Patent Publication Number: US-10776015-B1

Title: Limiting bandwidth usage for an asynchronous replication session in a storage system

Description:
FIELD 
     This application relates at least generally to devices, systems, and methods for data storage and data processing in computer systems. More particularly, this application relates at least to ways to improve efficiency of asynchronous replication in a storage system. 
     BACKGROUND 
     Computer data is vital to today&#39;s organizations, and content addressable storage systems (such as DELL EMC XTREMIO) (hereinafter “XtremIO”) can support a rich set of advanced data services such as single data instance, compression, snapshots, etc., by decoupling storage access, logical volume address space, and physical on-disk location of data. In systems such as this, volume and physical layout metadata can offer tremendous flexibility in decoupling and virtualization. Logical volume metadata used with these systems can provide flexible mapping from logical address to data content references, also known as a hash handle. The logical volume metadata also can make snapshot and single instance storage operations highly efficient. 
     In network environments where high-availability is a necessity, system administrators are constantly faced with the challenges of preserving data integrity and ensuring availability of critical system components, such as data systems and file systems. A significant part of protection of computer data against disasters is focused on data protection and on providing ways for computer systems to recover from disasters and other disruptions. Storage systems sometimes experience failures. For example, a storage device, such as a disk drive, may malfunction making the data stored therein inaccessible (at least temporarily). In addition, data and/or metadata stored on a storage system, or used by a storage system, may become corrupted. To protect against data loss as result of data and/or metadata corruption, file system corruption, and/or hardware failure, storage systems frequently use one or more protection strategies, such as mirroring and use of RAID (Redundant Array of Independent Disks), by taking systems offline to run diagnostic tools, perform manual corrections, etc. 
     In asynchronous replication sessions, data at a source cluster is replicated asynchronously to a target cluster. With multiple sessions replicating data from the source cluster to the target cluster, sessions compete for available resources, such as network bandwidth. Some sessions may proceed more aggressively than others. The sessions that share the same port with the aggressive sessions may lag on the recovery point objective (RPO) since they are not getting enough bandwidth to transfer their data. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form, to provide a basic understanding of one or more embodiments that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
     In certain embodiments, a method for implementing bandwidth control for an asynchronous replication session of a storage system is provided. The method includes receiving a request to set a bandwidth limit on the replication session, updating, by a system manager of the storage system, a replication session object with a value representing the bandwidth limit, and sending, by the system manager, the value representing the bandwidth limit to each of a plurality of control modules of the storage system. The method also includes calculating, by each of the control modules, a proportionate share of the bandwidth limit based on a number of slices owned by each of the control modules. The proportionate share of the bandwidth limit is indicated by a budget value via a budget byte window. The method further includes prior to initiating a scan of the slices by an initiator thread, identifying one or more budget byte windows of the plurality of control modules having a budget value equal to or exceeding a predetermined value, allocating the budget value for the one or more budget byte windows to the initiator thread, and reducing the budget value corresponding to the one or more budget byte windows by the predetermined value. The method also includes initiating, by a routing module for each of the control modules, the initiator thread for slices corresponding to the identified one or more budget byte windows, the initiator thread scanning the slices, sending, by the routing module, values representing a number of bytes transmitted and corresponding number of pages to respective control modules resulting from the scanning, and updating, by the control modules, corresponding budget byte windows based on the values sent by the routing module. 
     In certain embodiments, a system for implementing bandwidth control for an asynchronous replication session of a storage system is provided. The system includes a memory comprising computer-executable instructions and a processor executing the computer-executable instructions, the computer-executable instructions when executed by the processor cause the processor to perform operations. The operations include receiving a request to set a bandwidth limit on the replication session, updating, by a system manager of the storage system, a replication session object with a value representing the bandwidth limit, and sending, by the system manager, the value representing the bandwidth limit to each of a plurality of control modules of the storage system. The operations also include calculating, by each of the control modules, a proportionate share of the bandwidth limit based on a number of slices owned by each of the control modules. The proportionate share of the bandwidth limit is indicated by a budget value via a budget byte window. The operations also include prior to initiating a scan of the slices by an initiator thread, identifying one or more budget byte windows of the plurality of control modules having a budget value equal to or exceeding a predetermined value, allocating the budget value for the one or more budget byte windows to the initiator thread, and reducing the budget value corresponding to the one or more budget byte windows by the predetermined value. The operations further include initiating, by a routing module for each of the control modules, the initiator thread for slices corresponding to the identified one or more budget byte windows, the initiator thread scanning the slices, sending, by the routing module, values representing a number of bytes transmitted and corresponding number of pages to respective control modules resulting from the scanning, and updating, by the control modules, corresponding budget byte windows based on the values sent by the routing module. 
     In certain embodiments, a computer program product for implementing bandwidth control for an asynchronous replication session of a storage system is provided. The computer program product is embodied on a non-transitory computer readable storage medium, and the computer program product includes instructions that, when executed by a computer causes the computer to perform operations. The operations include receiving a request to set a bandwidth limit on the replication session, updating, by a system manager of the storage system, a replication session object with a value representing the bandwidth limit, and sending, by the system manager, the value representing the bandwidth limit to each of a plurality of control modules of the storage system. The operations also include calculating, by each of the control modules, a proportionate share of the bandwidth limit based on a number of slices owned by each of the control modules. The proportionate share of the bandwidth limit is indicated by a budget value via a budget byte window. The operations also include prior to initiating a scan of the slices by an initiator thread, identifying one or more budget byte windows of the plurality of control modules having a budget value equal to or exceeding a predetermined value, allocating the budget value for the one or more budget byte windows to the initiator thread, and reducing the budget value corresponding to the one or more budget byte windows by the predetermined value. The operations further include initiating, by a routing module for each of the control modules, the initiator thread for slices corresponding to the identified one or more budget byte windows, the initiator thread scanning the slices, sending, by the routing module, values representing a number of bytes transmitted and corresponding number of pages to respective control modules resulting from the scanning, and updating, by the control modules, corresponding budget byte windows based on the values sent by the routing module. 
     Details relating to these and other embodiments are described more fully herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       Objects, aspects, features, and advantages of embodiments disclosed herein will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements. Reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures without additional description in the specification to provide context for other features. For clarity, not every element may be labeled in every figure. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments, principles, and concepts. The drawings are not meant to limit the scope of the claims included herewith. 
         FIG. 1A  is a simplified diagram schematically illustrating a distributed storage system for data storage, having separate control and data planes, in accordance with at least one illustrative embodiment of the disclosure; 
         FIG. 1B  is an exemplary configuration of modules for the distributed storage system of  FIG. 1A , in accordance with at least one illustrative embodiment of the disclosure; 
         FIG. 1C  is a simplified block diagram showing a portion of a storage system in accordance with at least one illustrative embodiment of the disclosure; 
         FIG. 2  is a simplified block diagram of a replication system, in accordance with one illustrative embodiment of the disclosure; 
         FIG. 3  is a simplified diagram schematically illustrating four different node configurations for a replication system, in accordance with one illustrative embodiment; 
         FIG. 4  is a simplified schematic diagram showing the nodes of  FIG. 3  connected to a switch. 
         FIG. 5  is a simplified diagram showing a compute+SAN+store node for the device of  FIGS. 1A-1C and 2 ; 
         FIG. 6  is a simplified flowchart of a process for implementing bandwidth usage limits for an asynchronous replication session, in accordance with at least one disclosed embodiment; 
         FIG. 7  is simplified flowchart of a process for updating a bandwidth limit, in accordance with one embodiment; 
         FIG. 8  is a simplified flowchart of a process for limiting the number of requests generated during a slice scan, in accordance with one embodiment; 
         FIG. 9  is a simplified flowchart of a process for updating a budget byte window, in accordance with one embodiment; and 
         FIG. 10  is a simplified block diagram of an apparatus that may be used to implement at least a portion of the systems and processes of  FIGS. 1A-9 , in accordance with at least some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing embodiments of the concepts, structures, and techniques sought to be protected herein, some terms are explained, and some relevant background patents are referenced. The following description includes several terms for which the definitions are generally known in the art. However, the following glossary definitions are provided to clarify the subsequent description and may be helpful in understanding the specification and claims. 
