Storage system with continuous data verification for synchronous replication of logical storage volumes

An apparatus includes a processing device comprising a processor coupled to a memory, with the processing device being configured, in conjunction with synchronous replication of at least one logical storage volume between first and second storage systems, to acquire an address lock for a set of pages of the logical storage volume starting from a particular page of the logical storage volume, to determine content-based signatures for respective pages of the set of pages, to compute an additional signature as a function of the content-based signatures, and to send the additional signature and a pointer to the particular page from the first storage system to the second storage system, so as to permit the second storage system to verify consistency of the set of pages in the second storage system relative to the set of pages in the first storage system. Such operations are repeated for other sets of pages.

FIELD

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.

In synchronous replication, various malfunctions such as link loss, storage module restart or code issues can cause the source and target storage systems to lose their synchronization, and thereafter the target storage system becomes inconsistent, due to host writes being applied to data on the source but not on the target. Often, such problems are found only after the target data needs to be used, for example, after a disaster has destroyed the source storage system. By then it is of course too late to fix any problem.

Accordingly, a need exists for techniques that can discover and correct such inconsistencies between source and target data at an early stage, and without adversely impacting storage system performance.

SUMMARY

Illustrative embodiments provide techniques for continuous data verification during synchronous replication of one or more logical storage volumes from a source storage system to a target storage system. For example, in some embodiments, the “continuous” data verification is applied to a sequence of multiple sets of data pages of a given storage volume in a process that is performed repeatedly throughout at least a portion of an ongoing synchronous replication of the logical storage volume. The term “continuous” as used herein is therefore intended to be broadly construed.

These and other embodiments disclosed herein advantageously avoid data inconsistency problems that might otherwise arise due to loss of synchronization between source and target storage systems, by allowing inconsistent data to be detected and corrected in a particularly efficient manner, illustratively in real time, and without any adverse impact to storage system performance.

A given source or target storage system in some embodiments disclosed herein illustratively comprises a clustered implementation of a content addressable storage (CAS) system having a distributed storage controller. Similar advantages can be provided in other types of storage systems.

In one embodiment, an apparatus includes at least one processing device comprising a processor coupled to a memory, with the processing device being configured, in conjunction with synchronous replication of at least one logical storage volume between first and second storage systems, to acquire an address lock for a set of pages of the logical storage volume starting from a particular page of the logical storage volume, to determine content-based signatures for respective pages of the set of pages, to compute an additional signature as a function of the content-based signatures, and to send the additional signature and a pointer to the particular page from the first storage system to the second storage system, so as to permit the second storage system to verify consistency of the set of pages in the second storage system relative to the set of pages in the first storage system.

The acquiring, determining, computing and sending are illustratively repeated by the first storage system for each of one or more additional sets of pages of the logical storage volume starting from respective different particular pages of the logical storage volume. Such repetition continues in some embodiments until consistency of the logical storage volume in the second storage system relative to the logical storage volume in the first storage system has been successfully verified by the second storage system. After a designated waiting time, the page set based data verification for the logical storage volume can be restarted.

The first and second storage systems illustratively comprise respective source and target storage systems of the synchronous replication of said at least one logical storage volume, although the designation of the first storage system as the source and the second storage system as the target can be reversed in other embodiments. Accordingly, the first and second storage systems in some embodiments may comprise respective target and source storage systems of the synchronous replication of said at least one logical storage volume. Moreover, in some embodiments, a continuous data verification process of the type described above is simultaneously performed in two different directions, namely, both from source to target and from target to source, although any detected inconsistencies in such arrangements are illustratively corrected by sending the corresponding sets of pages from source to target.

The content-based signatures for respective ones of the pages are illustratively determined by accessing an address-to-hash table that stores logical addresses of the pages in association with respective hashes of the pages, with the respective hashes being generated by applying a secure hashing algorithm to content of the pages.

