top level 800 of the dense tree metadata structure. As noted, the top level (level 0) of the dense tree 700 is maintained in-core as a balanced tree, which is illustratively embodied as a B+ tree data structure. However, it will be apparent to those skilled in the art that other data structures, such as AVL trees, Red-Black trees, and heaps (partially sorted trees), may be advantageously used with the embodiments described herein. The B+ tree (top level 800) includes a root node 810, one or more internal nodes 820 and a plurality of leaf nodes (leaves) 830. The volume metadata stored on the tree is preferably organized in a manner that is efficient both to search, in order to service read requests and to traverse (walk) in ascending order of offset to accomplish merges to lower levels of the tree. The B+ tree has certain properties that satisfy these requirements, including storage of all data (i.e., volume metadata entries 600) in leaves 830 and storage of the leaves as sequentially accessible, e.g., as one or more linked lists. Both of these properties make sequential read requests for write data (i.e., extents) and read operations for dense tree merge more efficient. Also, since it has a much higher fan-out than a binary search tree, the illustrative B+ tree results in more efficient lookup operations. As an optimization, the leaves 830 of the B+ tree may be stored in a page cache 448, making access of data more efficient than other trees. In addition, resolution of overlapping offset entries in the B+ tree optimizes read requests of extents. Accordingly, the larger the fraction of the B+ tree (i.e., volume metadata) maintained in-core, the less loading (reading) of metadata from SSD is required so as to reduce read amplification.
FIG. 9 illustrates mappings 900 between levels of the dense tree metadata structure. Each level of the dense tree 700 includes one or more metadata pages 720, each of which contains multiple volume metadata entries 600. As noted, each volume metadata entry 600 has a fixed size, e.g., 12 bytes, such that a predetermined number of entries may be packed into each metadata page 720. The data entry (D) 610 is a map of (offset, length) to an address of (user) data which is retrievable using an extent key 618 (i.e., from an extent store instance). The (offset, length) illustratively specifies an offset range of a LUN. The index entry (I) 620 is a map of (offset, length) to a page key 628 of a metadata page 720 at the next lower level. Illustratively, the offset in the index entry (I) 620 is the same as the offset of the first entry in the metadata page 720 at the next lower level. The length 626 in the index entry 620 is illustratively the cumulative length of all entries in the metadata page 720 at the next lower level (including gaps between entries).
For example, the metadata page 720 of level 1 includes an index entry “I(2K,10K)” that specifies a starting offset 2K and an ending offset 12K (i.e., 12K=2K+10K); the index entry (I) illustratively points to a metadata page 720 of level 2 covering the specified range. An aggregate view of the data entries (D) packed in the metadata page 720 of level 2 covers the mapping from the smallest offset (e.g., 2K) to the largest offset (e.g., 12K). Thus, each level of the dense tree 700 may be viewed as an overlay of an underlying level. For instance the data entry “D(0,4K)” of level 1 overlaps 2K of the underlying metadata in the page of level 2 (i.e., the range 2K,4K).
In one or more embodiments, operations for volume metadata managed by the volume layer 340 include insertion of volume metadata entries, such as data entries 610, into the dense tree 700 for write requests. As noted, each dense tree 700 may be embodied as multiple levels of a search structure with possibly overlapping offset range entries at each level, wherein each level is a packed array of entries (e.g., sorted by offset) and where leaf entries have an offset range (offset, length) an extent key. FIG. 10 illustrates a workflow 1000 for inserting a volume metadata entry into the dense tree metadata structure in accordance with a write request. In an embodiment, volume metadata updates (changes) to the dense tree 700 occur first at the top level of the tree, such that a complete, top-level description of the changes is maintained in memory 220.
Operationally, the volume metadata process 710 applies the region key 762 to access the dense tree 700 (i.e., top level 800) of an appropriate region (e.g., offset range 440 as determined from the parameters 432 derived from a write request 410). Upon completion of a write request, the volume metadata process 710 creates a volume metadata entry, e.g., a new data entry 610, to record a mapping of offset/length-to-extent key (i.e., offset range-to-user data). Illustratively, the new data entry 610 includes an extent key 618 (i.e., from the extent store layer 350) associated with data (i.e., extent 470) of the write request 410, as well as offset 614 and length 616 (i.e., from the write parameters 432) and type 612 (i.e., data entry D). The volume metadata process 710 then updates the volume metadata by inserting (adding) the data entry D into the level 0 staging buffer 715, as well as into the top level 800 of dense tree 700 and the volume layer log 345, thereby signifying that the write request is stored on the storage array 150.
Dense Tree Volume Metadata Checkpointing
