Patent Publication Number: US-11386124-B2

Title: Snapshot rollback for synchronous replication

Description:
BACKGROUND 
     Many information processing systems are configured to replicate data from one storage system to another storage system, possibly at different physical sites. In some cases, such arrangements are utilized to support disaster recovery functionality within the information processing system. For example, an enterprise may replicate data from a production data center to a disaster recovery data center. In the event of a disaster at the production site, applications can be started at the disaster recovery site using the data that has been replicated to that site so that the enterprise can continue its business. 
     Data replication in these and other contexts can be implemented using asynchronous replication at certain times and synchronous replication at other times. For example, asynchronous replication may be configured to periodically transfer data in multiple cycles from a source site to a target site, while synchronous replication may be configured to mirror host writes from the source site to the target site as the writes are made at the source site. Storage systems participating in a replication process can therefore each be configured to support both asynchronous and synchronous replication modes. 
     Conventional approaches to data replication can be problematic under certain conditions. For example, as synchronous replication data transfer is triggered by input/output (IO) flow, and snapshot rollback operations are not part of IO handling, in order to rollback a synchronous replication source volume to an earlier snapshot, the synchronous replication session needs to be halted in order to perform the rollback followed by restarting the synchronous replication session. If there is a lot of data residing in the synchronous replication source volume, the restart of the synchronous replication session may trigger retransmission of a large quantity of data which can lead to a lengthy gap in the associated data protection window. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described herein 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. 
     One aspect provides a method for performing rollback of a snapshot between a source storage system and a target storage system in a synchronous replication session. The method includes reassigning a current replication source volume to a rollback source snapshot. The rollback source snapshot is generated for a consistency group that includes a plurality of volumes. The method also includes performing, for one or more snapshot trees maintained for the consistency group, a differential scan between a child of the rollback source snapshot and the current replication source volume, and calculating, from results of the differential scan, a dirty tree differential. For each difference identified in the dirty tree differential, the method further includes calculating a corresponding volume offset, and initiating a copy command for the current replication source volume and the rollback source snapshot. The copy command is translated to a remote metadata copy request in synchronous replication data transfer. 
     Another aspect provides a system for performing rollback of a snapshot between a source storage system and a target storage system in a synchronous replication session. The system includes a memory comprising computer-executable instructions, and a processor operable by a storage system. The processor is operable for executing the computer-executable instructions. The computer-executable instructions when executed by the processor cause the processor to perform operations. The operations include reassigning a current replication source volume to a rollback source snapshot. The rollback source snapshot is generated for a consistency group that includes a plurality of volumes. The operations also include performing, for one or more snapshot trees maintained for the consistency group, a differential scan between a child of the rollback source snapshot and the current replication source volume, and calculating, from results of the differential scan, a dirty tree differential. For each difference identified in the dirty tree differential, the operations further include calculating a corresponding volume offset, and initiating a copy command for the current replication source volume and the rollback source snapshot. The copy command is translated to a remote metadata copy request in synchronous replication data transfer. 
     A further aspect provides a computer program product for performing rollback of a snapshot between a source storage system and a target storage system in a synchronous replication session. The computer program product is embodied on a non-transitory computer readable medium. The computer program product includes instructions that, when executed by a computer, causes the computer to perform operations. The operations include reassigning a current replication source volume to a rollback source snapshot. The rollback source snapshot is generated for a consistency group that includes a plurality of volumes. The operations also include performing, for one or more snapshot trees maintained for the consistency group, a differential scan between a child of the rollback source snapshot and the current replication source volume, and calculating, from results of the differential scan, a dirty tree differential. For each difference identified in the dirty tree differential, the operations further include calculating a corresponding volume offset, and initiating a copy command for the current replication source volume and the rollback source snapshot. The copy command is translated to a remote metadata copy request in synchronous replication data transfer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 in order 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. 1  depicts a block diagram of an information processing system including source and target storage systems configured with functionality for snapshot rollback in a synchronous replication session according to an embodiment; 
         FIG. 2  is a flow diagram of a process for implementing snapshot rollback in a synchronous replication session according to an embodiment; 
         FIGS. 3A-3B  illustrate sample snapshot trees subject to respective normal and refreshed differential scan instances for a storage volume according to an embodiment; 
         FIG. 4  depicts a content addressable storage system having a distributed storage controller configured with functionality for implementing snapshot rollback in a synchronous replication session according to an embodiment; 
         FIG. 5  depicts a cloud infrastructure-based processing platform with physical and virtual processing resources for implementing snapshot rollback in a synchronous replication session in accordance with an embodiment; and 
         FIG. 6  depicts an alternative processing platform for implementing snapshot rollback in a synchronous replication session in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Performing rollback during a synchronous replication session provides certain challenges. As synchronous replication data transfer is triggered by input/output (IO) flow, and snapshot rollback operations are not part of IO handling, in order to rollback a synchronous replication source volume to an earlier snapshot, the synchronous replication session needs to be halted in order to perform the rollback followed by restarting the synchronous replication session. If there is a lot of data residing in the synchronous replication source volume, the restart of the synchronous replication session may trigger retransmission of a large quantity of data which can lead to a lengthy gap in the associated data protection window. Illustrative embodiments herein provide a storage system with functionality for improved snapshot rollback for snapshots generated for a consistency group of multiple storage volumes that is subject to a synchronous replication process involving source and target storage systems. Embodiments provide for rollback during an ongoing synchronous replication session without requiring termination and restart of the session. 
       FIG. 1  shows an information processing system  100  configured in accordance with an illustrative embodiment. The information processing system  100  comprises a plurality of host devices  101 , a source storage system  102 S and a target storage system  102 T (collectively referred to herein as “storage systems  102 ”), all of which are configured to communicate with one another over a network  104 . The source and target storage systems  102  are more particularly configured in this embodiment to participate in a synchronous replication process in which one or more storage volumes  110 S are synchronously replicated from the source storage system  102 S to the target storage system  102 T, possibly with involvement of at least one of the host devices  101 . The one or more storage volumes  110 S that are synchronously replicated from the source storage system  102 S to the target storage system  102 T may be part of a designated consistency group. 
     Each of the storage systems  102  is illustratively associated with a corresponding set of one or more of the host devices  101 . The host devices  101  illustratively comprise servers or other types of computers of an enterprise computer system, cloud-based computer system or other arrangement of multiple compute nodes associated with respective users. 
     The host devices  101  in some embodiments illustratively provide compute services such as execution of one or more applications on behalf of each of one or more users associated with respective ones of the host devices. Such applications illustratively generate input/output (IO) operations that are processed by a corresponding one of the storage systems  102 . The term IO as used herein refers to at least one of input and output. For example, IO operations may comprise write requests and/or read requests directed to stored data of a given one of the storage systems  102 . The storage systems  102  illustratively include respective processing devices of one or more processing platforms. For example, the storage systems  102  can each be configured as one or more processing devices each having a processor and a memory, possibly implementing virtual machines and/or containers, although numerous other configurations are possible. The storage systems  102  may be implemented on a common processing platform, or on separate processing platforms. 
     The host devices  101  are illustratively configured to write data to and read data from the storage systems  102  in accordance with applications executing on those host devices for system users. 
     As indicated above, the term “user” herein is intended to be broadly construed to encompass numerous arrangements of human, hardware, software or firmware entities, as well as combinations of such entities. Compute and/or storage services may be provided for users under a Platform-as-a-Service (PaaS) model, an Infrastructure-as-a-Service (IaaS) model and/or a Function-as-a-Service (FaaS) model, although it is to be appreciated that numerous other cloud infrastructure arrangements could be used. Also, illustrative embodiments can be implemented outside of the cloud infrastructure context, as in the case of a stand-alone computing and storage system implemented within a given enterprise. 
     In embodiments, the network  104  forms part of a global computer network such as the Internet, although other types of networks can be part of the network  104 , including a wide area network (WAN), a local area network (LAN), a satellite network, a telephone or cable network, a cellular network, a wireless network such as a WiFi or WiMAX network, or various portions or combinations of these and other types of networks. The network  104  in some embodiments therefore includes combinations of multiple different types of networks each including processing devices configured to communicate using Internet Protocol (IP) or other communication protocols. 
     As a more particular example, some embodiments may utilize one or more high-speed local networks in which associated processing devices communicate with one another utilizing Peripheral Component Interconnect express (PCIe) cards of those devices, and networking protocols such as InfiniBand, Gigabit Ethernet or Fibre Channel. Numerous alternative networking arrangements are possible in the embodiments, as will be appreciated by those skilled in the art. 