     As used herein, the term “storage system” is intended to be broadly construed to encompass, for example, private or public cloud computing systems for storing data as well as systems for storing data comprising virtual infrastructure and those not comprising virtual infrastructure. As used herein, the terms “client,” “host,” and “user” refer, interchangeably, to any person, system, or other entity that uses a storage system to read/write data. In some embodiments, the term “storage device” may also refer to a storage array including multiple storage devices. In certain embodiments, a storage medium may refer to one or more storage mediums such as a hard drive, a combination of hard drives, flash storage, combinations of flash storage, combinations of hard drives, flash, and other storage devices, and other types and combinations of computer readable storage mediums including those yet to be conceived. A storage medium may also refer both physical and logical storage mediums and may include multiple level of virtual to physical mappings and may be or include an image or disk image. A storage medium may be computer-readable and may also be referred to herein as a computer-readable program medium. 
     In certain embodiments, the term “I/O request” or simply “I/O” or “IO” may be used to refer to an input or output request, such as a data read or data write request, which can originate at a host, at a user, or at any other entity in operable communication with a computer system. 
     In certain embodiments, a storage device may refer to any non-volatile memory (NVM) device, including hard disk drives (HDDs), solid state drivers (SSDs), flash devices (e.g., NAND flash devices), and similar devices that may be accessed locally and/or remotely (e.g., via a storage attached network (SAN) (also referred to herein as storage array network (SAN)). 
     In certain embodiments, a storage array (sometimes referred to as a disk array) may refer to a data storage system that is used for block-based, file-based or object storage, where storage arrays can include, for example, dedicated storage hardware that contains spinning hard disk drives (HDDs), solid-state disk drives, and/or all-flash drives (e.g., the XtremIO all flash drive, available from DELL/EMC of Hopkinton Mass.). In certain embodiments, a data storage entity may be any one or more of a file system, object storage, a virtualized device, a logical unit, a logical unit number, a logical volume, a logical device, a physical device, and/or a storage medium. 
     In certain embodiments, a logical unit (LU) may be a logical entity provided by a storage system for accessing data from the storage system, and as used herein a logical unit is used interchangeably with a logical volume. In many embodiments herein, a LU or LUN (logical unit number) may be used interchangeable for each other. In certain embodiments, a LUN may be a logical unit number for identifying a logical unit; may also refer to one or more virtual disks or virtual LUNs, which may correspond to one or more Virtual Machines. LUNs can be divided into smaller logical areas, to balance the load between system modules, where each such small logical area is called a sub-LUN. 
     In certain embodiments, a physical storage unit may be a physical entity, such as a disk or an array of disks, for storing data in storage locations that can be accessed by address, where physical storage unit is used interchangeably with physical volume. In certain embodiments, a data storage entity may be any one or more of a file system, object storage, a virtualized device, a logical unit, a logical unit number, a logical volume, a logical device, a physical device, and/or a storage medium. 
     In certain embodiments, data replication includes processes by which storage data (e.g., data stored on a data storage entity) is duplicated to a remote or local system, to help provide an enhanced level of redundancy in case a main or primary storage backup system fails. In certain embodiments, an image may be a copy of a logical storage unit at a specific point in time. In certain embodiments, a clone may be a copy or clone of the image or images, and/or drive or drives of a first location at a second location. In some embodiments, a clone may be made up of a set of objects. 
     In certain embodiments, a snapshot may refer to differential representations of an image, i.e. the snapshot may have pointers to the original volume and may point to log volumes for changed locations. In certain embodiments, a snapshot may refer to differential representations of the state of a system. Snapshots may be combined into a snapshot array, which may represent different images over a time period or different states of a system over a time period. In certain embodiments, snapshots may be taken from the original source address range as well as from previous snapshots acting as new sources. Snapshots can be arranged into a hierarchy such as a tree, in certain embodiments, with each respective snapshot considered to be a leaf of the tree. Alternately, snapshots can be arranged into a type of tree where there is a tree per snapshot. In another alternative, snapshots can be viewed as part of an array and viewed “by row” or “by column.” With arrangement by row, the snapshots are arranged so that they are part of a tree of snapshots, where each leaf of the tree corresponds to another tree of addresses in that snapshot. With arrangement by column, snapshots can be arranged such that there is a tree of addresses, where each leaf contains a tree of snapshots which contain that address. In certain embodiments, a snapshot set consists of snapshots taken at the exact time on all volumes in a consistency group or, in other words, a snapshot on a consistency group. 
     In certain embodiments, a consistency group (CG) is a collection of base volumes in a storage array, where the base volumes that are the sources of snapshot images are referred to as member volumes of a consistency group. In certain embodiments, CGs are used to create a consistent image of a set of volumes, such as to create snapshots at both the production and the target locations. In certain embodiments, one purpose of a consistency group is to take simultaneous snapshot images of multiple volumes, thus ensuring crash-consistent (as opposed to application-consistent) copies of a collection of volumes at a particular point in time. In certain embodiments, where production volumes are written to target or copy volumes, a consistency group helps to ensure that all writes to the production volume(s) are also written to the copy(ies) in correct write-order and in consistent way, so the copy can always be used instead of production volume. For example, in some embodiments, a consistency group of data storage objects from different servers can be replicated across a network to one or more secondary servers; in the consistency group, file version objects from the different primary servers are kept consistent with each other even if applications or other user may access the original data storage object from more than one of the primary servers. In addition, the delta between successive snapshots of the consistency group can be created concurrently for data storage objects in different primary servers in such a way that the delta created at the different primary servers are consistent with each other even though an application may access the original data storage objects from more than one of the primary servers. 
     In certain embodiments, a journal may be a record of write transactions (e.g., I/O data) issued to a storage system, which may be used to maintain a duplicate storage system, and to roll back the duplicate storage system to a previous point in time. In some embodiments, each entry in a journal contains, apart from the I/O data itself, I/O metadata that can include information such as a volume identifier (ID), the I/O block offset within the volume, the I/O length, and a time stamp of the I/O. 
     In certain embodiments, a replication set may refer to an association created between a source or primary volume and a plurality of target volumes (which may be local or remote), where a consistency group may contain one or more replication sets. A consistency group can be journal protected, and in certain embodiments, all members of a consistency group can share a journal 
     In certain embodiments, XtremIO, available from Dell EMC of Hopkinton, Mass.) is a type of content addressable storage array that uses all flash technology. Flash, as is understood, is a solid-state (SS) random access media type that can read any address range with no latency penalty, in comparison to a hard disk drive (HDD) which has physical moving components which require relocation when reading from different address ranges and thus significantly increasing the latency for random I/O data. In an exemplary Content Addressable Storage (CAS) array (e.g., as described in U.S. Pat. No. 9,208,162 (hereinafter “&#39;162 patent”), which is hereby incorporated by reference), data is stored in blocks, for example of 4 KB, where each block has a unique large hash signature, for example of 20 bytes, saved on Flash memory. In certain embodiments, a long hash is a value computed based on a data packet, such as a SHA-1 hash that is 20 bytes in length, but this is not limiting. As described herein, hash signatures (also referred to herein as full hashes or long hashes) are accessed by small in-memory handles (Called herein, interchangeably, hash handles, short hash handles or short hashes)), for example of 6 bytes. These handles are unique to each array, but not necessarily unique across arrays. A hash signature is unique, meaning that if two hash signatures are the same then their corresponding data blocks are the same. In certain embodiments, a hash signature may be represented by a short hash generated by using, for example, processes shown in FIG. 12A of the &#39;162 patent. Short hashes are not necessarily world-wide unique, but in certain embodiments short hashes can be unique within a domain. In certain embodiments, the long hash can be computed or determined based on the short hash, using, for example, processes shown in FIG. 12C of the &#39;162 patent. 