When a level of the dense tree 700 is full, volume metadata entries 600 of the level are merged with the next lower level of the dense tree. As part of the merge, new index entries 620 are created in the level to point to new lower level metadata pages 720, i.e., data entries from the level are merged (and pushed) to the lower level so that they may be “replaced” with an index reference in the level. The top level 800 (i.e., level 0) of the dense tree 700 is illustratively maintained in-core such that a merge operation to level 1 facilitates a checkpoint to SSD 260. The lower levels (i.e., levels 1 and/or 2) of the dense tree are illustratively maintained on-flash and updated (e.g., merged) as a batch operation (i.e., processing the entries of one level with those of a lower level) when the higher levels are full. The merge operation illustratively includes a sort, e.g., a 2-way merge sort operation. A parameter of the dense tree 700 is the ratio K of the size of level N−1 to the size of level N. Illustratively, the size of the array at level N is K times larger than the size of the array at level N−1, i.e., sizeof(level N)=K*sizeof(level N−1). After K merges from level N−1, level N becomes full (i.e., all entries from a new, fully-populated level N−1 are merged with level N, iterated K times.)
FIG. 11 illustrates merging 1100 between levels, e.g., levels 0 and 1, of the dense tree metadata structure. In an embodiment, a merge operation is triggered when level 0 is full. When performing the merge operation, the dense tree metadata structure transitions to a “merge” dense tree structure (shown at 1120) that merges, while an alternate “active” dense tree structure (shown at 1150) is utilized to accept incoming data. Accordingly, two in-core level 0 staging buffers 1130, 1160 are illustratively maintained for concurrent merge and active (write) operations, respectively. In other words, an active staging buffer 1160 and active top level 1170 of active dense tree 1150 handle in-progress data flow (i.e., active user read and write requests), while a merge staging buffer 1130 and merge top level 1140 of merge dense tree 1120 handle consistency of the data during a merge operation. That is, a “double buffer” arrangement may be used to handle the merge of data (i.e., entries in the level 0 of the dense tree) while processing active operations.
During the merge operation, the merge staging buffer 1130, as well as the top level 1140 and lower level array (e.g., merge level 1) are read-only and are not modified. The active staging buffer 1160 is configured to accept the incoming (user) data, i.e., the volume metadata entries received from new put operations are loaded into the active staging buffer 1160 and added to the top level 1170 of the active dense tree 1150. Illustratively, merging from level 0 to level 1 within the merge dense tree 1120 results in creation of a new active level 1 for the active dense tree 1150, i.e., the resulting merged level 1 from the merge dense tree is inserted as a new level 1 into the active dense tree. A new index entry I is computed to reference the new active level 1 and the new index entry I is loaded into the active staging buffer 1160 (as well as in the active top level 1170). Upon completion of the merge, the region key 762 of volume superblock 760 is updated to reference (point to) the root, e.g., active top level 1170 and active level 0 header (not shown), of the active dense tree 1150, thereby deleting (i.e., rendering inactive) merge level 0 and merge level 1 of the merge dense tree 1120. The merge staging buffer 1130 (and the top level 1140 of the dense tree) thus becomes an empty inactive buffer until the next merge. The merge data structures (i.e., the merge dense tree 1120 including staging buffer 1130) may be maintained in-core and “swapped” as the active data structures at the next merge (i.e., “double buffered”).
FIG. 12 illustrates batch updating 1200 between lower levels, e.g., levels 1 and 2, of the dense tree metadata structure. Illustratively, as an example, a metadata page 720 of level 1 includes four data entries D and an index entry I referencing a metadata page 720 of level 2. When full, level 1 batch updates (merges) to level 2, thus emptying the data entries D of level 1, i.e., contiguous data entries are combined (merged) and pushed to the next lower level with a reference inserted in their place in the level. The merge of changes of layer 1 into layer 2 illustratively produces a new set of extents on SSD, i.e., new metadata pages are also stored, illustratively, in an extent store instance. As noted, level 2 is illustratively several times larger, e.g., K times larger, than level 1 so that it can support multiple merges. Each time a merge is performed, some older entries that were previously on SSD may be deleted. Advantageously, use of the multi-level tree structure lowers the overall frequency of volume metadata that is rewritten (and hence reduces write amplification), because old metadata may be maintained on a level while new metadata is accumulated in that level until it is full. Further, when a plurality of upper levels become full, a multi-way merge to a lower level may be performed (e.g., a three-way merge from full levels 0 and 1 to level 2).