     The source storage system  102 S includes a plurality of storage devices  106 S and an associated storage controller  108 S. The storage devices  106 S store the storage volumes  110 S. The storage volumes  110 S illustratively include respective logical units (LUNs) or other types of logical storage volumes. 
     Similarly, the target storage system  102 T includes a plurality of storage devices  106 T and an associated storage controller  108 T. The storage devices  106 T store storage volumes  110 T, at least a portion of which represent respective LUNs or other types of logical storage volumes that are replicated from the source storage system  102 S to the target storage system  102 T in accordance with a synchronous replication process. 
     The storage devices  106  of the storage systems  102  may be implemented as solid state drives (SSDs). Such SSDs are implemented using non-volatile memory (NVM) devices such as flash memory. Other types of NVM devices that can be used to implement at least a portion of the storage devices  106  include non-volatile random access memory (NVRAM), phase-change RAM PC-RAM) and magnetic RAM (MRAM). These and various combinations of multiple different types of NVM devices may also be used. For example, hard disk drives (HDDs) can be used in combination with or in place of SSDs or other types of NVM devices. 
     However, it is to be appreciated that other types of storage devices can be used in other embodiments. For example, a given storage system as the term is broadly used herein can include a combination of different types of storage devices, as in the case of a multi-tier storage system comprising a flash-based fast tier and a disk-based capacity tier. In such an embodiment, each of the fast tier and the capacity tier of the multi-tier storage system comprises a plurality of storage devices with different types of storage devices being used in different ones of the storage tiers. 
     For example, the fast tier may comprise flash drives or other types of SSDs while the capacity tier comprises HDDs. The particular storage devices used in a given storage tier may be varied in other embodiments, and multiple distinct storage device types may be used within a single storage tier. The term “storage device” as used herein is intended to be broadly construed, to encompass, for example, SSDs, HDDs, flash drives, hybrid drives or other types of storage devices. 
     In some embodiments, at least one of the storage systems  102  illustratively comprises a scale-out all-flash content addressable storage array such as an XtremIO storage array available from Dell EMC of Hopkinton, Mass. 
     The term “storage system” as used herein is therefore intended to be broadly construed and should not be viewed as being limited to content addressable storage systems or flash-based storage systems. A given storage system as the term is broadly used herein can comprise, for example, network-attached storage (NAS), storage area networks (SANs), direct-attached storage (DAS) and distributed DAS, as well as combinations of these and other storage types, including software-defined storage. 
     In some embodiments, communications between the host devices  101  and the storage systems  102  comprise Small Computer System Interface (SCSI) commands. Other types of SCSI or non-SCSI commands may be used in other embodiments, including commands that are part of a standard command set, or custom commands such as a “vendor unique command” or VU command that is not part of a standard command set. The term “command” as used herein is therefore intended to be broadly construed, to encompass, for example, a composite command that comprises a combination of multiple individual commands. Numerous other commands can be used in other embodiments. 
     The storage controller  108 S of source storage system  102 S in the  FIG. 1  embodiment includes a synchronous replication control engine  112 S and a snapshot generator  114 S. Similarly, the storage controller  108 T of target storage system  102 T includes a synchronous replication control engine  112 T and a snapshot generator  114 T. In some embodiments, one or more of the host devices  101  may implement a corresponding synchronous replication control engine  112 H. 
     Although not explicitly shown in the Figure, additional components can be included in the storage controllers  108 , such as signature generators utilized in generating content-based signatures of data pages. 
     The instances of synchronous replication control engines  112 H,  112 S, and  112 T are collectively referred to herein as synchronous replication control engines  112 . Such replication control engines are configured to implement synchronous replication control logic (not shown). The synchronous replication control logic controls performance of the synchronous replication process carried out between the storage systems, which as noted above in some embodiments further involves at least one of the host devices  101 . The data replicated from the source storage system  102 S to the target storage system  102 T can include all of the data stored in the source storage system  102 S, or only certain designated subsets of the data stored in the source storage system  102 S, such as particular designated sets of LUNs or other logical storage volumes. Different replication processes of different types can be implemented for different parts of the stored data. Also, the storage systems  102  can be configured to operate in different replication modes of different types at different times. A given storage volume designated for replication from the source storage system  102 S to the target storage system  102 T illustratively comprises a set of one or more LUNs or other instances of the storage volumes  110 S of the source storage system  102 S. Each such LUN or other storage volume illustratively comprises at least a portion of a physical storage space of one or more of the storage devices  106 S. The corresponding replicated LUN or other storage volume of the storage volumes  110 T of the target storage system  102 T illustratively comprises at least a portion of a physical storage space of one or more of the storage devices  106 T. 
     The synchronous replication control logic in some embodiments is configured to control the performance of corresponding portions of a synchronous replication process of the type illustrated in the flow diagram of  FIG. 2 . At least one of the host devices  101  in some embodiments can also include one or more instances of synchronous replication control logic and possibly also one or more snapshot generators, as well as additional or alternative components such as a signature generator. 
     The storage controllers  108  and  108 T of the storage systems  102  should also be understood to include additional modules and other components typically found in implementations of storage controllers and storage systems, although such additional modules and other components are omitted from the Figure for clarity and simplicity of illustration. 
     It will be assumed for the following description of the  FIG. 1  embodiment that there is an ongoing synchronous replication process being carried out between the source storage system  102 S and the target storage system  102 T (and in some instances one or more hosts  101 ) in the system  100 , utilizing their respective instances of synchronous replication control logic implemented by respective synchronous replication control engines  112 S and  112 T, and alternatively  112 H. 
     The synchronous replication process more particularly comprises a process in which a consistency group comprising one or more storage volumes is replicated from the source storage system  102 S to the target storage system  102 T during a synchronous replication session in which 10 data is mirrored between the source and target systems. The synchronous replication is illustratively implemented at least in part by or otherwise under the control of the source and target synchronous replication control engines  112 S and  112 T. Other types of replication arrangements can be used in other embodiments. 
     The storage systems  102  in the  FIG. 1  embodiment are assumed to be implemented using at least one processing platform each comprising one or more processing devices each having a processor coupled to a memory. Such processing devices can illustratively include particular arrangements of compute, storage and network resources. 
     The storage systems  102  may be implemented on respective distinct processing platforms, although numerous other arrangements are possible. At least portions of their associated host devices may be implemented on the same processing platforms as the storage systems  102  or on separate processing platforms. 
     The term “processing platform” as used herein is intended to be broadly construed so as to encompass, by way of illustration and without limitation, multiple sets of processing devices and associated storage systems that are configured to communicate over one or more networks. For example, distributed implementations of the system  100  are possible, in which certain components of the system reside in one data center in a first geographic location while other components of the system reside in one or more other data centers in one or more other geographic locations that are potentially remote from the first geographic location. Thus, it is possible in some implementations of the system  100  for the storage systems  102  to reside in different data centers. Numerous other distributed implementations of the storage systems  102  and their respective associated sets of host devices are possible. 
     Additional examples of processing platforms utilized to implement storage systems and possibly their associated host devices in illustrative embodiments will be described in more detail below in conjunction with  FIGS. 5 and 6 . 
     It is to be appreciated that these and other features of illustrative embodiments are presented by way of example only and should not be construed as limiting in any way. 
     Accordingly, different numbers, types and arrangements of system components such as host devices  101 , storage systems  102 , network  104 , storage devices  106 , storage controllers  108  and storage volumes  110  can be used in other embodiments. 
     It should be understood that the particular sets of modules and other components implemented in the system  100  as illustrated in  FIG. 1  are presented by way of example only. In other embodiments, only subsets of these components, or additional or alternative sets of components, may be used, and such components may exhibit alternative functionality and configurations. 
     For example, in other embodiments, at least portions of the above-described synchronous replication can be implemented in one or more host devices, or partially in a host device and partially in a storage system. Illustrative embodiments are not limited to arrangements in which all such functionality is implemented in source and target storage systems or a host device, and therefore encompass various hybrid arrangements in which the functionality is distributed over one or more storage systems and one or more associated host devices, each comprising one or more processing devices. References herein to “one or more processing devices” configured to implement particular operations or other functionality should be understood to encompass a wide variety of different arrangements involving one or more processing devices of at least one storage system and/or at least one host device. 
     As another example, it is possible in some embodiments that the source storage system and the target storage system can comprise different portions of the same storage system. In such an arrangement, a replication process is illustratively implemented to replicate data from one portion of the storage system to another portion of the storage system. The terms “source storage system” and “target storage system” as used herein are therefore intended to be broadly construed to encompass such possibilities. 