Responsive to receipt of the additional signature and the pointer to the particular page from the first storage system in the second storage system, the second storage system is illustratively configured to acquire an address lock for a set of pages of the logical storage volume starting from the particular page of the logical storage volume, to determine content-based signatures for respective pages of the set of pages, to compute an additional signature as a function of the content-based signatures, to compare the computed additional signature with the additional signature received from the first storage system, to generate a status indicator based at least in part on the comparing, and to send the status indicator to the first storage system to indicate whether or not consistency of the set of pages in the second storage system relative to the set of pages in the first storage system has been successfully verified by the second storage system.

The acquiring, determining, computing, comparing and sending are illustratively repeated by the second storage system for each of one or more additional sets of pages of the logical storage volume starting from respective different particular pages of the logical storage volume responsive to receipt of respective additional signatures and respective pointers to the particular pages from the first storage system.

The address locks acquired for the set of pages of the logical storage volume in the first storage system are illustratively released responsive to receipt in the first storage system of a positive status indicator from the second storage system indicating that consistency of the set of pages in the second storage system relative to the set of pages in the first storage system has been successfully verified by the second storage system.

In some embodiments, such as those in which the first and second storage systems comprise respective source and target storage systems of the synchronous replication, the set of pages of the logical storage volume are sent from the source storage system to the target storage system responsive to receipt in the source storage system of a negative status indicator from the target storage system indicating that consistency of the set of pages in the target storage system relative to the set of pages in the source storage system has not been successfully verified by the target storage system.

Computing an additional signature as a function of the content-based signatures in some embodiments comprises applying a secure hashing algorithm to hashes of the respective data pages of the set of data pages.

As another example, computing an additional signature as a function of the content-based signatures alternatively comprises storing in a buffer a plurality of logical addresses and respective content-based signatures for respective non-zero pages of the set of pages, and responsive to the buffer being empty, generating the additional signature as a predetermined signature, and otherwise generating the additional signature at least in part by applying a secure hashing algorithm to contents of the buffer.

In some embodiments, the first and second storage systems comprise respective CAS systems having respective sets of non-volatile memory storage devices. For example, the first and second storage systems are illustratively associated with respective source and target sites of a replication process, with the source site comprising a production site data center and the target site comprising a disaster recovery site data center, although a wide variety of other arrangements are possible.

The processing device in some embodiments comprises at least a portion of a storage controller of one of the first and second storage systems.

As another example, the processing device illustratively comprises at least one of a plurality of storage nodes of a distributed storage system, with each such storage node comprising a set of processing modules configured to communicate with corresponding sets of processing modules on other ones of the storage nodes. The sets of processing modules of the storage nodes of the distributed storage system collectively comprise at least a portion of a storage controller of the storage system.

Numerous other clustered and non-clustered storage system arrangements are possible in other embodiments.

These and other illustrative embodiments include, without limitation, apparatus, systems, methods and processor-readable storage media.

DETAILED DESCRIPTION

FIG. 1shows an information processing system100configured in accordance with an illustrative embodiment. The information processing system100comprises a plurality of host devices101, a source storage system102S and a target storage system102T, all of which are configured to communicate with one another over a network104. The source and target storage systems102are more particularly configured in this embodiment to participate in a synchronous replication process in which one or more storage volumes are synchronously replicated from the source storage system102S to the target storage system102T, possibly with involvement of at least one of the host devices101. The one or more storage volumes that are synchronously replicated from the source storage system102S to the target storage system102T are illustratively part of a designated consistency group.

The synchronous replication process can be initiated from another replication process of a different type, such as an asynchronous replication process. Accordingly, the storage systems102can transition from asynchronous to synchronous replication, and vice versa.

Each of the storage systems102is illustratively associated with a corresponding set of one or more of the host devices101. The host devices101illustratively 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 devices101in 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 (TO) operations that are processed by a corresponding one of the storage systems102. The term “input-output” as used herein refers to at least one of input and output. For example, TO operations may comprise write requests and/or read requests directed to logical addresses of a particular logical storage volume of a given one of the storage systems102. These and other types of TO operations are also generally referred to herein as TO requests.

The storage systems102illustratively comprise respective processing devices of one or more processing platforms. For example, the storage systems102can each comprise 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 systems102can additionally or alternatively be part of cloud infrastructure such as an Amazon Web Services (AWS) system. Other examples of cloud-based systems that can be used to provide at least portions of the storage systems102include Google Cloud Platform (GCP) and Microsoft Azure.