     When replicating between two CAS arrays, it is much more efficient to use hash signatures instead of sending the full block. If the target already has the data block corresponding to the hash signature, there is no need to send the corresponding data. However, reading the hash signatures may be expensive, and is wasteful if the target does not have the data (in this case it can be faster to send the data without a hash signature, and let the target calculate the hash signature.) While the hash handles are readily available without the need to read from Flash, since the hash handles are not unique, they cannot be easily used to check if a target contains a hash signature. In some implementations, hash handles are shortcuts for hash signatures, and can give a reliable hint of the existence of a hash signature in an array. 
     In certain embodiments, an X-page is a predetermined-size aligned chunk as the base unit for memory and disk operations. In certain embodiments described in the present description, the X-Page size is referred to as having 4 KB; however other smaller or larger values can be used as well, and nothing in the design is limited to a specific value. 
     In certain embodiments, a logical X-page address is the logical address of an X-page, containing a LUN identifier as well as the offset of the X-page within the LUN. 
     In certain embodiments, deduplication of data is a technique that ensures that the same data is not intentionally stored twice in different places. Advantageously, using content-based mapping of data to data modules (D-modules) and within D-modules (as described further herein in connection with  FIGS. 1A-1C ) provides inherent deduplication. 
     In certain embodiments, asynchronous replication is a technique where, after a user or other entity writes data to the primary (also referred to as “source”) storage array first, the asynchronous replication process commits data to be replicated to memory or a disk-based journal, including managing all the needed metadata for the replication process. It then copies the data at scheduled (e.g., periodic) intervals to replication targets. Some types of asynchronous replication can send I/Os in batches even while waiting for acknowledgement from a replication target that replication was successful. In contrast, with some types of synchronous replication, the synchronous replication process cannot continue serving I/Os until the target sends an acknowledgement for the I/Os already sent. Asynchronous replication can be advantageous for replication of data over long distances, to maintain a replica of the data at a destination site. Updates to the destination image can be issued manually, or automatically (e.g., at predetermined times) based on a customizable Recovery Point Objective (RPO) (e.g., the acceptable amount of data, measured in units of time, that may be lost in a failure). 
     In certain embodiments, a cluster is a system comprising two or more computers or systems (called nodes) which work together as a system, e.g., to execute applications or perform other tasks, so that entities that user or access them, have the impression that only a single system responds to them, thus creating an illusion of a single resource (virtual machine). 
     In certain embodiments, a recovery point objective (RPO) may refer to a maximum acceptable lag time between the time data is committed to a source site and the time the data is committed to a target site or an acceptable amount of data loss measured in time. The frequency with which to perform replication cycles may be determined by the RPO and/or a retention policy. For example, a policy may define an RPO setting of x seconds, where x may be a positive integer, to ensure that the most recent available replica stored on the target site reflects the state of data stored on source site no longer than x seconds ago. 
     It is envisioned that at least some embodiments described herein are usable with one or more of the embodiments described in certain commonly owned U.S. patents, and background information useful in understanding one or more embodiments described herein can be found in certain commonly owned U.S. patents, including but not limited to: U.S. Pat. No. 7,765,187 (“Replication of a Consistency Group of Data Storage Objects from Servers in a Data Network”); U.S. Pat. No. 9,104,326 (“Scalable Block Data Storage Using Content Addressing”); and U.S. Pat. No. 9,208,162 (“Generating a Short Hash Handle”), each of which patents are hereby incorporated by reference. 
     While vendor-specific terminology may be used herein to facilitate understanding, it is understood that the concepts, techniques, and structures sought to be protected herein are not limited to use with any specific commercial products. In addition, to ensure clarity in the disclosure, well-understood methods, procedures, circuits, components, and products are not described in detail herein. 
     The phrases, “such as,” “for example,” “e.g.,” “exemplary,” and variants thereof, are used herein to describe non-limiting embodiments and are used herein to mean “serving as an example, instance, or illustration.” Any embodiments herein described via these phrases and/or variants are not necessarily to be construed as preferred or advantageous over other embodiments and/or to exclude the incorporation of features from other embodiments. In addition, the word “optionally” is used herein to mean that a feature or process, etc., is provided in some embodiments and not provided in other embodiments.” Any particular embodiment may include a plurality of “optional” features unless such features conflict. 
     Before describing further aspects of various embodiments herein, one or more environments in which the embodiments can be implemented, are now described. 
       FIG. 1A  illustrates a system  10  for scalable block data storage and retrieval using content addressing, which is usable in accordance with certain embodiments described herein. The system  10  includes data storage devices  12  on which the data blocks are stored. The storage devices  12  are networked to computing modules, there being several kinds of modules, including control modules  14  and data modules  16 . The modules carry out content addressing for storage and retrieval under the control and supervision of a management module  15 , and the network defines separate paths or planes, control paths or a control plane which goes via the control modules  14  and data paths or a data plane which goes via the data modules  16 . 
     The control (C) modules  14  may control execution of read and write commands. The data (D) modules  16  are connected to the storage devices  20  and, under control of a respective control module, pass data to or from the storage devices. Both the C and D modules  14 ,  16 , respectively, may retain extracts of the data stored in the storage device, and the extracts may be used for the content addressing. Typically, the extracts may be computed by cryptographic hashing of the data, as will be discussed in greater detail below, and hash modules (H) ( FIG. 1B ) may specifically be provided for this purpose. That is, the hash modules calculate hash values for data which is the subject of storage commands, and the hash values calculated may later be used for retrieval. 
     Routing modules  18  may terminate storage and retrieval operations and distribute command parts of any operations to control modules that are explicitly selected for the operation in such a way as to retain balanced usage within the system  10 . The routing modules  18  may use hash values, calculated from data associated with the operations, to select the control module for the distribution. More particularly, selection of the control module may use hash values, but typically relies on the user address and not on the content (hash). The hash value is, however, typically used for selecting the Data (D) module  16 , and for setting the physical location for data storage within a D module  16 . 
     The storage devices  12  may be solid-state random-access storage devices, as opposed to spinning disk devices; however, disk devices may be used instead or in addition. A deduplication feature may be provided. The routing modules  18  and/or data modules  16  may compare the extracts or hash values of write data with hash values of already stored data, and where a match is found, simply point to the matched data and avoid rewriting. The modules are combined into nodes  20  on the network, and the nodes are connected over the network by a switch  22 . The use of content addressing with multiple data modules selected based on the content hashing, and a finely grained mapping of user addresses to control modules, allows for a scalable distributed architecture. 
       FIG. 1B  is a functional block diagram of a system  50  in which a Hash (H) module  52  is connected to a routing (R) module  54 . The R module  54  is connected to both control  56  and data  58  modules. The data module  58  is connected to any number of memory devices (e.g., solid-state devices (SSD))  60 . A management module  62  supervises the operations of the modules in the system  50 . 
     A function of the H module  52  is to calculate the Hash function value for a given block of data, e.g., data which is the subject of storage commands. The hash values calculated may later be used for retrieval. The Hash function may be based on standards-based hash functions such as SHA-1 and MD5, or based on a proprietary function, but this is not limiting. The hash function is selected, in certain embodiments, to generate a uniformly distributed output over the range of potential input values. In certain embodiments, H modules  52  share nodes with an R module  54 , but that is not limiting. More generally, the H modules  52  can reside in certain nodes, in all nodes, together with R modules  54 , or together with C modules  56  or D modules  58 . 