FIG. 13 is an example simplified procedure 1300 for merging between levels of the dense tree metadata structure. The procedure starts at step 1305 and proceeds to step 1310 where incoming data received at the dense tree metadata structure is inserted into level 0, i.e., top level 800, of the dense tree. Note that the incoming data is inserted into the top level 800 as a volume metadata entry. At step 1315, a determination is made as whether level 0, i.e., top level 800, of the dense tree is rendered full. If not, the procedure returns to step 1310; otherwise, if the level 0 is full, the dense tree transitions to a merge dense tree structure at step 1320. At step 1325, incoming data is loaded into an active staging buffer of an active dense tree structure and, at step 1330, the level 0 merges with level 1 of the merge dense tree structure. In response to the merge, a new active level 1 is created for the active dense tree structure at step 1335. At step 1340, an index entry is computed to reference the new active level 1 and, at step 1345, the index entry is loaded into the active dense tree structure. At step 1350, a region key of a volume superblock is updated to reference the active dense tree structure and, at step 1355, the level 0 and level 1 of the merge dense tree structure are rendered inactive (alternatively, deleted). The procedure then ends at step 1360.
In an embodiment, as the dense tree fills up, the volume metadata is written out to one or more files on SSD in a sequential format, independent of when the volume layer log 345 is de-staged and written to SSD 260, i.e., logging operations may be independent of merge operations. When writing volume metadata from memory 220 to SSD, direct pointers to the data, e.g., in-core references to memory locations, may be replaced with pointers to an index block in the file that references a location where the metadata can be found. As the files are accumulated, they are illustratively merged together in a log-structured manner that continually writes the metadata sequentially to SSD. As a result, the lower level files grow and contain volume metadata that may be outdated because updates have occurred to the metadata, e.g., newer entries in the dense tree may overlay older entries, such as a hole entry overlaying an underlying data entry. The updates (i.e., layered LBA ranges) are “folded” into the lower levels, thereby overwriting the outdated metadata. The resulting dense tree structure thus includes newly written metadata and “holes” where outdated metadata has been deleted.
Quantifying Trapped Logical Storage Space
Embodiments described herein are directed to a technique for quantifying logical storage space trapped in an extent store due to overlapping write requests associated with volume metadata managed by the volume layer. As noted, the volume metadata is illustratively organized as a multi-level dense tree metadata structure, wherein each level of the dense tree metadata structure (dense tree) includes volume metadata entries for storing the volume metadata. When a level of the dense tree is full, the volume metadata entries of the level are merged with a next lower level of the dense tree in accordance with a merge operation. Illustratively, the technique may be invoked during the merge operation to process the volume metadata entries at each level of the dense tree involved in the merge and determine the offset range overlap of the entries. To that end, the technique may include an algorithm configured to calculate the overlapping space per level and then aggregate the overlapping space of all levels involved in the merge operation to arrive at a result that quantifies the logical storage space trapped in the extent store. As used herein, the extent store may be viewed as a global pool of extents stored on storage arrays of the cluster. Processing of the overlapping write requests during the merge operation may manifest as partial overwrites of one or more existing extents which, in turn, may result in logical storage space being trapped in the extent store. The algorithm may be employed to calculate an amount of trapped logical storage space of the existing extents including, e.g., de-duplicated extents and, if the amount of trapped space exceeds a threshold, read-modify-write operations may be performed on the existing extents to recapture (i.e., free) that trapped space by, e.g., purging the extents consuming the trapped space and writing new extents having non-overlapped data.
In an embodiment, the technique may implement the algorithm according to a depth-first tree traversal of the volume metadata entries of the dense tree to compute trapped space using specific rules and data structures, such as a space lockup list embodied as a push-pop stack and a space lockup counter (i.e., accumulator). As the algorithm progresses through the tree traversal, the specific rules are applied to add and/or remove items from the space lockup list, depending upon a previous extent key in the list, and accumulate amounts of trapped space to the space lockup counter. The algorithm is illustratively invoked during the merge operation, e.g., as a subroutine call to perform a space lockup calculation as each level of the tree is processed. The technique relies on the dense tree to perform appropriate traversal and merge operations of the list, e.g., the merge operation may provide sorted ordering of the volume metadata entries per level of the dense tree as the entries are compared and merged. Notably, the algorithm may apply to any system and associated data structure configured to process variable length, overlapping write requests in the form of (type, length, value and original length) fields to calculate overlapping logical storage space generated by a workload.