The storage systems102may be implemented on a common processing platform, or on separate processing platforms.

The host devices101are illustratively configured to write data to and read data from the storage systems102in accordance with applications executing on those host devices for system users.

The term “user” herein is intended to be broadly construed so as 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.

The source storage system102S comprises a plurality of storage devices106S and an associated storage controller108S. The storage devices106S store storage volumes110S. The storage volumes110S illustratively comprise respective logical units (LUNs) or other types of logical storage volumes.

Similarly, the target storage system102T comprises a plurality of storage devices106T and an associated storage controller108T. The storage devices106T store storage volumes110T, at least a portion of which represent respective LUNs or other types of logical storage volumes that are replicated from the source storage system102S to the target storage system102T in accordance with a synchronous replication process.

The storage devices106of the storage systems102illustratively comprise 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 devices106include non-volatile random access memory (NVRAM), phase-change RAM (PC-RAM), magnetic RAM (MRAM), resistive RAM, spin torque transfer magneto-resistive RAM (STT-MRAM), and Intel Optane™ devices based on 3D XPoint™ memory. 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 while the capacity tier comprises hard disk drives. 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, so as 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 systems102illustratively comprises a scale-out all-flash content addressable storage array such as an XtremIO™ storage array from Dell EMC of Hopkinton, Mass. A wide variety of other types of storage arrays can be used in implementing a given one of the storage systems102in other embodiments, including by way of example one or more VNX®, VIVIAX®, Unity™ or PowerMax™ storage arrays, commercially available from Dell EMC. Additional or alternative types of storage products that can be used in implementing a given storage system in illustrative embodiments include software-defined storage, cloud storage, object-based storage and scale-out storage. Combinations of multiple ones of these and other storage types can also be used in implementing a given storage system in an illustrative embodiment.

In some embodiments, communications between the host devices101and the storage systems102comprise Small Computer System Interface (SCSI) or Internet SCSI (iSCSI) 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, so as to encompass, for example, a composite command that comprises a combination of multiple individual commands. Numerous other commands can be used in other embodiments.

For example, although in some embodiments certain commands used by the host devices101to communicate with the storage systems102illustratively comprise SCSI or iSCSI commands, other embodiments can implement10operations utilizing command features and functionality associated with NVM Express (NVMe), as described in the NVMe Specification, Revision 1.3, May 2017, which is incorporated by reference herein. Other storage protocols of this type that may be utilized in illustrative embodiments disclosed herein include NVMe over Fabric, also referred to as NVMeoF, and NVMe over Transmission Control Protocol (TCP), also referred to as NVMe/TCP.

The storage controller108S of source storage system102S in theFIG. 1embodiment includes replication control logic112S and data verification logic114S. It can also include additional elements, such as a signature generator for generating content-based signatures of respective data pages.

Similarly, the storage controller108T of target storage system102T includes replication control logic112T and data verification logic114T. The storage controller108T, like the storage controller108S, can also include additional elements, such as a signature generator for generating content-based signatures of respective data pages.

The instances of replication control logic112S and112T are collectively referred to herein as replication control logic112. Such replication control logic instances are also referred to herein as individually or collectively comprising at least a portion of a “replication engine” of the system100.

The replication control logic112of the storage systems102controls performance of the synchronous replication process carried out between those storage systems, which as noted above in some embodiments further involves at least one of the host devices101. The data replicated from the source storage system102S to the target storage system102T can include all of the data stored in the source storage system102S, or only certain designated subsets of the data stored in the source storage system102S, 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.

A given storage volume designated for replication from the source storage system102S to the target storage system102T illustratively comprises a set of one or more LUNs or other instances of the storage volumes110S of the source storage system102S. Each such logical storage volume illustratively comprises at least a portion of a physical storage space of one or more of the storage devices106S. The corresponding replicated logical storage volume of the storage volumes110T of the target storage system102T illustratively comprises at least a portion of a physical storage space of one or more of the storage devices106T.