     A function of the R module  54  is to terminate storage area network (SAN) Read/Write commands and route them to appropriate C and D modules  56 ,  58 , for execution by these modules. By doing so, the R module  54  can distribute workload over multiple C and D modules  56 ,  58 , and at the same time create complete separation of the control and data planes, that is, provide separate control and data paths. In certain embodiments, the R module  54  routes SCSI I/O request to the C modules  56 , guarantees execution, and returns the result. In certain embodiments, the R module  54  maintains an up to date data structure called an address-to-control module (A→C or A2C) table, coordinated with the management back end (MBD), indicating which C module  56  is responsible for each logical X-page address (LXA), and also showing a balance a range of all possible LXAs between available C modules  56 . In certain embodiments, for write operations, the R module  54  instructs the calculation of a hash digest for each X-page by requesting such calculation from a hash calculation module (e.g., the H module  52 ). 
     A function of the C module  56  is to control the execution of a Read/Write (R/W) command, as well as other storage functions implemented by the system. The C module  56  also may maintain and manage key metadata elements. In certain embodiments, the C module  56  receives an I/O request from an R module  54  on a certain sub-LUN (SL), guaranteeing its atomic execution (i.e., execution independent of other processes) and returns the result to the R module  54 . The C module  56  also communicates with D modules  58  to execute the I/O requests. In addition, the C module  56  monitors the disk content of its logical space by associating each LXA with its hash digest; and balances the work load between the D modules for the SLs that the C module  56  is maintaining. The C module  56  and data module  58  each maintains certain data structures and corresponding metadata journals for those data structures. For example, the C module  56  maintains an “address to hash” table (A2H table, also referred to herein as A→H table) and corresponding A2H metadata journal, in the C module  56 . The D module  58  maintains the “hash to physical location of data” table (H2P table, also referred to herein as H→P table) and its corresponding metadata journal in the D module  58 . That is, in certain embodiments, the in-memory copy of a journal is in the same module as the corresponding metadata table. In certain embodiments, the on-disk journal copy is persisted and mirrored across nodes in the cluster in journal disk chunks. 
     An A2H table maps each LXA that belongs to the SLs that the C module  56  is responsible for, to the hash digest representing the X-page Data that currently resides in that address. The C module  56  maintains the A2H table in a persistent way. The C module  56  may initiate requests to D modules  58  to save table pages to disk and read them from disk. In addition, to avoid frequent disk operations, the C module  56  maintains a journal certain of the latest table operations. These journals include (but are not limited to) the A2H metadata journals (A2H Hash tree) and dirty tree update metadata journals. As discussed below, the data module (D) takes charge of Hash Metadata (HMD), physical layout (PL) metadata, hash to physical layout (H2P) mapping, H2P metadata journals, on disk block allocation (3WBM) and disk block allocation bitmap (3WBM) journals, as discussed further herein. For example, in certain embodiments, the metadata journals include information associated with time-based changes to information in the respective A2H and H2P tables and time-based changes to the disk block allocation bitmap. 
     The H2P table maps each range of hash digests to the corresponding D module  58  responsible for that range. The H2P table balances the range of all possible hash digests between the available D modules  58 . 
     A function of the D module  58  is to perform the actual R/W operation by accessing the storage devices  60  attached to it. The D module  58  may maintain metadata related with the physical location of data blocks. In certain embodiments, the D module  58  is responsible for: maintaining a set of LUNs which are attached locally and performing all I/O operations on these LUN; managing the physical layout of the attached LUNs; managing the mapping between X-Page Data hash digests and their physical location in a persistent way; managing deduplication of X-Page Data in a persistent way; and receiving disk I/O requests from C modules  56 , perform them and returning a result. 
     In certain embodiments, the D module  58  is also responsible for, for each write operation, backing up the X-Page Data in the designated D backup module and performing read-modify operations for writes that are smaller than X-Page size (This process also involves, in certain embodiments, computing a hash digest for these X-Pages). In certain embodiments, the D module  58  maintains an up-to-date H→(D, D backup ) table coordinated with the MBE, where the H→(D, D backup ) table is expected to balance the range of all possible hash digests between the available D modules  58 . 
     Balancing between the D modules is based on hashing of the content. For example, in certain embodiments, the D module  58  makes use of a hash digest metadata table. The hash digest metadata table maps each in use hash digest, that represents actual X-Page Data, to its meta data information including its physical page on the storage media (SSD), its memory copy (if exists), a mapping to any backup memory copy and a reference count for the purpose of deduplication. The D modules  58  manage a separate nonvolatile memory pool (NVRAM or UPS protected) for X-Page data backup purposes. The backup holds X-Pages that are held in memory of the D primary and have not yet been destaged. This is an example of the user data destage cache (UDC). There are dirty X-pages waiting to be persisted on disk. When re-balancing between D modules  58  occurs (due to a D module  58  failure for example), the D module  58  may communicate with other D modules  58  to create new backup copies or move a primary ownership as required. 
     The D modules  58  allow deduplication per X-Page data by maintaining a persistent reference count that guarantees only one copy per X-Page data. The D modules  58  manage the hash digest metadata table in a persistent way. The table is coordinated with the physical layout for physical pages allocation, with the memory pointer, memory backup pointer, and deduplication reference count. 
     As will be appreciated, the H, R, C, D, and M modules, respectively,  52 ,  54 ,  56 ,  62 , and  58  may be implemented in software and executed on a physical node. In addition, the aforementioned U.S. Pat. No. 9,104,326 patent provides information relating to additional functionality of the R, C, D, and H modules, in certain embodiments. 
       FIG. 1C  shows a storage system  100  according to an illustrative embodiment of the disclosure. The storage system  100  may be the same as or similar to a node  20  within the distributed storage system  10  of  FIG. 1A  and/or the system  50  of  FIG. 1B . The storage system  100  may include a plurality of subsystems  102   a - 102   d  (generally denoted  102  herein), a storage array  106  comprising a plurality of storage devices  108   a  . . .  108   n  (generally denoted  108  herein), and a primary memory  118 . In some embodiments, the storage devices  108  may be provided as random-access storage devices, such as solid-state devices (SSDs). 
     The primary memory  118  can be any type of memory having access times that are significantly faster compared to the storage devices  108 . In some embodiments, primary memory  118  may be provided as dynamic random-access memory (DRAM). In certain embodiments, primary memory  118  may be provided as synchronous DRAM (SDRAM). In one embodiment, primary memory  118  may be provided as double data rate SDRAM (DDR SDRAM), such as DDR3 SDRAM. 
     In the embodiment shown, the subsystems  102  include a routing subsystem  102   a , a control subsystem  102   b , a data subsystem  102   c , and a management subsystem  102   d . The routing subsystem, control subsystem  102   b , data subsystem  102   c , and management subsystem  102   d  correspond to the R, C, D, and M modules shown in  FIGS. 1A-1B . A single control subsystem and a single routing subsystem is shown in  FIG. 1C  for simplicity; however, it will be understood that multiple control subsystems and routing systems may be included in the system of  FIG. 1C  in order to realize the advantages of the embodiments described herein. 
     In one embodiment, subsystems  102  may be provided as software components, i.e., computer program code that, when executed on a processor, may cause a computer to perform functionality described herein. In a certain embodiment, the storage system  100  includes an operating system (OS) and one or more of the subsystems  102  may be provided as user space processes executable by the OS. In other embodiments, the subsystems  102  may be provided, at least in part, as hardware, such as digital signal processor (DSP) or an application specific integrated circuit (ASIC) configured to perform functionality described herein. 