     The operation of the information processing system  100  will now be described in further detail with reference to the flow diagram of the illustrative embodiment of  FIG. 2 , which implements a synchronous replication process. The steps of the process illustratively involve interactions between a source storage system and a target storage system, referred to as respective “source” and “target” in these Figures, illustratively utilizing synchronous replication control logic instances and snapshot generators of storage controllers of the source and target. For example, synchronous replication control logic of the source interacts with synchronous replication control logic of the target in performing synchronous replication for a consistency group. It is possible in other embodiments that at least one of the storage systems does not include synchronous replication control logic and a snapshot generator, and in such embodiments these components are instead implemented in one or more host devices. 
     In a synchronous replication session, the source storage system  102 S generates a current snapshot set for a consistency group comprising a plurality of storage volumes subject to replication from the source storage system  102 S to the target storage system  102 T, and may schedule a differential scan of the current snapshot set relative to a previous snapshot set generated for the consistency group. The term “scheduling” as used herein is intended to be broadly construed, so as to encompass, for example, initiating, triggering or otherwise controlling performance a differential scan in conjunction with a given replication cycle. 
     It is assumed for the present embodiment that the given replication cycle is a non-initial replication cycle of an ongoing synchronous replication process, such that there is a previous snapshot set already available from a previous cycle. In an initial replication cycle, the entire contents of the current snapshot set are illustratively transferred to from the source to the target, and differential scanning is not utilized. The current snapshot set for the initial replication cycle becomes the previous snapshot set for the next replication cycle. 
     It is assumed for the present embodiment that the given replication cycle is a non-initial replication cycle of an ongoing synchronous replication process, such that there is a previous snapshot set already available from a previous cycle. In an initial replication cycle, the entire contents of the current snapshot set are illustratively transferred to from the source to the target, and differential scanning is not utilized. The current snapshot set for the initial replication cycle becomes the previous snapshot set for the next replication cycle. 
     The current snapshot set and other snapshot sets referred to in the context of some embodiments herein are illustratively generated for a consistency group that comprises multiple storage volumes. A snapshot tree of the consistency group in such embodiments illustratively comprises multiple individual snapshot trees for respective ones of the storage volumes, each generally having the same topology of nodes. Accordingly, generation of a snapshot set for a consistency group illustratively comprises generating a plurality of snapshots for respective ones of the multiple storage volumes. Such snapshot sets and associated versions of the consistency group vary over time and are represented by nodes of the snapshot tree of the consistency group. Again, the snapshot tree for the consistency group may be viewed as illustratively comprising multiple superimposed snapshot trees for the respective storage volumes of the consistency group with each such storage volume snapshot tree having substantially the same topology as the consistency group snapshot tree. 
     A given one of the snapshot trees corresponding to a particular one of the storage volumes more particularly comprises a root node, at least one branch node, and a plurality of leaf nodes, with a given one of the branch nodes representing a particular version of the storage volume from which a corresponding snapshot is taken. A first one of the leaf nodes which is a child of the given branch node represents a subsequent version of the storage volume, and a second one of the leaf nodes which is a child of the given branch node comprises the corresponding snapshot providing a point-in-time (PIT) copy of the particular version of the storage volume. 
     Illustrative examples of consistency group snapshot trees of the type described above are shown in  FIGS. 3A and 3B  and will be described in greater detail herein in conjunction with the flow diagram of  FIG. 2 . 
     In some embodiments, the snapshot trees comprise or are otherwise associated with additional information also arranged in the form of a tree structure. For example, a given one of the snapshot trees may be associated with one or more additional trees including at least one of a “dirty” tree that characterizes updates to logical addresses of the corresponding storage volume, and a hash tree comprising content-based signatures of respective ones of the logical addresses of the corresponding storage volume. All nodes of a given snapshot tree in some embodiments, including both branch nodes and leaf nodes, may each be associated with corresponding metadata of both a dirty tree and a hash tree. 
     An instance of the differential scan performed for the given snapshot tree in embodiments of this type can further comprise aggregating information of at least one of the dirty tree and the hash tree between start and stop nodes of the given snapshot tree. The start and stop nodes are examples of what are more generally referred to herein as first and second nodes corresponding to respective current and previous snapshot sets. Start and stop nodes can comprise branch nodes or leaf nodes associated with those branch nodes. Also, a given one of the first and second nodes can comprise a leaf node rather than a branch node. Terms such as “start node,” “stop node,” “first node” and “second node” are therefore intended to be broadly construed and should not be viewed as being restricted to either branch nodes or leaf nodes. 
     A wide variety of other types of snapshot trees and possibly one or more associated additional trees can be used in other embodiments. Also, the term “tree” as used herein is intended to be broadly construed to comprise any type of data structure characterizing a plurality of nodes and a plurality of edges interconnecting respective pairs of the nodes. 
     The content-based signatures of the above-noted hash tree associated with a given storage volume in some embodiments comprise hash digests of their respective pages, each generated by application of a hash function such as Secure Hashing Algorithm 1 (SHA1) to the content of its corresponding page. Other types of secure hashing algorithms, such as SHA2 or SHA256, or more generally other hash functions, can be used in generating content-based signatures herein. 
     A given hash digest in illustrative embodiments is unique to the particular content of the page from which it is generated, such that two pages with exactly the same content will have the same hash digest, while two pages with different content will have different hash digests. It is also possible that other types of content-based signatures may be used, such as hash handles of the type described elsewhere herein. A hash handle generally provides a shortened representation of its corresponding hash digest. More particularly, the hash handles are shorter in length than respective hash digests that are generated by applying a secure hashing algorithm to respective ones of the data pages. Hash handles are considered examples of “content-based signatures” as that term is broadly used herein. 
     In embodiments in which the storage systems  102  comprise content addressable storage systems, address metadata is illustratively utilized to provide content addressable storage functionality within those systems. The address metadata in some embodiments comprises at least a portion of one or more logical layer mapping tables that map logical addresses of respective ones of the data pages of the storage volume to corresponding content-based signatures of the respective data pages. Examples of logical layer mapping tables and other metadata structures maintained by at least the storage controller  108 T of target storage system  102 T will be described elsewhere herein. The manner in which the source storage system  102 S processes the snapshot trees of respective storage volumes in conjunction with synchronous replication of the consistency group will now be described in further detail. 
     For each of one or more snapshot trees maintained for the consistency group, the source storage system  102 S determines if a first node corresponding to the previous snapshot set is an ancestor of a second node corresponding to the current snapshot set. The source storage system  102 S then alters a manner in which an instance of the differential scan is performed for the snapshot tree responsive to a result of the determination. The first and second nodes may comprise, for example, respective branch nodes of the snapshot tree. Alternatively, at least one of the first and second nodes may comprise a leaf node. The differential scan process is further described in commonly assigned U.S. patent application Ser. No. 16/357,957 entitled “Storage System with Differential Scanning of Non-Ancestor Snapshot Pairs in Asynchronous Replication,” which was filed on Mar. 19, 2019, the contents of which are incorporated herein by reference it their entirety. 
     In some embodiments, for example, the current snapshot set is derived from a restored version of at least a portion of the consistency group, such that the first node corresponding to the previous snapshot set is not an ancestor of the second node corresponding to the current snapshot set. Such situations can arise, for example, when one or more storage volumes have been restored to previous versions during an ongoing replication process. Illustrative embodiments advantageously allow efficient differential scanning to be performed in these and other situations in which the first node is not an ancestor of the second node. 
     Determining if the first node corresponding to the previous snapshot set is an ancestor of the second node corresponding to the current snapshot set in some embodiments more particularly comprises inspecting a path from the second node towards a root node of the snapshot tree. Responsive to the first node being part of the path, a determination is made that the first node is an ancestor of the second node. Responsive to the first node not being part of the path, a determination is made that the first node is not an ancestor of the second node. The first node is illustratively considered “part of the path” if it is encountered in traversing the path and is otherwise not considered “part of the path.” 