The data verification logic114of the storage systems102is illustratively configured to control the performance of a process for continuous data verification in synchronous replication, such as that shown in the flow diagrams ofFIGS. 4A and 4B. At least one of the host devices101in some embodiments can also include one or more instances of data verification logic and possibly also one or more instances of replication control logic and one or more signature generators.

The storage controllers108of the storage systems102should also be understood to include additional modules and other components typically found in conventional 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 theFIG. 1embodiment that there is an ongoing synchronous replication process being carried out between the source storage system102S and the target storage system102T in the system100, utilizing their respective instances of replication control logic112S and112T.

An exemplary synchronous replication process more particularly comprises a synchronous replication process in which host writes to a consistency group comprising one or more storage volumes are mirrored from the source storage system102S to the target storage system102T as the host writes are made at the source storage system102S.

Other types of replication arrangements can be used in other embodiments. For example, the storage systems may be configurable to operate in both asynchronous and synchronous replication modes, with transitions between the modes controlled by their respective instances of replication control logic112S and112T.

A given such asynchronous replication mode illustratively comprises a cycle-based asynchronous replication process in which a consistency group comprising one or more storage volumes is replicated from the source storage system102S to the target storage system102T over a plurality of asynchronous replication cycles.

Other examples of replication processes that can be used in illustrative embodiments include active-active replication, in which one of the storage systems operates as a “leader” relative to another one of the storage systems operating as a “follower” in implementing consistent synchronous writes to both storage systems. Such active-active replication is considered a type of synchronous replication as that term is broadly used herein.

The system100is illustratively configured to provide what is referred to herein as “continuous data verification in synchronous replication.” For example, such continuous data verification in synchronous replication is illustratively performed as part of a replication process carried out between the source storage system102S and the target storage system102T. These and other operations related to continuous data verification in synchronous replication as disclosed herein are illustratively implemented at least in part by or otherwise under the control of the source and target instances of data verification logic114S and114T. One or more such operations can be additionally or alternatively controlled by one or more other system components in other embodiments.

In accordance with the functionality for continuous data verification in synchronous replication, the storage controller108S of source storage system102S is configured, in conjunction with synchronous replication of at least one logical storage volume between the source and target storage systems102S and102T, to acquire an address lock for a set of pages of the logical storage volume starting from a particular page of the logical storage volume, to determine content-based signatures for respective pages of the set of pages, to compute an additional signature as a function of the content-based signatures, and to send the additional signature and a pointer to the particular page to the target storage system102T, so as to permit the target storage system102T to verify consistency of the set of pages in the target storage system102T relative to the set of pages in the source storage system102S.

These source-side operations are repeated for each of one or more additional sets of pages of the logical storage volume starting from respective different particular pages of the logical storage volume, illustratively until consistency has been verified and any detected inconsistencies corrected, for all pages of the logical storage volume. For example, any inconsistencies detected by the target storage system102T for one or more sets of pages for which additional signatures and pointers are sent by the source storage system102S to the target storage system102T are illustratively corrected by sending the corresponding sets of pages from the source storage system102S to the target storage system102T.

Such an arrangement is an example of what is referred to herein as “continuous data verification” in synchronous replication of one or more logical storage volumes from source storage system102S to target storage system102T. As mentioned previously, the “continuous” data verification is illustratively applied to a sequence of multiple sets of data pages of a given storage volume in a process that is performed repeatedly throughout at least a portion of an ongoing synchronous replication of the logical storage volume. The term “continuous” as used herein is therefore intended to be broadly construed.

The source storage system102S and target storage system102T are examples of what are more generally referred to herein as respective “first and second storage systems.” In other embodiments, the designation of first and second storage systems as respective source and target storage systems can be reversed. Moreover, in some embodiments, a continuous data verification process of the type described above is simultaneously performed in two different directions, namely, both from source to target and from target to source, although any detected inconsistencies in such arrangements are illustratively corrected by sending the corresponding sets of pages from source to target. The sets of pages that are sent from source to target to correct detected discrepancies are themselves eventually subject to data verification by running the data verification process in the direction from target to source.