     The routing subsystem  102   a  may be configured to receive I/O operations from clients  118  using, for example, an external application-programming interface (API) and to translate client I/O operations into internal commands. In some embodiments, the routing subsystem  102   a  is configured to receive commands from small computer system interface (SCSI) clients  118 . In certain embodiments, the system  100  may store data in fixed-size chunks, for example 4K chunks, where each chunk may have a unique hash value (referred to herein as a “chunk hash,” which in certain embodiments is the same as a hash digest). In such embodiments, the routing subsystem  102   a  may be configured to split data into fixed-size chunks and to calculate the corresponding chunk hashes. In one embodiment, chunk hashes are calculated using Secure Hash Algorithm 1 (SHA-1) processing. In some embodiments, a chunk corresponds to a fixed number of contiguous blocks within a storage device. 
     The control subsystem  102   b  may be configured to maintain a mapping between I/O addresses associated with data and the corresponding chunk hashes. As shown in  FIG. 1C , this mapping may be maintained using a data structure  112 , referred to herein as an “I/O address to chunk hash mapping table” or “A2H table,” (also known as A→H table) according to some embodiments. In one embodiment, I/O addresses may be logical addresses used by clients  118  to access data within the storage system  100 . 
     The data subsystem  102   c  may be configured to maintain a mapping between chunk hashes and physical storage addresses (i.e., storage locations within the storage array  106  and/or within individual storage devices  108 ). This mapping may be maintained using a data structure  114 , referred to herein as a “hash to physical address mapping table” or “H2P table,” or “H→P table,” according to some embodiments, where this table, in certain embodiments, includes information similar to that of the aforementioned HMD (hash metadata) and PL (physical layout) tables. In certain embodiments, as described, for example, in the incorporated by reference patents, there also may be a mapping referred to as the H2D or H→D table, where D stands for disk physical layout. In certain embodiments, the H2P table is maintained to route data with different hashes to different D modules. The data subsystem  102   c  may be also be configured to read and write data from/to the storage array  106  (and/or to individual storage devices  108  therein). 
     It will be appreciated that combinations of the A2H  112  and H2P  114  tables may provide multiple levels of indirection between the logical (or “I/O”) address a client  118  uses to access data and the physical address where that data is stored. Among other advantages, this can give the storage system  100  freedom to move data within the storage array  106  without affecting a client&#39;s  118  access to that data (e.g., if a storage device  108  fails). 
     The management subsystem  102   d  (also referred to herein as system manager or SYM) may be configured to monitor and track the status of various hardware and software resources within the storage system  100 . In some embodiments, the management subsystem  102   d  may manage the allocation of memory by other subsystems (e.g., subsystems  102   a - 102   c ) using techniques described below in conjunction with  FIGS. 2-10 , described further herein. In some embodiments, the management subsystem  102   d  can also be configured to monitor other subsystems  102  (e.g., subsystems  102   a - 102   c ) and to use this information to determine when the storage system  100  may begin processing client I/O operations after a restart. 
     In certain embodiments, the system  100  enables control of the usage of bandwidth during asynchronous replication. The management subsystem  102   d  maintains a replication session object  120  with a predetermined maximum bandwidth limit value that is permitted for a replication session across all control modules. In addition, the control subsystem  102   b  maintains a data structure  122  that provides a value specifying the control module&#39;s allocated share of the bandwidth limit. The control subsystem  102   b  also includes a data structure  124  that provides a value indicating whether the control module is subject to the bandwidth limit process. For example, the data structure  124  may represent a flag that lets the system know whether the control module is under the bandwidth limit. In addition, the control subsystem  102   b  also includes a data structure  126  that provides updated data (e.g., in a table format) relating to replication session object slices during the bandwidth control processes. Finally, the control subsystem  102   b  includes a data structure  128  that provides a slice bitmap usable during the bandwidth control processes. These elements are described further herein. 
       FIG. 2  is a simplified block diagram of an exemplary replication system  200 , in accordance with one illustrative embodiment of the disclosure, which may be implemented using the system  10  of  FIG. 1A . The system  200  includes a host  210 , a replicator module  208 , a source storage module  202  (e.g., a storage array) on a source side and a target storage module  204  (e.g., a storage array), on a target side, which are advantageously in operable communication over a network (not shown). The host  210  may include an application (not shown) that writes to the source storage  202 . The replicator module  208  ensures that all the writes made to the source storage  202  are also eventually made to the target storage device  204 . The replicator module  208  may reside outside the storage arrays  202  and  204 , or it may be inside one of them, or it may be in an independent system, or it may be inside the host  210 . 
     The source storage  202  includes consistency groups (e.g., a consistency group  220   a , a consistency group  222   a  and a consistency group  224   a ) and the target storage device  204  also includes respective consistency groups (e.g., for target  204  a consistency group  220   b , a consistency group  222   b , and a consistency group  224   b ). The consistency group  220   a  includes volumes (e.g., a volume  228   a  and a volume  230   a ) and the consistency group  220   b  includes volumes (e.g., volumes  228   b  and  230   b ). The consistency group  222   a  includes a volume  232   a  and the consistency group  222   b  includes volume  232   b . In certain embodiments, the storage volumes  228   b ,  230   b ,  232   b , are replicas of the respective consistency groups  220   a ,  222   a  on the source storage side. In certain embodiments, the target storage modules  204  can include one or more target clusters, e.g., target cluster  240 . 
     The system  200  also can, for example, be similar to the storage system  100  of  FIG. 1C . The source storage  202  can be located at a production site and the target storage system  204  can be located at a replication site. Although the system  200  shows a plurality of consistency groups  220 ,  222 ,  224  at the source side, at least some systems  200  can have a single consistency group (e.g., consistency group  220   a ) that is replicated to a target cluster (e.g., target cluster  240 ). In certain embodiments, one or more of the source side consistency groups are replicated asynchronously to target storage device. 
     In certain embodiments, the consistency group  220   a  and the consistency group  220   b  (in each respective target cluster) may form first and second replication pairs, where the consistency group  220   a  is replicated asynchronously to the consistency group  220   b . The consistency group  222   a  and the consistency group  222   b  also form a replication pair wherein the consistency group  222   a  is replicated asynchronously to the consistency group  222   b . It will be appreciated that, in certain embodiments, a replication set can include, for example, two or more elements (e.g., a primary, such as consistency group  220   a , and a target, e.g.,  220   b ). 
     In certain embodiments, the volumes for consistency group  220   a  and the consistency group  220   b  form volume pairs, so that each volume in consistency group  220   a  corresponds to exactly one volume in consistency group  220   b . That is, the volume  228   a  is paired with the volume  228   b  and the volume  230   a  is paired with the volume  230   b . In particular, in certain embodiments, the volume  228   a  is replicated at the volume  228   b  and the volume  230   a  is replicated at the volume  230   b . In one example, a replication pair includes information on the volume pairs and the replication, and fully describes a replication instance between a source storage and a target storage. 
     Referring still to  FIG. 2 , in one example, one or more of the storage volumes on the source side and the target side are each on a respective disk. For example, in one embodiment, the storage volumes  228   a  and  230   a  in consistency group  220   a  is on a first respective flash memory device and is configured for replication to replicate to target cluster  240 , which is on a flash memory device. Thus, in an illustrative replication configuration, in accordance with certain embodiments herein, a single consistency group (CG) is replicated into a target cluster. In the asynchronous replication case, this can be done by taking snapshots, calculating differences (deltas) between snapshot generations, and rebuilding the snapshots on targets. Examples of how snapshots and deltas between snapshots are created and used in exemplary replication and storage systems are described, for example, in U.S. Pat. No. 7,764,187, which is hereby incorporated by reference. In certain embodiments, for systems such as those shown in  FIG. 2 , asynchronous replication is advantageous for primary storage (e.g., including a consistency group (CG)) that is replicated to targets at a greater distance. 
       FIG. 3  is a simplified diagram schematically illustrating four different node configurations for a replication system usable with at least some embodiments herein, in accordance with one illustrative embodiment. In  FIG. 3  each node type shows the functional modules that execute, in at least one copy, within the node, and functional modules that may optionally execute within this node. Optional modules are shown in dashed line. 