     In some embodiments, altering a manner in which an instance of the differential scan is performed for the snapshot tree responsive to a result of the determination includes performing a first type of differential scan comprising aggregating node metadata of a path from the second node to the first node responsive to the first node being an ancestor of the second node, and performing a second type of differential scan different than the first type of differential scan responsive to the first node not being an ancestor of the second node. For example, the second type of differential scan illustratively comprises identifying a first path from the first node to a root node, identifying a second path from the second node to the root node, determining a lowest common ancestor between the first and second paths, combining the first and second paths at the lowest common ancestor, and aggregating node metadata of the combined first and second paths. In the above-described instance of the differential scan, the aggregated metadata is utilized to determine all changed data pages of each storage volume of the consistency group between the current and previous snapshot sets. The resulting differential data is provided from the source storage system  102 S to the target storage system  102 T. 
     The instance of the differential scan is therefore illustratively performed by the source storage system  102 S in the given replication cycle of the consistency group. In addition, results of performing the instance of the differential scan are transmitted from the source storage system  102 S to the target storage system in the given replication cycle. 
     As mentioned previously, the term “storage volume” as used herein is intended to be broadly construed and should not be viewed as being limited to any particular format or configuration. The term “consistency group” as used herein is also intended to be broadly construed and may comprise one or more storage volumes. 
     A more particular example of the differential scanning functionality described above will now be presented. In this example, the replication control logic instances  112 S and  112 T are assumed to cooperate to facilitate differential scanning of non-ancestor snapshot pairs in conjunction with ongoing synchronous replication from the source to the target. Like other embodiments herein, the process utilized in the present example advantageously provides efficient differential scanning even in those situations in which one or more storage volumes have been subject to restoration from a previous snapshot as part of an ongoing synchronous replication process. 
     In the present example, it is assumed that the source and target storage systems  102  are configured via their respective instances of replication control logic to perform efficient differential scanning in situations in which the first node is an ancestor of the second node, including the situation illustrated in  FIG. 3A . The efficient differential scanning generally involves aggregating node metadata between the first and second nodes, using the above-described dirty trees and hash trees which fully characterize any data changes between the corresponding snapshots. Such node metadata provides all the information needed to perform the differential scanning in a particularly efficient manner, thereby enhancing the performance of the overall synchronous replication process. 
     The illustrative embodiments allow such efficient differential scanning to also be performed even in those situations in which the first node is not an ancestor of the second node, including the situation illustrated in  FIG. 3B . This includes situations in which one or more storage volumes of the consistency group are “refreshed” or otherwise restored to previous versions during the ongoing synchronous replication process, also referred to herein as “live” restoration of the one or more storage volumes. 
     Turning now to  FIG. 2 , a flow diagram describing a process  200  for performing rollback of a snapshot between a source storage system and a target storage system during a synchronous replication session will now be described according to embodiments. 
     In block  202 , the process  200  suspends host input/output (IO) operations (e.g., from one of the hosts  101  to the storage system  102 S). In block  204 , the process  200  reassigns a current replication source volume (e.g., one of storage volumes  110 ) to a rollback source snapshot. The rollback source snapshot reflects a snapshot taken at an earlier point in time and to which the process seeks to roll back the source volume. The rollback source snapshot may be generated for a consistency group that includes a plurality of storage volumes subject to the synchronous replication session. Upon reassigning the current replication source volume to the rollback source snapshot, a child of the rollback source snapshot inherits a volume identifier of the current replication source volume, and the current replication source volume receives a new volume identifier. For example, when a volume is newly created, it is assigned an external volume ID (e.g., sid) 1 and internal volume id (iid) 2. At some point in time, a snapshot is created with external ID (sid)-1 and internal volume ID (iid) 5. Sid-1 is invalid, meaning it is not exposed to the host. Upon determining a snap reassignment should be performed, the snapshot is given the external ID 1, and the internal ID remains as 5. The original volume is assigned a new extern ID iid10. The system may decide to keep the volume with the external ID 10 or delete it. From the host point of view, it will see the volume with the external ID 1 having the content of iid 5. 
     In block  206 , a differential scan is performed for one or more snapshot trees maintained for the consistency group. The differential scan is performed between a child of the rollback source snapshot and the current replication source volume. The differential scan may be implemented via the techniques described above and referenced in U.S. patent application Ser. No. 16/357,957. In block  208 , the process  200  calculates a dirty tree differential from results of the differential scan. 
     The dirty tree of a volume may be implemented as a bitmap tree to describe which blocks have been written/allocated for the volume. Each volume owns its own dirty tree. In the snapshot case, the original dirty tree of the volume is transferred to the snap, and the new dirty tree of the volume after snap describes blocks that have been changed since snapshot taken. When the dirty tree differential is calculated between a volume and a snap, we accumulate the dirty tree bitmap changes between the volume and snap, the resulting bitmap describes all the blocks that are different between a volume and its snap. 
     A given one of the above-identified snapshot trees is associated with one or more additional trees including at least one of the dirty tree that characterizes updates to logical addresses of a corresponding one of the storage volumes, and a hash tree that includes content-based signatures of respective ones of the logical addresses of the corresponding storage volumes. As indicated above, an instance of the differential scan performed for the snapshot trees includes aggregating information of at least one of the dirty tree and the hash tree between designated nodes of the one or more snapshot trees. 
     As described, e.g., in the above-referenced U.S. patent application Ser. No. 16/357,957, performing the differential scan includes determining if a first node to a previous snapshot is an ancestor of a second node corresponding to the current replication source volume. This determination is implemented by inspecting a path from the second node towards a root node of the snapshot tree, responsive to the first node being part of the path, determining that the first node is an ancestor of the second node, and responsive to the first node not being part of the path, determining that the first node is not an ancestor of the second node. 
     Performing the differential scan further includes altering a manner in which an instance of the differential scan is performed for the snapshot trees responsive to a result of the determination. This altering includes performing a first type of differential scan comprising aggregating node metadata of a path from the second node to the first node responsive to the first node being an ancestor of the second node, and performing a second type of differential scan different than the first type of differential scan responsive to the first node not being an ancestor of the second node. The second type of differential scan includes identifying a first path from the first node to a root node, identifying a second path from the second node to the root node, determining a lowest common ancestor between the first and second paths, combining the first and second paths at the lowest common ancestor; and aggregating node metadata of the combined first and second paths. 
     Blocks  210 - 212  are performed for each difference identified in the dirty tree differential. In block  210 , a volume offset is calculated. The volume offset is used as described further herein. In block  212 , the process  200  initiates a copy command for current replication source volume and the rollback source snapshot. The copy command is translated to a remote metadata copy request in a synchronous replication data transfer. Initiation and execution of the copy command is described further in commonly assigned U.S. patent application Ser. No. 16/177,782 entitled “Method to Support Hash Based XCopy Synchronous Replication,” which was filed on Nov. 1, 2018, the contents of which are incorporated herein in their entirety. 
     In an embodiment, the copy command is a Small Computer System Interface (SCSI)-based extended copy command that includes the volume offset, e.g., from block  210 , a source logical unit, a source logical block address range, a target logical unit, and a target logical block address range. In embodiments, the copy command is received from an initiator (e.g., from the synchronous replication control engine  112 S of  FIG. 1  at the source or from the synchronous replication control engine  112 H implemented by the host device  101 ) and is executed by the target storage system (e.g., system  102 T). Completion of the copy command for each difference in the dirty tree differential results in the target storage system containing identical content as the source storage system after the rollback (e.g., the target uses the differential data to keep its version updated and consistent with the source). 
     Once the rollback operations are completed, the host is notified and host IO access to synchronous replication is resumed on the source volume in block  214 . 
       FIG. 3A  shows an example of a snapshot tree  300 - 1  for the consistency group that includes the selected storage volume. Such a snapshot tree illustratively represents a combination of multiple superimposed snapshot trees for respective ones of the storage volumes of the consistency group, with each of the storage volume snapshot trees having substantially the same format as the snapshot tree  300 - 1 . Thus, although the snapshot tree format illustrated in the Figure is for a consistency group, it is also representative of multiple individual snapshot trees for respective storage volumes of the consistency group. A given such snapshot tree for the selected storage volume is subject to the first type of differential scan. 
     The snapshot tree  300 - 1  comprises a root node and a plurality of branch nodes denoted CGn-2, CGn-1, CGn and CG. The root node represents a version of the consistency group from which an initial point-in-time (PIT) copy is captured as snapshot set S0. The branch nodes CGn-2, CGn-1 and CGn represent subsequent versions of the consistency group from which respective PIT copies are captured as subsequent snapshot sets Sn-2, Sn-1 and Sn, as the storage volumes of the consistency group change over time responsive to execution of IO operations. The snapshot sets Sn-1 and Sn are associated with respective previous and current replication cycles denoted as cycle n−1 and cycle n. 