FIGS. 14a-c illustrate types of trapped logical space: three-way split, back overlap, and front overlap. Illustratively, a three-way split (FIG. 14a) of a first volume metadata entry 610a (having an extent key K1 associated with extent 470a) occurs when second (later) volume metadata entry 610b (having an extent key K2 associated with second (later) extent 470b) is inserted into the dense tree such that an offset range (e.g., 512-1536) of the second entry 610b is within the offset range (e.g., 0-2048) of first (earlier) entry 610a. Accordingly, the first volume metadata entry 610a may be split (e.g., during a merge operation) into 610a′ at the beginning (i.e., front) of the offset range (e.g., 0-511) and volume data entry 610c at the end (i.e., back) of the offset range (e.g., 1537-2048) with the second metadata entry 610b occupying an intermediate portion of the offset range (e.g., 512-1536) in between those of entries 610a′ and 610c. As result, storage space of the first extent 470a may be overlapped, i.e., “trapped,” by the second extent 470b. 
As used herein, trapped (i.e., locked-up) logical space denotes one or more portions (i.e., fragments) of data extents that are inaccessible as a user request. Note that trapped logical space may include de-duplicated extents. For example, consider a LUN and a write request having data spanning half an offset range of the LUN. If the data is written (via a first user request) to a lower half of the LUN offset range and written again (via a second user request) to an upper half of the LUN offset range, the LUN may be considered to be logically full, i.e., the entire offset range of the LUN has been logically written. Notably, the data may be de-duplicated such that only half of an actual storage space provision for the LUN is occupied. However, if the same data is written yet again (via a third user request) in the middle of the offset range (i.e., yielding front and back overlaps respectively to lower and upper halves of the data), half of the logical space (i.e., middle portion of the offset range) may be consider trapped (i.e., three-way split), even though the same data has been written three times (and de-duplicated twice) with only half the actual storage space remaining occupied. Note further than trapped logical space may be computed according to any volume metadata organization, e.g., per region (i.e., dense tree), per volume (i.e., group of regions), and per user visible entities, such as LUNs.
In an embodiment, a volume metadata entry 610 (e.g., data entry 610) may be extended to include additional content: an original length 619 indicating an original length of the data entry prior to an overlap, and a relative offset 617 indicating an offset (relative to a start) into an extent associated with the volume metadata entry. For example, after a three way split, entry 610c (i.e., “tail”) inherits the original length (e.g., 2048) of the split entry 610a since the same extent 470a is referenced in both data entries 610a,c via the same key K1. Entry 610c also includes a relative offset (e.g., 1536) referencing a latter portion of the extent 470a. Note, however, that entry 610a′ has a relative offset (e.g., zero) that still references a beginning of the extent 470a, but has a reduced length (e.g., 512). According to the technique, entries that reference different offsets within a same extent, e.g., K1 (0-511) and K1 (1537-2048), are referred to as fragments (i.e., portions of an extent).
Illustratively, a back overlap (FIG. 14b) occurs when a later volume metadata entry 610d is inserted into the dense tree such that an offset range (e.g., 1024-3072) of the later entry overlaps and extends above (i.e., higher than) the offset range (e.g., 0-2048) of the first (earlier) entry 610a. Accordingly, the length 616 of the first entry 610a may be reduced (i.e., “shortened”) by the amount of offset range overlap with the later entry 610d (e.g., length of 2048 reduced to 1024) to yield the new volume metadata entry 610e, i.e., fragment K1 offset (0-1023).
A front overlap (FIG. 14c) illustratively occurs when a later volume metadata entry 610f is inserted into the dense tree such that an offset range (e.g., 0-2048) of the later entry overlaps and extends below (i.e., lower than) the offset range (e.g., 1024-3072) of the first (earlier) entry 610a. Accordingly, the length 616 of the first entry 610a may be reduced (i.e., “shortened”) by the amount of offset range overlap with the later entry 610f (e.g., length of 2048 reduced to 1024) to yield a new volume metadata entry 610g. Additionally, the relative offset 617 of the new entry 610g may be increased, i.e., “shifted,” by the amount offset range overlap (e.g., relative offset of 0 shifted to 1024) from the first entry 610a, i.e., fragment K1 offset (2049-3072).
FIG. 15 illustrates the technique for quantifying logical space trapped for an exemplary sequence of write requests that are aggregated into extents for storage in the extent store. Illustratively, a volume metadata entry 610h corresponding to an extent pointed to (i.e., referenced) by key K1 has an LBA range of 0-5120 bytes of which of only 2048 bytes are used, e.g., in the upper and lower address ranges (fragments [1] and [8]). Other offset ranges are covered by entries 610i,j,k,n,m,p corresponding to extents referenced by keys K2, K3 (fragments [2-4] and [6-7]) and K4 (fragment [5]). After accounting for all fragments associated with an extent, a determination may be rendered as to the amount of trapped (i.e., locked-up) logical space in the extent due to overlapping write requests. That is, the technique proceeds fragment-by-fragment traversing depth first (i.e., overlap first) among the volume metadata entries while accumulating the amount of trapped logical space.
The following assumptions, based in part on properties of the dense tree, may be applied to the algorithm of the technique described herein:

    
    
        1. Extents with the same key have the same data and the same original length, constituting de-duped extents.
        2. Extents of the same offset range overwritten with the same data (i.e., exact overlap with de-duped data) have no duplicate metadata entries in the lower levels (i.e., the merged levels) of the dense tree for the same range, i.e., no identical metadata entries.
        3. Volume metadata entries resulting from overlapping write requests within the same offset range are adjusted (i.e., three-way split, back overlap, front overlap) such that non-overlapping start and end offsets (i.e., offset and length of entries) occur without any gaps, i.e., there should be no holes/gaps in the range between an earlier extent overlapped (i.e., partially overwritten) by a later extent.
    
    


Accordingly, in the example of FIG. 15, entries 610h-q (respectively for keys K1, K2, K3, K4, K2, and K1) have non-overlapping start and end offsets (start offset+length) without any gaps.
The following variables and data structures may be used in the algorithm:
(i) Original end offset (start offset+original length);
(ii) Original length;
(iii) Relative end offset within the extent (relative offset+length);
(iv) Entry space lockup value (original length of entry−length of entry);
(v) Total space lockup value (i.e., running space lockup counter of trapped logical space); and
(vi) Space lockup list (e.g., a space lockup stack) having items as a tuple of a metadata entry and a space lockup value for that entry.
The following rules for processing volume metadata entries may be applied to the algorithm of the technique:
Rule I (start a null list)—If the space lockup list (e.g., space lockup stack) is empty, add a tuple (current entry, entry space lockup value) as an item to the end of the list, e.g., push {current entry, entry space lockup value} on the space lockup stack. The space lockup value for the entry is computed from the next immediate overlap, assuming a front or back overlap. Note correction for a three-way “tail” fragment is made in Rule III(a) below.
Rule II (traverse to the next overlap)—If the space lockup list (e.g., space lockup stack) is not empty and the key of the last item (i.e., entry) in the list is different from the key of the current item (i.e., entry), then a tuple (current entry, entry space lockup value) is added to the end of the list, e.g., push {entry, entry space lockup value} on the space lockup stack.
Rule III (handle entries with the same key)—If the list space lockup (e.g., the space lockup stack) is not empty and the key of the last item (entry) in the list is the same as the key of the current item (entry), then one of two sub-rules III(a) or III(b) below applies. Note this rule handles entries with the same key as a result of a three-way split or front/back overlaps of de-duplicated data having the same key:
Rule III (a) (three-way split): If the original stop offset of the current item (entry) is equal to the end offset of the last item (entry) in the list (e.g., stack), then the current item is a fragment of an extent referenced by the key produced from a middle overlapping write request (i.e., three-way split). That is, the space lockup values for overlapping extents (and fragments) have been accumulated in the total space lockup value of the space lockup counter. Accordingly, the space lockup value of the item (entry) corresponding to this key in the list is updated to remove a space corresponding to the end (i.e., tail) fragment, which was assumed to be locked-up when the last item was added to the list. Note this rule handles split of an entry into two entries with the same key (i.e., split of an extent).
Rule III (b) (back or front overlap of de-duped extents): If the original end offset of the current item (entry) is not equal to the end offset of the last item (entry) in the list, then (i) remove (e.g., pop) the existing item from the list (e.g., space lockup stack), (ii) accumulate its space lockup value to the total space lockup value of the space lockup counter, and (iii) add (e.g., push) the tuple (current entry, entry space lockup value) to the end of the list (e.g., stack). Note this rule handles front or back overlapping write requests for de-duplicated extents, i.e., having the same extent key.
Rule IV (accumulate locked-up space)—If the original end offset of the current item (entry) is greater than or equal to the end offset of the last item (entry) in the list (stack), then overlapping fragments of the current entry have been added to the list (e.g., pushed on the stack). That is, the space lockup values for overlapping extents (and fragments) have been captured in the list (stack). Accordingly, iteratively remove (e.g., pop) those items from the list (e.g., space lockup stack) and accumulate their space lockup value in the total space lockup value of the space lockup counter.