     All nodes include a switch interface  308 , to allow interconnecting with a switch (see  FIG. 4 ) in a multi-node system configuration. A node that contains a SAN function includes at least one SAN Interface module  310  and at least one R nodule. A bode that contains a Store function includes at least one SSD driver module  312  and at least one D module. Hence, in certain embodiments, Compute+SAN and Compute+SAN+STORE nodes contain a SAN interface, to interface with the external SAN. The interface may typically use a SCSI-based protocol running on any of a number of interfaces including Fiber Channel, Ethernet, and others, through which read/write and other storage function commands are being sent to the system. Compute+Store and Compute+SAN+Store Nodes contain an SSD driver  312  to interface with SSDs  208  attached to that specific node, where data is stored and accessed. 
       FIG. 4  is a simplified schematic diagram  450  showing the nodes of  FIG. 3  connected to a switch, in a high-level system block diagram. A system implementation includes one or more nodes  400 ,  402 . In certain embodiments where a system contains more than two nodes, all physical nodes are interconnected by a switch  404  which may be based on any of a number of networking technologies including Ethernet, InfiniBand and so forth. In the specific case of a 2-node system, the two nodes can be interconnected directly without a need for a switch. The interconnections between each node and the switch may include redundancy, to achieve high system availability with no single point of failure. In such a case, each node may contain two or more switch interface modules  406 , and the switch may contain two or more ports per physical node. 
       FIG. 5  is a simplified diagram  550  showing a compute+SAN+store node for the device of  FIGS. 1A-4 . In particular,  FIG. 5  illustrates a single node system configuration, in which R, C and D modules,  500 ,  502  and  504  respectively are together in a compute+SAN+Store node  506 . A switch interface  508  links to a switch. A SAN interface  510  provides an interface for storage area networking. An SSD driver  512  interfaces with the storage devices. 
     A four-node system configuration is shown in  FIGS. 1A and 3  above. The configuration includes two compute and store nodes and two compute+SAN nodes. A system that is built from multiple physical nodes can inherently support a high availability construction, where there is no single point of failure. This means that any node or sub-node failure can be compensated for by redundant nodes, having a complete copy of the system&#39;s metadata, and a complete redundant copy of stored data (or parity information allowing recovery of stored data). The distributed and flexible architecture allows for seamless support of failure conditions by simply directing actions to alternate nodes. 
     The R module is responsible for routing SCSI I/O requests to the C modules, guarantee execution and return the result; and balancing the work load between the C modules for the requests it is routing. An A→C table indicates which C module is responsible for each logical X-page address (LXA). Each C module is responsible for a list of Sub LUNs (SLs). The R module receives requests for I/O&#39;s from the SAN interface routes them to the designated C modules and returns the result to the SAN interface. 
     If an I/O operation spans across multiple sub LUNs (SLs), and perhaps multiple C modules, then the R module has the responsibility of breaking the big I/O operation into multiple smaller independent operations according to the sub LUN unit size (SLUS). Since the atomic unit size (AUS) is never larger than the SLUS, as explained in greater detail below, each such I/O is treated as an independent operation throughout the system. The results may then be aggregated before returning to the SAN interface. The R module is responsible for maintaining an up-to-date A→C table coordinated with the MBE. The A→C table is expected to balance the range of all possible LXAs between the available C modules. For write operations, the R module instructs the calculation of the hash digest for each X-Page by requesting such calculation from a hash calculation module. 
     The C module is responsible for: receiving an I/O request from an R module on a certain SL, guaranteeing its atomic execution and returning the result; communicating with D modules to execute the I/O requests; monitoring the disk content of its SLs&#39; logical space by associating each LXA with its hash digest; and balancing the work load between the D modules for the SLs it is maintaining. 
     An H→D table maps each range of hash digests to the corresponding D module responsible for this range. An A→H table maps each LXA that belongs to the SLs C is responsible for, to the hash digest representing the X-Page data that currently resides in this address. The C module receives I/O requests from R modules, distributes the work to the D modules, aggregates the results and guarantees an atomic operation. The result is returned to the R module. The C module maintains an up-to-date H→D table coordinated with the MBE. The table is expected to balance the range of all possible hash digests between the available D modules. 
     The C module maintains an A→H table in a persistent way. The C module may initiate I/O requests to D modules to save table pages to disk and read them from disk. To avoid frequent disk operations, a journal of the latest table operations may be maintained. Data is balanced between the C modules based on the logical address, at the granularity of sub-LUNs. 
     The D module is responsible for: maintaining a set of LUNs which are attached locally and performing all I/O operations on these LUNs; managing the physical layout of the attached LUNs; managing the mapping between X-Page data hash digests and their physical location in a persistent way; managing deduplication of X-Page data in a persistent way; and receiving disk I/O requests from C modules, perform them and returning a result. The D module is also responsible for, for each write operation, backing up the X-Page data in the designated D backup module and performing read-modify operations for writes that are smaller than X-Page size (This process also involves computing a hash digest for these X-Pages). The D module is further responsible for maintaining an up-to-date H→(D, D backup ) table coordinated with the MBE. The H→(D, D backup ) table is expected to balance the range of all possible hash digests between the available D modules. The D module does not communicate directly with R modules. The only interaction with R modules involves RDMA read/write operations of X-Page data. Balancing between the D modules is based on hashing of the content. The D module makes use of a hash digest metadata table. The hash digest metadata table maps each in use hash digest, that represents actual X-Page data, to its metadata information including its physical page on the storage media (SSD), its memory copy (if exists), a mapping to any backup memory copy and a reference count for the purpose of deduplication. 
     A further structure used is the H→(D, D backup ) table. The H→(D, D backup ) table maps each range of hash digests to the corresponding D module responsible for the range as well as the D backup  module responsible for the range. The D modules allocate a physical page for each X-Page. The D modules also manage the memory for the physical storage. They allocate memory pages for read/write operations and perform background destaging from memory to storage media when necessary, for example, when running low on memory. The D modules manage a separate nonvolatile memory pool (NVRAM or UPS protected) for X-Page data backup purposes. The backup holds X-Pages that are held in memory of the D primary and have not yet been destaged. When re-balancing between D modules occur (due to a D module failure for example), the D module may communicate with other D modules to create new backup copies or move a primary ownership as required. 
     The D modules allow deduplication per X-Page data by maintaining a persistent reference count that guarantees only one copy per X-Page data. The D modules manage the hash digest metadata table in a persistent way. The table is coordinated with the physical layout for physical pages allocation, with the memory pointer, memory backup pointer and deduplication reference count. The D modules receive I/O requests from C modules, perform the requests while supporting deduplication and return the result. The D modules may perform RDMA read/write operations on memory that resides in other modules, such as R modules as mentioned above, as part of the I/O operation. When a write operation smaller than the size of an X-Page is received, the D module may read the entire X-Page to memory and perform partial X-Page modification on that memory. In this case race conditions may occur, for example when two small writes to the same X-Page occur in parallel, and the D module may be required to compute the hash digest of the resulting X-Page. This is discussed in greater detail below. 
     The H-Module calculates the hash function of a given block of data, effectively mapping an input value to a unique output value. The hash function may be based on standards-based hash functions such as SHA-1 and MD5 or based on a proprietary function. The hash function is selected to generate a uniformly distributed output over the range of potential input values. The H modules usually share nodes with an R module but more generally, the H modules can reside in certain nodes, in all nodes, together with R modules, or together with C or D modules. 
       FIGS. 6-9  are simplified flowcharts of a method of implementing bandwidth control for an asynchronous replication session at a storage system, in accordance with at least one disclosed embodiment, which method is usable, for example, with the systems of  FIGS. 1C and 2  and in connection with  FIGS. 6-9 . The bandwidth limit is imposed on a per-session basis. 