     A given storage volume snapshot tree having a format of the type shown in  FIG. 3A  represents a storage volume and its snapshots over time. Each leaf node represents a particular version of the storage volume or a snapshot of the storage volume, and each branch node represents a shared ancestor between a version of the storage volume, a snapshot of the storage volume, or a child branch node. When a given snapshot of the storage volume is created, two child leaf nodes are created, one representing new updates to the storage volume after creation of the snapshot, and the other representing the snapshot. The volume node from which the snapshot was created therefore becomes a branch node in the snapshot tree. When a given snap set of the consistency group is created for its member storage volumes, two new leaf nodes are created in each of the snapshot trees of the respective storage volumes. 
     The given storage volume snapshot tree having a format of the type shown in  FIG. 3A  illustratively corresponds to a particular storage volume that has not been subject to live restoration during the replication process. The instance of the differential scan performed in this example utilizes as its start node the non-root node corresponding to snapshot set Sn-1 of the previous replication cycle and utilizes as its stop node the non-root node corresponding to snapshot set Sn of the current cycle. These start and stop nodes are associated with respective branch nodes CGn-1 and CGn. References herein to first and second nodes in illustrative embodiments refer to respective branch nodes of a snapshot tree. A given such branch node generally has at least one corresponding leaf node. It is also possible that at least one of the first and second nodes can alternatively comprise a leaf node. 
     A second type of differential scan is performed by aggregating node metadata for paths combined at a lowest common ancestor in the manner described elsewhere herein. It is to be appreciated that terms such as “aggregating” and “aggregate” as used herein are intended to be broadly construed, and can include multiple different types of aggregation, such as aggregation of dirty tree metadata followed by aggregation of hash tree metadata, with each such aggregation type possibly proceeding in different directions through at least portions of a given node chain and in some cases involving different node chains potentially having different sets of nodes. 
       FIG. 3B  shows an example of a snapshot tree  300 - 2  that is subject to the second type of differential scan. The snapshot tree  300 - 2  has a structure similar to that of snapshot tree  300 - 1 , but the consistency group was “refreshed” or otherwise restored to a previous version corresponding to snapshot set Sn-m′ during the ongoing replication process. In this example, snapshot set Sn-1 represents a previous snapshot set generated for the consistency group in replication cycle n−1. It is apparent that, at some point after generation of the snapshot set Sn-1, the consistency group was restored using snapshot set Sn-m′ which was generated from the version of the consistency group represented by branch node CGn-m′. Snapshot set Sn-m is a snapshot set generated from the consistency group as restored using snapshot set Sn-m′. Branch node CGn′ represents a version of the consistency group subsequent to the restoration, from which snapshot set Sn is generated in cycle n of the replication process. Any changes to the storage volumes of the consistency group after generation of snapshot set Sn are reflected in the current version denoted by node CG. 
     The snapshot set Sn in this example is therefore a current snapshot set, and the snapshot set Sn-1 is a previous snapshot set, but the branch node CGn-1′ corresponding to the previous snapshot set Sn-1 is not an ancestor of the branch node CGn′ corresponding to the current snapshot set Sn. The branch nodes CGn-‘ and CGn’ are representative of what are more generally referred to herein as respective first and second nodes corresponding to respective previous and current snapshot sets. 
     As indicated above, the second type of differential scan in this example combines paths from the first node to the root node and from the second node to the root node, at a lowest common ancestor illustratively represented by the branch node CGn-m′ in this example, and aggregates node metadata for the combined path in the manner described elsewhere herein. 
     A combined path for aggregation of node metadata in the  FIG. 3B  example illustratively comprises the branch nodes CGn-1′, . . . CGn-m′, Sn-m′, and CGn′. The combined path in some embodiments can additionally include the leaf nodes Sn-1 and Sn corresponding to respective branch nodes CGn-1‘ and CGn’. In some implementations, this combined path is used for aggregation of dirty tree metadata, and a different combined path is used for aggregation of hash tree metadata. For example, the hash tree metadata is illustratively aggregated along a combined path that includes nodes CGn′, Sn-m′, CGn-m′ . . . Root. Numerous other arrangements are possible. 
     Terms such as “root node” and “non-root node,” “start node” and “stop node,” and “first node” and “second node” as used herein are all intended to be broadly construed. A non-root node is considered to be any snapshot tree node that is not a root node. Start node and stop node designations for a given snapshot tree in some embodiments can be reversed relative to the designation arrangements referred to above in conjunction with the examples of  FIGS. 3A and 3B . Accordingly, such terms should not be construed as requiring a particular directionality for scanning the snapshot tree. It should also be understood that a wide variety of other snapshot tree arrangements may be used. 
     The particular processing operations and other system functionality described in conjunction with the flow diagram of  FIG. 2  are presented by way of illustrative example only and should not be construed as limiting the scope of the disclosure in any way. Alternative embodiments can use other types of processing operations to provide efficient differential scanning of non-ancestor snapshots in conjunction with synchronous replication. For example, the ordering of the process steps may be varied in other embodiments, or certain steps may be performed at least in part concurrently with one another rather than serially. Also, one or more of the process steps may be repeated periodically, or multiple instances of the process can be performed in parallel with one another in order to implement a plurality of different synchronous replication processes for respective different consistency groups comprising different sets of storage volumes or for different storage systems or portions thereof within a given information processing system. 
     Functionality such as that described in conjunction with the flow diagram of  FIG. 2  can be implemented at least in part in the form of one or more software programs stored in memory and executed by a processor of a processing device such as a computer or server. As will be described below, a memory or other storage device having executable program code of one or more software programs embodied therein is an example of what is more generally referred to herein as a “processor-readable storage medium.” 
     For example, storage controllers such as storage controllers  108  of storage systems  102  that are configured to control performance of one or more steps of the  FIG. 2  process in their corresponding system  100  can be implemented as part of what is more generally referred to herein as a processing platform comprising one or more processing devices each comprising a processor coupled to a memory. A given such processing device may correspond to one or more virtual machines or other types of virtualization infrastructure. The storage controllers  108 , as well as other system components, may be implemented at least in part using processing devices of such processing platforms. For example, in a distributed implementation of a given one of the storage controllers  108 , respective distributed modules of such a storage controller can be implemented in respective containers running on respective ones of the processing devices of a processing platform. 
     In some implementations of the  FIG. 2  process, the source and target storage systems comprise content addressable storage systems configured to maintain various metadata structures that are utilized in the differential scanning. Examples of metadata structures maintained by the source and target storage systems in illustrative embodiments include the logical layer and physical layer mapping tables described below. It is to be appreciated that these particular tables are only examples, and other tables or metadata structures having different configurations of entries and fields can be used in other embodiments. 
     An address-to-hash (“A2H”) utilized in some embodiments comprises a plurality of entries accessible utilizing logical addresses as respective keys, with each such entry of the A2H table comprising a corresponding one of the logical addresses, a corresponding hash handle, and possibly one or more additional fields. 
     A hash-to-data (“H2D”) table utilized in some embodiments comprises a plurality of entries accessible utilizing hash handles as respective keys, with each such entry of the H2D table comprising a corresponding one of the hash handles, a physical offset of a corresponding one of the data pages, and possibly one or more additional fields. 
     A hash metadata (“HMD”) table utilized in some embodiments comprises a plurality of entries accessible utilizing hash handles as respective keys. Each such entry of the HMD table comprises a corresponding one of the hash handles, a corresponding reference count and a corresponding physical offset of one of the data pages. A given one of the reference counts denotes the number of logical pages in the storage system that have the same content as the corresponding data page and therefore point to that same data page via their common hash digest. The HMD table may also include one or more additional fields. 
     A physical layer based (“PLB”) table utilized in some embodiments illustratively comprises a plurality of entries accessible utilizing physical offsets as respective keys, with each such entry of the PLB table comprising a corresponding one of the physical offsets, a corresponding one of the hash digests, and possibly one or more additional fields. 
     As indicated above, the hash handles are generally shorter in length than the corresponding hash digests of the respective data pages, and each illustratively provides a short representation of the corresponding full hash digest. For example, in some embodiments, the full hash digests are 20 bytes in length, and their respective corresponding hash handles are illustratively only 4 or 6 bytes in length. 
     Also, it is to be appreciated that terms such as “table” and “entry” as used herein are intended to be broadly construed, and the particular example table and entry arrangements described above can be varied in other embodiments. For example, additional or alternative arrangements of entries can be used. 