Exemplary operation of the technique may be described to compute a total (logical) space lockup value associated with a merge operation of metadata entries 610h-p involving a plurality of new write requests (corresponding to metadata entries 610i-p) with varying lengths of data (i.e., extents) that overlap portions of an offset range of an existing extent (associated metadata entry 610h) in the extent store. The existing extent may be formed by a write request with an offset range of, e.g., 0-5120 (entry 610h), with the later (new) write requests (entries 610i-p) overlapping portions of that range. Illustratively, volume metadata entries corresponding to the extents may be represented by “key (offset range, original length)” fields and include the following (8) entries:
[1] K1 (0-1023, 5120) (entry 610h)
[2] K2 (1024-1535, 1024) (entry 610i)
[3] K2 (1536-2047, 1024) (entry 610j)
[4] K3 (2048-2559, 1024) (entry 610n)
[5] K4 (2560-3071, 512) (entry 610k)
[6] K2 (3072-3583, 1024) (entry 610m)
[7] K2 (3584-4095, 1024) (entry 610p)
[8] K1 (4096-5119, 5120) (entry 610q)
In FIG. 15, the large existing extent (corresponding to entry 610h and referenced by K1) has an offset range of 0-5120 and multiple smaller extents overlap that existing extent, thereby creating fragments (corresponding to entries 610i-p). Once a last fragment ([8]) of the existing extent is processed by the algorithm, the space-lockup list is empty and the space lockup counter contains the total space lockup value indicating the total logical storage space trapped in the extents of the extent store pertaining to the volume. Note that fragments [2], [3], [6], and [7] reference the same (de-duped) extent associated with K2 and that fragments [1] and [8] result from a three-way split of the extent associated with K1.
Initially, the space lockup list is empty and the total space lockup value is set to zero; thus Rule I applies. A first item [1] and associated tuple are added (pushed) to the list (stack). The tuple includes the volume metadata entry, i.e., key (offset range, original length) and a space lockup value, which is computed as “space lockup=original length of entry−length of entry”. Thus, for item [1] the space lockup value=5120−1024 (len)=4096, and the tuple is {K1 (0-1023, 5120), 4096}. Note, the space lockup computed for the entry assumes a back overlap for the remaining offset range (1024-5119). Correction for the tail fragment (i.e., item [8]) from the three-way split of entry 610h occurs later.
The next current item [2] includes K2. The space lockup list is not empty and the key of the last item in the list, e.g., K1, is different from the key of the current item, e.g., K2, so Rule II applies. The tuple {K2 (1024-1535, 1024), 1024−512 (=512)} associated with the current item [2] is added (pushed) to the list.
The next current item [3] includes K2. The space lockup list is not empty and the key of the last item in the list, e.g., K2, is the same as the key of the current item, e.g., K2. Moreover, the current item [3] has an original stop offset that is greater than the original stop offset of item [2], so Rule III (b), i.e., de-duplicate for back overlapping write request, applies. Accordingly, the previous item [2] is removed (popped) from the list and its space-lockup value is added to the space lockup counter, e.g., 0+512=512. Notably, the previous item is removed (popped) from the list and its space lockup value is added to the space lockup counter, because all fragments (e.g., [2] and [3]) associated with the extent of the volume metadata entry 610i of the item have been processed, and the space lockup value represents the trapped logical storage space. In addition, the tuple {K2 (1536-2048, 1024), 1024−512 (=512)} associated with the current item [3] is added to the list.
The next current item [4] includes K3. The space lockup list is not empty and the key of the last item in the list, e.g., K2, is different from the key of the current item, e.g., K3, so Rule II applies. Accordingly, the tuple {K3 (2048-2559, 1024), 1024−512 (=512)} associated with the current item [4] is added (pushed) to the list.