     Referring to  FIGS. 1C, 2, and 6-9 , the system receives a request to set a bandwidth limit (also referred to as a maximum bandwidth limit) on the replication session in block  602  of process  600  in  FIG. 6 . The bandwidth limit reflects a total maximum bandwidth permitted for all control modules operating in furtherance of a replication session. This request can be a user-tunable parameter and may be implemented, e.g., using a ‘modify-remote-protection-session’ command at the storage system. The bandwidth limit may be specified as megabytes per second (MB/s). A zero value entered by a user indicates that no bandwidth limit on the session is desired. 
     In block  604 , a system manager (SYM  102   d  of  FIG. 1C ) of the storage system updates a replication session object  120  in a management object module (MOM) with a value representing this bandwidth limit. The MOM may reside in the management module  102   d  of  FIG. 1C . The system manager sends the value representing the bandwidth limit to each of the control modules of the storage system. The control modules, in turn, process this information as described in  FIG. 7 . 
     In block  606 , each of the control modules uses an apportioned share of the bandwidth limit to control the number of requests generated during a slice scan. The slice scan process is described further in  FIG. 8 . 
     In block  608 , each of the control modules updates a budget byte window  122  value based on results of slice scan. This update process is described further in  FIG. 9 . 
     Turning to  FIG. 7 , once the system manager sends the bandwidth limit to the control modules, each of the control modules calculates a proportionate (individual) share of the bandwidth limit based on a number of slices owned by each of the control modules via process  700  of  FIG. 7 . In other words, each control module calculates its own share of the bandwidth limit that it can generate based on the number of slices it owns. The proportionate share of the bandwidth limit for each of the control modules may be calculated by dividing the value representing the bandwidth limit by a number of the plurality of control modules of the storage system. For example, if the maximum bandwidth limit for the replication session is 100 MB/s and there are four control modules each of which owns 256 slices then each control module will at most generate 25 MB/s. In an embodiment, the budget byte window may be represented as a page size multiplied by the 256 slices. In a further embodiment, the budget byte window reflects a maximum number of address/hash pairs for a control-to-routing (C2R) task. A C2R task refers to messages transmitted between control modules and routing modules. For example, a message can list the addresses and hashes that a routing module needs to send to a target system. C2R tasks and responses transmitted between the control module and the routing module are shown generally in  FIG. 1C  at  130 . 
     In the control module, the granularity of the budget byte window  122  may be represented in bytes/msec. Thus, using the above bandwidth limit example of 25 MB/s per control module, the maximum bandwidth per control module may be translated from MB/s to bytes/msec as:
         budget-byte-window-per-msec=max bandwidth per control module in bytes per msec,   where the budget-byte-window is per session (replication session object)       

     Thus, in this example, a control module having 25 MB/s will be translated to 25,000 B/ms in the budget byte window. 
     In block  704 , each of the control modules populates its own budget byte window  122  with a value (referred to as budget value) reflecting its proportionate share of the total bandwidth limit (e.g., 25,000 B/ms). 
     In block  706 , the replication session object flag structure  124  is flagged for each control module to indicate whether the control module is subject to the maximum bandwidth limit process. This may be, e.g., a True/False value. If the flag is not set, then there is no need to use the budget byte window during the scanning process to prevent other sessions not under the limit from waiting on or applying the budget byte window. 
     As indicated above in block  606  of  FIG. 6 , the control modules use the bandwidth limit to control the number of requests generating during a slice scan.  FIG. 8  describes this process  800 . 
     In block  802 , prior to initiating a scan of the slices for a control module by an initiator thread, for each control module, the number of bytes in the budget byte window is ascertained. In block  804 , the process determines whether the number of bytes in the budget byte window is less than a predetermined value. For example, the predetermined value may be set as a page size*256 slices, which is the maximum number of address/hash pairs one C2R task can have. In block  806 , if the budget byte window is less than the predetermined value (e.g., page size*256), the iterator thread does not proceed. In other words, in block  806 , no scan is performed for the control module. Each control module performs the scan of slices it owns independent of other control modules. When the iterator thread does not proceed, this is because the particular control module waits to start the scan until it has enough bytes in the budget byte window. Other control modules may or may not proceed depending upon how much budge they have. 
     If the number of bytes in the budget byte window is equal to or exceeds the predetermined value, the process allocates the number of bytes (budget value) to the initiator thread for scanning and reduces the budget byte window by the number of allocated bytes in block  808 . For example, the budget byte window is reduced by 256*page size under spinlock. 
     During the slice scan, a routing module may initiate the initiator thread for slices corresponding to the budget byte window. The initiator thread starts the slice scan. 
     In block  810 , C2R tasks may be created for pages up to a specified size limit. For example, during the scan C2R tasks are created for pages up to 256. 
     If the slice scan is not completed after the specified limit has been reached (256 pages) in block  812 , the slice is marked as not done, or incomplete, in block  814  and the slice/last-offset pair is saved in the replication session object slice table  126  and return (e.g., the control module will stop the scan until it has enough budget to begin again). If the control module finds more than 256 pages to be transferred to a target in one slice, it cannot finish the scan of that slice. The control module will thus save the slice number and last address pair to identify where to start the next time. 
     In block  816 , if the scan has completed before the specified limit (e.g., 256 pages) has been reached, the budget byte window is updated to reflect the number of pages used. For example, if 100 pages were used, then 256−100=156*page size is added to the budget byte window. 
     When the iterator thread returns to check for more work, instead of relaying a slice cursor, the slice bitmap  128  may be checked to determine if all the slices have been scanned. 
     In block  818 , the process determines (e.g., from the bitmap) whether the scan has completed for all slices for the control module. If so, the number of bytes transmitted and the number of pages used are sent to the control module in block  820 . 
     However, in block  822 , if the scan has not completed for all slices of the control module, the next slice is selected for scanning. A list in the replication session object slice table  126  is checked for the last offset and the scan continues. 
     After completing the processing of the C2R task, the routing module sends the number of bytes transmitted and number of pages back to the control modules in a C2R response  130 . 
     As indicated above in block  608  of  FIG. 6 , the process updates the budget byte window via the control module. The flow diagram  900  of  FIG. 9  further describes this process. 
     Upon completion of the scanning of all of the control modules, the corresponding budget byte windows for each of the plurality of control modules are updated. In block  902 , the process determines a number of bytes currently sent during the replication session as a running sum of bytes transmitted from routing modules plus pages inflight multiplied by a running average of bytes per page from the routing modules (running sum of bytes transmitted from all routing modules+pages inflight*running average of bytes per page from all routing modules). 
     In block  904 , the process determines an effective number of bytes to send as: time elapsed since a last reset in milliseconds multiplied by the budget value minus the number of bytes currently sent ((time since last reset in msec*control module max bandwidth in bytes per msec)−number of bytes currently sent); 
     In block  906 , if effective number of bytes is equal to zero, the budget byte window is updated with a zero value in block  908 . If the effective number of bytes is greater than zero, the budget byte window is updated with the effective number of bytes in block  910 . The updated budget byte window is used to determine whether sufficient budget exists to send a selected C2R task to a respective routing module. 
     In embodiments, all of the dynamic parameters (e.g., budget byte window, bytes transmitted, bytes per page, etc.) may be reset at frequent intervals (e.g., every minute). This can be useful, e.g., in situations where host write patterns change (e.g., data can become more compressible or less, more deduplicated or less, etc.), so the bytes-per-page can change at any time. This may also be useful in situations where there are network glitches that won&#39;t affect the budget byte window calculation beyond a minute. This can be particularly important in case the cycles are very long, e.g., in hours. 