     As indicated above, in some embodiments, the storage system may comprise an XtremIO storage array or other type of content addressable storage system suitably modified to incorporate functionality for differential scanning of non-ancestor snapshots in conjunction with an ongoing synchronous replication process as disclosed herein. 
     An illustrative embodiment of such a content addressable storage system will now be described with reference to  FIG. 4 . In this embodiment, a content addressable storage system  405  comprises a plurality of storage devices  406  and an associated storage controller  408 . The content addressable storage system  405  may be viewed as a particular implementation of a given one of the storage systems  102 , and accordingly is assumed to be coupled to the other one of the storage systems  102  and to one or more host devices of a computer system within information processing system  100 . 
     Although it is assumed that both the source storage system  102 S and the target storage system  102 T are content addressable storage systems in some embodiments, other types of storage systems can be used for one or both of the source storage system  102 S and the target storage system  102 T in other embodiments. For example, it is possible that at least one of the storage systems  102  in an illustrative embodiment need not be a content addressable storage system and need not include an ability to generate content-based signatures. In such an embodiment, at least portions of the differential scanning functionality of the one or more storage systems can be implemented in a host device. 
     The storage controller  408  in the present embodiment is configured to implement functionality for efficient differential scanning of non-ancestor snapshots of the type previously described in conjunction with  FIGS. 1 through 3 . For example, the content addressable storage system  405  illustratively participates as a source storage system in a synchronous replication process with a target storage system that may be implemented as another instance of the content addressable storage system  405 . 
     The storage controller  408  includes distributed modules  412  and  414 , which are configured to operate in a manner similar to that described above for respective corresponding replication control logic  112  and snapshot generators  114  of the storage controllers  108  of system  100 . Module  412  is more particularly referred to as distributed replication control logic, and illustratively comprises multiple replication control logic instances on respective ones of a plurality of distinct nodes. Module  414  is more particularly referred to as a distributed snapshot generator, and illustratively comprises multiple snapshot generation instances on respective ones of the distinct nodes. 
     The content addressable storage system  405  in the  FIG. 4  embodiment is implemented as at least a portion of a clustered storage system and includes a plurality of storage nodes  415  each comprising a corresponding subset of the storage devices  406 . Such storage nodes  415  are examples of the “distinct nodes” referred to above, and other clustered storage system arrangements comprising multiple storage nodes and possibly additional or alternative nodes can be used in other embodiments. A given clustered storage system may therefore include not only storage nodes  415  but also additional storage nodes, compute nodes or other types of nodes coupled to network  104 . Alternatively, such additional storage nodes may be part of another clustered storage system of the system  100 . Each of the storage nodes  415  of the storage system  405  is assumed to be implemented using at least one processing device comprising a processor coupled to a memory. 
     The storage controller  408  of the content addressable storage system  405  is implemented in a distributed manner to comprise a plurality of distributed storage controller components implemented on respective ones of the storage nodes  415 . The storage controller  408  is therefore an example of what is more generally referred to herein as a “distributed storage controller.” In subsequent description herein, the storage controller  408  is referred to as distributed storage controller  408 . 
     Each of the storage nodes  415  in this embodiment further comprises a set of processing modules configured to communicate over one or more networks with corresponding sets of processing modules on other ones of the storage nodes  415 . The sets of processing modules of the storage nodes  415  collectively comprise at least a portion of the distributed storage controller  408  of the content addressable storage system  405 . 
     The modules of the distributed storage controller  408  in the present embodiment more particularly comprise different sets of processing modules implemented on each of the storage nodes  415 . The set of processing modules of each of the storage nodes  415  comprises at least a control module  408 C, a data module  408 D and a routing module  408 R. The distributed storage controller  408  further comprises one or more management (“MGMT”) modules  408 M. For example, only a single one of the storage nodes  415  may include a management module  408 M. It is also possible that management modules  408 M may be implemented on each of at least a subset of the storage nodes  415 . A given set of processing modules implemented on a particular one of the storage nodes  415  therefore illustratively includes at least one control module  408 C, at least one data module  408 D and at least one routing module  408 R, and possibly a management module  408 M. 
     Communication links may be established between the various processing modules of the distributed storage controller  408  using well-known communication protocols such as IP, Transmission Control Protocol (TCP), and remote direct memory access (RDMA). For example, respective sets of IP links used in data transfer and corresponding messaging could be associated with respective different ones of the routing modules  408 R. 
     Although shown as separate modules of the distributed storage controller  408 , the modules  412  and  414  in the present embodiment are assumed to be distributed at least in part over at least a subset of the other modules  408 C,  408 D,  408 R and  408 M of the storage controller  408 . Accordingly, at least portions of the differential scanning functionality of the modules  412  and  414  may be implemented in one or more of the other modules of the storage controller  408 . In other embodiments, the modules  412  and  414  may be implemented as stand-alone modules of the storage controller  408 . 
     The storage devices  406  are configured to store metadata pages  420  and user data pages  422  and may also store additional information not explicitly shown such as checkpoints and write journals. The metadata pages  420  and the user data pages  422  are illustratively stored in respective designated metadata and user data areas of the storage devices  406 . Accordingly, metadata pages  420  and user data pages  422  may be viewed as corresponding to respective designated metadata and user data areas of the storage devices  406 . A given “page” as the term is broadly used herein should not be viewed as being limited to any particular range of fixed sizes. In some embodiments, a page size of 8 kilobytes (KB) is used, but this is by way of example only and can be varied in other embodiments. For example, page sizes of 4 KB, 16 KB or other values can be used. Accordingly, illustrative embodiments can utilize any of a wide variety of alternative paging arrangements for organizing the metadata pages  420  and the user data pages  422 . 
     The user data pages  422  are part of a plurality of LUNs configured to store files, blocks, objects or other arrangements of data, each also generally referred to herein as a “data item,” on behalf of users of the content addressable storage system  405 . Each such LUN may comprise particular ones of the above-noted pages of the user data area. The user data stored in the user data pages  422  can include any type of user data that may be utilized in the system  100 . The term “user data” herein is therefore also intended to be broadly construed. 
     A given storage volume for which content-based signatures are generated using modules  412  and  414  illustratively comprises a set of one or more LUNs, each including multiple ones of the user data pages  422  stored in storage devices  406 . The content addressable storage system  405  in the embodiment of  FIG. 4  is configured to generate hash metadata providing a mapping between content-based digests of respective ones of the user data pages  422  and corresponding physical locations of those pages in the user data area. Content-based digests generated using hash functions are also referred to herein as “hash digests.” Such hash digests or other types of content-based digests are examples of what are more generally referred to herein as “content-based signatures” of the respective user data pages  422 . The hash metadata generated by the content addressable storage system  405  is illustratively stored as metadata pages  420  in the metadata area. The generation and storage of the hash metadata is assumed to be performed under the control of the storage controller  408 . 
     Each of the metadata pages  420  characterizes a plurality of the user data pages  422 . For example, a given set of user data pages representing a portion of the user data pages  422  illustratively comprises a plurality of user data pages denoted User Data Page 1, User Data Page 2, . . . User Data Page n. Each of the user data pages in this example is characterized by a LUN identifier, an offset and a content-based signature. The content-based signature is generated as a hash function of content of the corresponding user data page. Illustrative hash functions that may be used to generate the content-based signature include the above-noted SHA1 secure hashing algorithm, or other secure hashing algorithms known to those skilled in the art, including SHA2, SHA256 and many others. The content-based signature is utilized to determine the location of the corresponding user data page within the user data area of the storage devices  406 . 
     Each of the metadata pages  420  in the present embodiment is assumed to have a signature that is not content-based. For example, the metadata page signatures may be generated using hash functions or other signature generation algorithms that do not utilize content of the metadata pages as input to the signature generation algorithm. Also, each of the metadata pages is assumed to characterize a different set of the user data pages. 
     A given set of metadata pages representing a portion of the metadata pages  420  in an illustrative embodiment comprises metadata pages denoted Metadata Page 1, Metadata Page 2, . . . Metadata Page m, having respective signatures denoted Signature 1, Signature 2, . . . Signature m. Each such metadata page characterizes a different set of n user data pages. For example, the characterizing information in each metadata page can include the LUN identifiers, offsets and content-based signatures for each of the n user data pages that are characterized by that metadata page. It is to be appreciated, however, that the user data and metadata page configurations described above are examples only, and numerous alternative user data and metadata page configurations can be used in other embodiments. 
     Ownership of a user data logical address space within the content addressable storage system  405  is illustratively distributed among the control modules  408 C. 