The next current item [5] includes K4. The original length of the item (entry) equals the length of the entry, thus there is no overlap of write requests and no fragments corresponding to the extent referenced by K4. Accordingly, the current item [5] is not added to the space lockup list, since it does not contribute to any space lockup due to overlapping write requests. In addition, the end offset of the current item [5] corresponds to the end offset of item [4] in the list; hence all fragments corresponding to the extent referenced by K3 of item [4] have been processed (e.g., traversed and added to the list), so Rule IV applies. As a result, all items that have original end offsets less than or equal to the original end offset of the current item [5] may be removed from the list and their space lockup values are accumulate to the space lockup counter. Therefore, items [3] and [4] are removed (popped) from the list and their space lockup values are accumulated to the space lockup counter, i.e., 512+512+512=1536.
The next current item [6] includes K2. The space lockup list is not empty and the key of the last item in the list, e.g., K1, is different from the key of the current item, e.g., K2, so Rule II applies. The tuple {K2 (3072-3583, 1024), 1024−512 (=512)} associated with the current item [6] is added to the list.
The next current item [7] includes K2. The space lockup list is not empty and the key of the last item in the list, e.g., K2, is the same as the key of the current item, e.g., K2. Moreover, the current item [7] has an original end offset that is greater than the original end offset of item [6], so Rule III (b), i.e., de-duplicate for front overlapping write request, applies. Accordingly the previous item [6] is removed from the list and its space lockup value is accumulated to the space lockup counter, i.e., 1536+512=2048. In addition, the tuple {K2 (3584-4095, 1024), 1024−512 (=512)} associated with the current item [7] is added (pushed) to the list.
However, the original stop offset of item [7] is less than or equal to the end offset of the last remaining item [1] in the space lockup list, so item [7] is removed (popped) from the list and its space lockup value is accumulated to the space lockup counter, i.e., 2048+512=2560.
The next current item [8] includes key K1. The space lockup list is not empty and the key of the last item in the list, e.g., K1, is the same as the key of the current item, e.g., K1. Moreover, the original stop offset of current item [8] is equal to that of the last item [1] in the list, so Rule III (a) applies. Since the current item [8] is a fragment of the extent pointed to by K1 produced from a middle overlapping write, the offset range value corresponding to the current item [8], i.e., 4096−5120=1024, is removed from the space lockup value of the last item [1] on the list to reduce the locked-up space assumed when item [1] was first added to the list. Thus the space lockup value of item [1] is updated, i.e., 4096−1024=3072, and is accumulated to the space lockup counter, i.e., 2560+3072=5632. In addition, the last item [1] is removed (popped) from the list. The final space lockup value representing the total trapped logical space is thus 5632 and the algorithm completes, thereby quantifying the trapped logical space.
While there have been shown and described illustrative embodiments directed to a technique for quantifying logical storage space trapped in an extent store due to overlapping write requests associated with metadata managed by the volume layer of the storage I/O stack executing on one or more nodes of the cluster, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the embodiments herein. For example, embodiments have been shown and described herein with relation to an algorithm of the technique configured to perform a space lockup calculation as each level of the dense tree is processed during a merge operation. However, the embodiments in their broader sense are not so limited, and may, in fact, also allow for use of the algorithm to determine trapped logical space according to any volume metadata organization, e.g., per region (i.e., dense tree), per volume (i.e., group of regions), and per user visible entities, such as LUNs.
The foregoing description has been directed to specific embodiments. It will be apparent, however, that other variations and modifications may be made to the described embodiments, with the attainment of some or all of their advantages. For instance, it is expressly contemplated that the components and/or elements described herein can be implemented as software encoded on a tangible (non-transitory) computer-readable medium (e.g., disks and/or CDs) having program instructions executing on a computer, hardware, firmware, or a combination thereof. Accordingly this description is to be taken only by way of example and not to otherwise limit the scope of the embodiments herein. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the embodiments herein.