     In embodiments, the control module maximum bandwidth may not be changed unless a user explicitly sets a new bandwidth limit or there is a redistribution of slices. If some of the links are down, the system may attempt to use the other links with available bandwidth. If some routing modules have links with impairments or have lower bandwidth than others, then C2R tokens for these routing modules won&#39;t be available as fast as others and will end up using faster links to transfer the data. 
     In the above-described flow charts of  FIGS. 6-9 , rectangular elements, herein denoted “processing blocks,” represent computer software instructions or groups of instructions. Alternatively, the processing blocks may represent steps performed by functionally equivalent circuits such as a digital signal processor (DSP) circuit or an application specific integrated circuit (ASIC). The flow diagrams do not depict the syntax of any particular programming language but rather illustrate the functional information one of ordinary skill in the art requires to fabricate circuits or to generate computer software to perform the processing required of the particular apparatus. It should be noted that many routine program elements, such as initialization of loops and variables and the use of temporary variables may be omitted for clarity. The particular sequence of blocks described is illustrative only and can be varied without departing from the spirit of the concepts, structures, and techniques sought to be protected herein. Thus, unless otherwise stated, the blocks described below are unordered meaning that, when possible, the functions represented by the blocks can be performed in any convenient or desirable order. 
     Further, the processes and operations described herein can be performed by a computer especially configured for the desired purpose or by a general-purpose computer especially configured for the desired purpose by another computer program stored in a computer readable storage medium or in memory. 
       FIG. 10  is a simplified block diagram of an apparatus that may be used to implement at least a portion of the systems and method of  FIGS. 1A-9B , in accordance with at least some embodiments. As shown in  FIG. 10 , computer  1000  may include processor  1002 , volatile memory  1004  (e.g., RAM), non-volatile memory  1006  (e.g., one or more hard disk drives (HDDs), one or more solid state drives (SSDs) such as a flash drive, one or more hybrid magnetic and solid state drives, and/or one or more virtual storage volumes, such as a cloud storage, or a combination of physical storage volumes and virtual storage volumes), graphical user interface (GUI)  1010  (e.g., a touchscreen, a display, and so forth) and input and/or output (I/O) device  1008  (e.g., a mouse, a keyboard, etc.). Non-volatile memory  1004  stores, e.g., journal data  1004   a  and metadata  1004   b . The non-volatile memory,  1006  can include, in some embodiments, an operating system  1014 , and computer instructions  1012 , and data  1016 . In certain embodiments, the computer instructions  1012  are configured to provide several subsystems, including a routing subsystem  1012 A, a control subsystem  1012   b , a data subsystem  1012   c , and a management subsystem  1012   d . In certain embodiments, the computer instructions  1012  are executed by the processor/CPU  1002  out of volatile memory  1004  to perform at least a portion of the processes shown in  FIGS. 6-9 . Program code also may be applied to data entered using an input device or GUI  1010  or received from I/O device  1008 . 
     The systems and processes of  FIGS. 1A-9  are not limited to use with the hardware and software described and illustrated herein and may find applicability in any computing or processing environment and with any type of machine or set of machines that may be capable of running a computer program. The processes described herein may be implemented in hardware, software, or a combination of the two. The logic for carrying out the method may be embodied as part of the system described in  FIG. 10 , which is useful for carrying out a method described with reference to embodiments shown in, for example,  FIGS. 1A-9 . The processes and systems described herein are not limited to the specific embodiments described. For example, the processes of  FIGS. 6-9  are not limited to the specific processing order shown. Rather, any of the blocks of the processes may be re-ordered, combined, or removed, performed in parallel or in serial, as necessary, to achieve the results set forth herein. 
     Processor  1002  may be implemented by one or more programmable processors executing one or more computer programs to perform the functions of the system. As used herein, the term “processor” describes an electronic circuit that performs a function, an operation, or a sequence of operations. The function, operation, or sequence of operations may be hard coded into the electronic circuit or soft coded by way of instructions held in a memory device. A “processor” may perform the function, operation, or sequence of operations using digital values or using analog signals. In some embodiments, the “processor” can be embodied in one or more application specific integrated circuits (ASICs). In some embodiments, the “processor” may be embodied in one or more microprocessors with associated program memory. In some embodiments, the “processor” may be embodied in one or more discrete electronic circuits. The “processor” may be analog, digital, or mixed-signal. In some embodiments, the “processor” may be one or more physical processors or one or more “virtual” (e.g., remotely located or “cloud”) processors. 
     Various functions of circuit elements may also be implemented as processing blocks in a software program. Such software may be employed in, for example, one or more digital signal processors, microcontrollers, or general-purpose computers. Described embodiments may be implemented in hardware, a combination of hardware and software, software, or software in execution by one or more physical or virtual processors. 
     Some embodiments may be implemented in the form of methods and apparatuses for practicing those methods. Described embodiments may also be implemented in the form of program code, for example, stored in a storage medium, loaded into and/or executed by a machine, or transmitted over some transmission medium or carrier, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation. A non-transitory machine-readable medium may include but is not limited to tangible media, such as magnetic recording media including hard drives, floppy diskettes, and magnetic tape media, optical recording media including compact discs (CDs) and digital versatile discs (DVDs), solid state memory such as flash memory, hybrid magnetic and solid-state memory, non-volatile memory, volatile memory, and so forth, but does not include a transitory signal per se. When embodied in a non-transitory machine-readable medium and the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the method. 
     When implemented on one or more processing devices, the program code segments combine with the processor to provide a unique device that operates analogously to specific logic circuits. Such processing devices may include, for example, a general-purpose microprocessor, a digital signal processor (DSP), a reduced instruction set computer (RISC), a complex instruction set computer (CISC), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a programmable logic array (PLA), a microcontroller, an embedded controller, a multi-core processor, and/or others, including combinations of one or more of the above. Described embodiments may also be implemented in the form of a bitstream or other sequence of signal values electrically or optically transmitted through a medium, stored magnetic-field variations in a magnetic recording medium, etc., generated using a method and/or an apparatus as recited in the claims. 
     For example, when the program code is loaded into and executed by a machine, such as the computer of  FIG. 10 , the machine becomes an apparatus for practicing the invention. When implemented on one or more general-purpose processors, the program code combines with such a processor to provide a unique apparatus that operates analogously to specific logic circuits. As such a general-purpose digital machine can be transformed into a special purpose digital machine.  FIG. 10  shows Program Logic  1024  embodied on a computer-readable medium  1020  as shown, and wherein the Logic is encoded in computer-executable code configured for carrying out the reservation service process of this invention and thereby forming a Computer Program Product  1022 . The logic may be the same logic on memory loaded on processor. The program logic may also be embodied in software modules, as modules, or as hardware modules. A processor may be a virtual processor or a physical processor. Logic may be distributed across several processors or virtual processors to execute the logic. 
     In some embodiments, a storage medium may be a physical or logical device. In some embodiments, a storage medium may consist of physical or logical devices. In some embodiments, a storage medium may be mapped across multiple physical and/or logical devices. In some embodiments, storage medium may exist in a virtualized environment. In some embodiments, a processor may be a virtual or physical embodiment. In some embodiments, logic may be executed across one or more physical or virtual processors. 
     For purposes of illustrating the present embodiment, the disclosed embodiments are described as embodied in a specific configuration and using special logical arrangements, but one skilled in the art will appreciate that the device is not limited to the specific configuration but rather only by the claims included with this specification. In addition, it is expected that during the life of a patent maturing from this application, many relevant technologies will be developed, and the scopes of the corresponding terms are intended to include all such new technologies a priori. 
     The terms “comprises,” “comprising”, “includes”, “including”, “having” and their conjugates at least mean “including but not limited to”. As used herein, the singular form “a,” “an” and “the” includes plural references unless the context clearly dictates otherwise. Various elements, which are described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. It will be further understood that various changes in the details, materials, and arrangements of the parts that have been described and illustrated herein may be made by those skilled in the art without departing from the scope of the following claims.