     The differential scanning functionality provided by modules  412  and  414  in this embodiment is assumed to be distributed across multiple distributed processing modules, including at least a subset of the processing modules  408 C,  408 D,  408 R and  408 M of the distributed storage controller  408 . 
     For example, the management module  408 M of the storage controller  408  may include a replication control logic instance that engages corresponding replication control logic instances in all of the control modules  408 C and routing modules  408 R in order to implement a synchronous replication process. As indicated above, in some embodiments, the content addressable storage system  405  comprises an XtremIO storage array suitably modified to incorporate differential scanning functionality as disclosed herein. 
     In arrangements of this type, the control modules  408 C, data modules  408 D and routing modules  408 R of the distributed storage controller  408  illustratively comprise respective C-modules, D-modules and R-modules of the XtremIO storage array. The one or more management modules  408 M of the distributed storage controller  408  in such arrangements illustratively comprise a system-wide management module (“SYM module”) of the XtremIO storage array, although other types and arrangements of system-wide management modules can be used in other embodiments. Accordingly, differential scanning functionality in some embodiments is implemented under the control of at least one system-wide management module of the distributed storage controller  408 , utilizing the C-modules, D-modules and R-modules of the XtremIO storage array. 
     In the above-described XtremIO storage array example, each user data page has a fixed size such as 8 KB and its content-based signature is a 20-byte signature generated using the SHA1 secure hashing algorithm. Also, each page has a LUN identifier and an offset, and so is characterized by &lt;lun_id, offset, signature&gt;. 
     The content-based signature in the present example comprises a content-based digest of the corresponding data page. Such a content-based digest is more particularly referred to as a “hash digest” of the corresponding data page, as the content-based signature is illustratively generated by applying a hash function such as the SHA1 secure hashing algorithm to the content of that data page. The full hash digest of a given data page is given by the above-noted 20-byte signature. The hash digest may be represented by a corresponding “hash handle,” which in some cases may comprise a particular portion of the hash digest. The hash handle illustratively maps on a one-to-one basis to the corresponding full hash digest within a designated cluster boundary or other specified storage resource boundary of a given storage system. In arrangements of this type, the hash handle provides a lightweight mechanism for uniquely identifying the corresponding full hash digest and its associated data page within the specified storage resource boundary. The hash digest and hash handle are both considered examples of “content-based signatures” as that term is broadly used herein. 
     Examples of techniques for generating and processing hash handles for respective hash digests of respective data pages are disclosed in U.S. Pat. No. 9,208,162, entitled “Generating a Short Hash Handle,” and U.S. Pat. No. 9,286,003, entitled “Method and Apparatus for Creating a Short Hash Handle Highly Correlated with a Globally-Unique Hash Signature,” both of which are incorporated by reference herein. 
     As mentioned previously, storage controller components in an XtremIO storage array illustratively include C-module, D-module and R-module components. For example, separate instances of such components can be associated with each of a plurality of storage nodes in a clustered storage system implementation. 
     The distributed storage controller in this example is configured to group consecutive pages into page groups, to arrange the page groups into slices, and to assign the slices to different ones of the C-modules. For example, if there are 1024 slices distributed evenly across the C-modules, and there are a total of 16 C-modules in a given implementation, each of the C-modules “owns” 1024/16=64 slices. In such arrangements, different ones of the slices are assigned to different ones of the control modules  408 C such that control of the slices within the storage controller  408  of the storage system  405  is substantially evenly distributed over the control modules  408 C of the storage controller  408 . 
     The D-module allows a user to locate a given user data page based on its signature. Each metadata page also has a size of 8 KB and includes multiple instances of the &lt;lun_id, offset, signature&gt; for respective ones of a plurality of the user data pages. Such metadata pages are illustratively generated by the C-module but are accessed using the D-module based on a metadata page signature. 
     The metadata page signature in this embodiment is a 20-byte signature but is not based on the content of the metadata page. Instead, the metadata page signature is generated based on an 8-byte metadata page identifier that is a function of the LUN identifier and offset information of that metadata page. 
     If a user wants to read a user data page having a particular LUN identifier and offset, the corresponding metadata page identifier is first determined, then the metadata page signature is computed for the identified metadata page, and then the metadata page is read using the computed signature. In this embodiment, the metadata page signature is more particularly computed using a signature generation algorithm that generates the signature to include a hash of the 8-byte metadata page identifier, one or more ASCII codes for particular predetermined characters, as well as possible additional fields. The last bit of the metadata page signature may always be set to a particular logic value so as to distinguish it from the user data page signature in which the last bit may always be set to the opposite logic value. 
     The metadata page signature is used to retrieve the metadata page via the D-module. This metadata page will include the &lt;lun_id, offset, signature&gt; for the user data page if the user page exists. The signature of the user data page is then used to retrieve that user data page, also via the D-module. 
     Write requests processed in the content addressable storage system  405  each illustratively comprise one or more IO operations directing that at least one data item of the storage system  405  be written to in a particular manner. A given write request is illustratively received in the storage system  405  from a host device over a network. In some embodiments, a write request is received in the distributed storage controller  408  of the storage system  405  and directed from one processing module to another processing module of the distributed storage controller  408 . For example, a received write request may be directed from a routing module  408 R of the distributed storage controller  408  to a particular control module  408 C of the distributed storage controller  408 . Other arrangements for receiving and processing write requests from one or more host devices can be used. 
     As indicated above, the term “write request” as used herein is intended to be broadly construed, so as to encompass one or more IO operations directing that at least one data item of a storage system be written to in a particular manner. A given write request is illustratively received in a storage system from a host device. 
     In the XtremIO context, the C-modules, D-modules and R-modules of the storage nodes  415  communicate with one another over a high-speed internal network such as an InfiniBand network. The C-modules, D-modules and R-modules coordinate with one another to accomplish various IO processing tasks. 
     The write requests from the host devices identify particular data pages to be written in the storage system  405  by their corresponding logical addresses each comprising a LUN ID and an offset. 
     As noted above, a given one of the content-based signatures illustratively comprises a hash digest of the corresponding data page, with the hash digest being generated by applying a hash function to the content of that data page. The hash digest may be uniquely represented within a given storage resource boundary by a corresponding hash handle. 
     The content addressable storage system  405  utilizes a two-level mapping process to map logical block addresses to physical block addresses. The first level of mapping uses an address-to-hash (“A2H”) table and the second level of mapping uses a hash metadata (“HMD”) table, with the A2H and HMD tables corresponding to respective logical and physical layers of the content-based signature mapping within the content addressable storage system  405 . The HMD table or a given portion thereof in some embodiments disclosed herein is more particularly referred to as a hash-to-data (“H2D”) table. 
     The first level of mapping using the A2H table associates logical addresses of respective data pages with respective content-based signatures of those data pages. This is also referred to as logical layer mapping. The second level of mapping using the HMD table associates respective ones of the content-based signatures with respective physical storage locations in one or more of the storage devices  106 . This is also referred to as physical layer mapping. 
     Examples of these and other metadata structures utilized in illustrative embodiments were described above in conjunction with  FIG. 2 . These particular examples include respective A2H, H2D, HMD and PLB tables. In some embodiments, the A2H and H2D tables are utilized primarily by the control modules  408 C, while the HMD and PLB tables are utilized primarily by the data modules  408 D. 
     For a given write request, hash metadata comprising at least a subset of the above-noted tables is updated in conjunction with the processing of that write request. The A2H, H2D, HMD and PLB tables described above are examples of what are more generally referred to herein as “mapping tables” of respective distinct types. Other types and arrangements of mapping tables or other content-based signature mapping information may be used in other embodiments. Such mapping tables are still more generally referred to herein as “metadata structures” of the content addressable storage system  405 . It should be noted that additional or alternative metadata structures can be used in other embodiments. References herein to particular tables of particular types, such as A2H, H2D, HMD and PLB tables, and their respective configurations, should be considered non-limiting and are presented by way of illustrative example only. Such metadata structures can be implemented in numerous alternative configurations with different arrangements of fields and entries in other embodiments. 
     The logical block addresses or LBAs of a logical layer of the storage system  405  correspond to respective physical blocks of a physical layer of the storage system  405 . The user data pages of the logical layer are organized by LBA and have reference via respective content-based signatures to particular physical blocks of the physical layer. 
     Each of the physical blocks has an associated reference count that is maintained within the storage system  405 . The reference count for a given physical block indicates the number of logical blocks that point to that same physical block. 