## Claims

1. A system comprising:
a central processing unit (CPU) adapted to execute a storage input/output (I/O) stack;
one or more solid state drives (SSDs) coupled to the CPU; and
a memory coupled to the CPU and configured to store the storage I/O stack and a multi-level dense tree metadata structure, wherein each level of the dense tree metadata structure includes entries for storing metadata having mappings from offset ranges of a logical unit (LUN) to extent keys associated with storage locations for extents stored on an extent store of the SSDs, wherein the storage I/O stack is configured to:
receive overlapping write requests directed to the LUN, the overlapping write requests representing overlapping offset ranges of the LUN, the overlapping offset ranges stored as overlapping entries of the dense tree metadata structure;
process the mappings of the overlapping entries to determine an offset range overlap of the overlapping entries per level; and
aggregate the offset range overlap of the overlapping entries between adjacent levels of the multi-level dense tree metadata structure to quantify trapped logical storage space in overlapping extents of the extent store due to the overlapping write requests.

2. The system of claim 1 wherein the adjacent levels of the dense tree metadata structure include an upper level and a next lower level.

3. The system of claim 2 wherein the storage I/O stack is further configured to perform a merge operation to merge the entries of the upper level of the dense tree metadata structure with the entries of the next lower level of the dense tree metadata structure when the upper level is full.

4. The system of claim 1 wherein the storage I/O stack is further configured to:
calculate an amount of the trapped logical storage space of the overlapping extents; and
determine whether the amount of trapped logical storage space exceeds a threshold.

5. The system of claim 4 wherein, if the amount of trapped logical space exceeds the threshold, the storage I/O stack is further configured to:
perform read-modify-write operations on the overlapping extents to recapture the trapped logical storage space.

6. The system of claim 5 wherein the storage I/O stack when configured to perform the read-modify-write operations is further configured to:
purge the extents consuming the trapped logical storage space; and
write new non-overlapping extents.

7. The system of claim 1 wherein the storage I/O stack is further configured to:
implement a depth-first tree traversal of the entries of the dense tree metadata structure to compute the trapped logical storage space using predetermined rules and data structures.

8. The system of claim 7 wherein the data structures comprise a space lockup list and a space lockup counter.

9. The system of claim 8 wherein the space lockup list is embodied as a push-pop stack.

10. The system of claim 8 wherein the storage I/O stack when configured to implement the depth-first tree traversal is further configured to:
apply the predetermined rules to one of add and remove items from the space lockup list depending on a previous extend key in the list; and
accumulate amounts of the trapped logical storage space to the space lockup counter.

11. A method comprising:
storing a dense tree metadata structure in a memory coupled to a processor of a storage system, the dense tree metadata structure having multiple levels wherein each level of the dense tree metadata structure includes entries for storing metadata having mappings from offset ranges of a logical unit (LUN) to extent keys associated with storage locations for extents stored on an extent store of the storage system;
receiving overlapping write requests directed to the LUN at the storage system, the overlapping write requests representing overlapping offset ranges of the LUN, the overlapping offset ranges stored as overlapping entries of the dense tree metadata structure;
processing the mappings of the overlapping entries to determine an offset range overlap of the overlapping entries per level; and
aggregating the offset range overlap of the overlapping entries between adjacent levels of the multi-level dense tree metadata structure to quantify trapped logical storage space in overlapping extents of the extent store due to the overlapping write requests.

12. The method of claim 11 wherein the adjacent levels of the dense tree metadata structure include an upper level and a next lower level.

13. The method of claim 12 further comprising:
performing a merge operation to merge the entries of the upper level of the dense tree metadata structure with the entries of the next lower level of the dense tree metadata structure when the upper level is full.

14. The method of claim 11 further comprising:
calculating an amount of the trapped logical storage space of the overlapping extents; and
determining whether the amount of trapped logical storage space exceeds a threshold.

15. The method of claim 14 further comprising:
performing read-modify-write operations on the overlapping extents to recapture the trapped logical storage space if the amount of trapped logical space exceeds the threshold.

16. The method of claim 15 wherein performing the read-modify-write operations comprises:
purging the extents consuming the trapped logical storage space; and
writing new non-overlapping extents.

17. The method of claim 11 further comprising:
implementing a depth-first tree traversal of the entries of the dense tree metadata structure to compute the trapped logical storage space using predetermined rules and data structures.

18. The method of claim 17 wherein the data structures comprise a space lockup list and a space lockup counter.

19. The method of claim 18 wherein implementing the depth-first tree traversal comprises:
applying the predetermined rules to one of add and remove items from the space lockup list depending on a previous extent key in the list; and
accumulating amounts of the trapped logical storage space to the space lockup counter.

20. A non-transitory computer readable medium including program instructions for execution on a processor, the program instructions when executed configured to:
store a dense tree metadata structure in a memory coupled to the processor, the dense tree metadata structure having multiple levels wherein each level of the dense tree metadata structure includes entries for storing metadata having mappings from offset ranges of a logical unit (LUN) to extent keys associated with storage locations for extents stored on an extent store;
receive overlapping write requests directed to the LUN, the overlapping write requests representing overlapping offset ranges of the LUN, the overlapping offset ranges stored as overlapping entries of the dense tree metadata structure;
process the mappings of the overlapping entries to determine an offset range overlap of the overlapping entries per level; and
aggregate the offset range overlap of the overlapping entries between adjacent levels of the multi-level dense tree metadata structure to quantify trapped logical storage space in overlapping extents of the extent store due to the overlapping write requests.