     In releasing logical address space in the storage system, a dereferencing operation is generally executed for each of the LBAs being released. More particularly, the reference count of the corresponding physical block is decremented. A reference count of zero indicates that there are no longer any logical blocks that reference the corresponding physical block, and so that physical block can be released. 
     It should also be understood that the particular arrangement of storage controller processing modules  408 C,  408 D,  408 R and  408 M as shown in the  FIG. 4  embodiment is presented by way of example only. Numerous alternative arrangements of processing modules of a distributed storage controller may be used to implement differential scanning functionality in a clustered storage system in other embodiments. 
     Additional examples of content addressable storage functionality implemented in some embodiments by control modules  408 C, data modules  408 D, routing modules  408 R and management module(s)  408 M of distributed storage controller  408  can be found in U.S. Pat. No. 9,104,326, entitled “Scalable Block Data Storage Using Content Addressing,” which is incorporated by reference herein. Alternative arrangements of these and other storage node processing modules of a distributed storage controller in a content addressable storage system can be used in other embodiments. 
     Illustrative embodiments of a storage system with differential scanning functionality as disclosed herein can provide a number of significant advantages relative to conventional  10  arrangements. For example, some embodiments allow efficient differential scanning to be performed even in those situations in which a first node of a snapshot tree is not an ancestor of a second node of the snapshot tree. This includes situations in which one or more storage volumes of a consistency group are “refreshed” or otherwise restored to previous versions during an ongoing synchronous replication process, using what is also referred to herein as “live” restoration of the one or more storage volumes. 
     Accordingly, illustrative embodiments can provide efficient differential scanning between any two snapshots in a snapshot tree, regardless of whether or not the two snapshots are an ancestor snapshot pair or a non-ancestor snapshot pair. Such arrangements therefore avoid the need to terminate and subsequently restart the replication process in order to restore one or more storage volumes of a consistency group using previous versions. 
     In some embodiments, the source and target storage systems are illustratively implemented as respective content addressable storage systems, but in other embodiments one or more of the storage systems can instead be a traditional storage array, which does not support any type of content addressable storage functionality, with any missing functionality being provided by a host device. Accordingly, functionality for differential scanning of non-ancestor snapshot pairs in synchronous replication as disclosed herein can be implemented in a storage system, in a host device, or partially in a storage system and partially in a host device. 
     It is to be appreciated that the particular advantages described above and elsewhere herein are associated with particular illustrative embodiments and need not be present in other embodiments. Also, the particular types of information processing system features and functionality as illustrated in the drawings and described above are exemplary only, and numerous other arrangements may be used in other embodiments. 
     Illustrative embodiments of processing platforms utilized to implement host devices and storage systems with differential scanning functionality will now be described in greater detail with reference to  FIGS. 5 and 6 . Although described in the context of system  100 , these platforms may also be used to implement at least portions of other information processing systems in other embodiments. 
       FIG. 5  shows an example processing platform comprising cloud infrastructure  500 . The cloud infrastructure  500  comprises a combination of physical and virtual processing resources that may be utilized to implement at least a portion of the information processing system  100 . The cloud infrastructure  500  comprises multiple virtual machines (VMs) and/or container sets  502 - 1 ,  502 - 2 , . . .  502 -L implemented using virtualization infrastructure  504 . The virtualization infrastructure  504  runs on physical infrastructure  505 , and illustratively comprises one or more hypervisors and/or operating system level virtualization infrastructure. The operating system level virtualization infrastructure illustratively comprises kernel control groups of a Linux operating system or other type of operating system. 
     The cloud infrastructure  500  further comprises sets of applications  510 - 1 ,  510 - 2 , . . .  510 L running on respective ones of the VMs/container sets  502 - 1 ,  502 - 2 , . . .  502 -L under the control of the virtualization infrastructure  504 . The VMs/container sets  502  may comprise respective VMs, respective sets of one or more containers, or respective sets of one or more containers running in VMs. 
     In some implementations of the  FIG. 5  embodiment, the VMs/container sets  502  comprise respective VMs implemented using virtualization infrastructure  504  that comprises at least one hypervisor. Such implementations can provide differential scanning functionality of the type described above for one or more processes running on a given one of the VMs. For example, each of the VMs can implement replication control logic and/or snapshot generators for providing differential scanning functionality in the system  100 . 
     An example of a hypervisor platform that may be used to implement a hypervisor within the virtualization infrastructure  504  is the VMware® vSphere® which may have an associated virtual infrastructure management system such as the VMware® vCenter™. The underlying physical machines may comprise one or more distributed processing platforms that include one or more storage systems. In other implementations of the  FIG. 5  embodiment, the VMs/container sets  502  comprise respective containers implemented using virtualization infrastructure  504  that provides operating system level virtualization functionality, such as support for Docker containers running on bare metal hosts, or Docker containers running on VMs. The containers are illustratively implemented using respective kernel control groups of the operating system. Such implementations can also provide differential scanning functionality of the type described above. For example, a container host device supporting multiple containers of one or more container sets can implement one or more instances of replication control logic and/or snapshot generators for providing differential scanning functionality in the system  100 . 
     As is apparent from the above, one or more of the processing modules or other components of system  100  may each run on a computer, server, storage device or other processing platform element. A given such element may be viewed as an example of what is more generally referred to herein as a “processing device.” The cloud infrastructure  500  shown in  FIG. 5  may represent at least a portion of one processing platform. Another example of such a processing platform is processing platform  600  shown in  FIG. 6 . 
     The processing platform  600  in this embodiment comprises a portion of system  100  and includes a plurality of processing devices, denoted  602 - 1 ,  602 - 2 ,  602 - 3 , . . .  602 -K, which communicate with one another over a network  604 . 
     The network  604  may comprise any type of network, including by way of example a global computer network such as the Internet, a WAN, a LAN, a satellite network, a telephone or cable network, a cellular network, a wireless network such as a WiFi or WiMAX network, or various portions or combinations of these and other types of networks. 
     The processing device  602 - 1  in the processing platform  600  comprises a processor  610  coupled to a memory  612 . The processor  610  may comprise a microprocessor, a microcontroller, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), graphics processing unit (GPU) or other type of processing circuitry, as well as portions or combinations of such circuitry elements. 
     The memory  612  may comprise random access memory (RAM), read-only memory (ROM), flash memory or other types of memory, in any combination. The memory  612  and other memories disclosed herein should be viewed as illustrative examples of what are more generally referred to as “processor-readable storage media” storing executable program code of one or more software programs. 
     Articles of manufacture comprising such processor-readable storage media are considered illustrative embodiments. A given such article of manufacture may comprise, for example, a storage array, a storage disk or an integrated circuit containing RAM, ROM, flash memory or other electronic memory, or any of a wide variety of other types of computer program products. The term “article of manufacture” as used herein should be understood to exclude transitory, propagating signals. Numerous other types of computer program products comprising processor-readable storage media can be used. 
     Also included in the processing device  602 - 1  is network interface circuitry  614 , which is used to interface the processing device with the network  604  and other system components and may comprise conventional transceivers. 
     The other processing devices  602  of the processing platform  600  are assumed to be configured in a manner similar to that shown for processing device  602 - 1  in the Figure. 
     Again, the particular processing platform  600  shown in the figure is presented by way of example only, and system  100  may include additional or alternative processing platforms, as well as numerous distinct processing platforms in any combination, with each such platform comprising one or more computers, servers, storage devices or other processing devices. 
     It should therefore be understood that in other embodiments different arrangements of additional or alternative elements may be used. At least a subset of these elements may be collectively implemented on a common processing platform, or each such element may be implemented on a separate processing platform. 
     As indicated previously, components of an information processing system as disclosed herein can be implemented at least in part in the form of one or more software programs stored in memory and executed by a processor of a processing device. For example, at least portions of the differential scanning functionality of one or more components of a storage system as disclosed herein are illustratively implemented in the form of software running on one or more processing devices. 
     It should again be emphasized that the above-described embodiments are presented for purposes of illustration only. Many variations and other alternative embodiments may be used. For example, the disclosed techniques are applicable to a wide variety of other types of information processing systems, host devices, storage systems, storage nodes, storage devices, storage controllers, synchronous replication processes, snapshot generators and associated control logic and metadata structures. Also, the particular configurations of system and device elements and associated processing operations illustratively shown in the drawings can be varied in other embodiments. Moreover, the various assumptions made above in the course of describing the illustrative embodiments should also be viewed as exemplary rather than as requirements or limitations of the disclosure. Numerous other alternative embodiments within the scope of the appended claims will be readily apparent to those skilled in the art.