The source storage system102S illustratively determines the content-based signatures for respective ones of the pages in a given set of pages by accessing an address-to-hash or A2H table that stores logical addresses of the pages in association with respective hashes of the pages, with the respective hashes being generated by applying a secure hashing algorithm to content of the pages, as described in more detail elsewhere herein. Other techniques can be used to determine content-based signatures for respective pages in other embodiments. In some embodiments, each of the sets of pages has the same number of pages, denoted by a number N in some example processes herein, wherein a given set of N pages may be 16 pages or another suitable number of pages, such as 8 pages or 32 pages, appropriate to a given implementation.

The above-noted address locks acquired in the source storage system102S for a given set of pages of the logical storage volume are illustratively released responsive to receipt in the source storage system102S of a positive status indicator from the target storage system102T, with the positive status indicator indicating that consistency of the set of pages in the target storage system102T relative to the set of pages in the source storage system102S has been successfully verified by the target storage system102T.

The operation of the target storage system102T responsive to receipt of the additional signature and the pointer to the particular page from the source storage system102S is more particularly as follows. The target storage system102T acquires an address lock for a set of pages of the logical storage volume starting from the particular page of the logical storage volume, determines content-based signatures for respective pages of the set of pages, computes an additional signature as a function of the content-based signatures, compares the computed additional signature with the additional signature received from the source storage system102S, generates a status indicator based at least in part on the comparing, and sends the status indicator to the source storage system102S to indicate whether or not consistency of the set of pages in the target storage system102T relative to the set of pages in the source storage system102S has been successfully verified by the target storage system102T.

The target storage system102T thus verified consistency in this embodiment for a given set of pages by comparing the additional signature that it computes for that set of pages on the target side with the received additional signature that it received from the source storage system102S as computed for that set of pages on the source side. Other arrangements for “verifying consistency” of sets of pages between source and target sides can be used in other embodiments, and that term is therefore intended to be broadly construed.

If for some reason the target storage system102T is unable to acquire the address lock for the set of pages, the target storage system102T illustratively sends a retry code to the source storage system102S. Responsive to receipt of the retry code from the target storage system102T, the source storage system102S repeats the acquiring, determining, computing and sending for the same set of pages of the logical storage volume starting from the particular page of the logical storage volume.

The target storage system102T repeats the above-noted target-side acquiring, determining, computing, comparing and sending operations for each of one or more additional sets of pages of the logical storage volume, starting from respective different particular pages of the logical storage volume, responsive to receipt of respective additional signatures and respective pointers to the particular pages from the source storage system102S. For example, such target-side operations can be repeated until consistency of the entire logical storage volume in the target storage system102T relative to the entire logical storage volume in the source storage system102S has been successfully verified by the target storage system102T, including appropriate correction of any detected inconsistencies.

If the target storage system102T detects an inconsistency between its computed additional signature and the additional signature received from the source storage system102S, it generates a negative status indicator and sends it to the source storage system102S, to indicate that consistency of the set of pages in the target storage system102T relative to the set of pages in the source storage system102S has not been successfully verified by the target storage system102T. As indicated previously, the source storage system102S responds to this detected inconsistency by sending the set of pages to the target storage system102T, thereby correcting the detected inconsistency. Other techniques can be used to correct detected inconsistencies in other embodiments.

In some embodiments, the above-noted computing of an additional signature as a function of the content-based signatures comprises applying a secure hashing algorithm to hashes of the respective data pages of the set of data pages.

In other embodiments, computing an additional signature as a function of the content-based signatures comprises storing in a buffer a plurality of logical addresses and respective content-based signatures for respective non-zero pages of the set of pages. Responsive to the buffer being empty, the additional signature is generated as a predetermined signature, and otherwise the additional signature is generated at least in part by applying a secure hashing algorithm to contents of the buffer.

Examples of particular secure hashing algorithms that may be used in these and other embodiments are described elsewhere herein. However, other types of techniques not necessarily utilizing secure hashing algorithms can be used to generate content-based signatures and associated additional signatures herein.

The above-described operations associated with continuous data verification in synchronous replication are illustratively performed at least in part by or under the control of the replication control logic112S operating in cooperation with the data verification logic114S.

More detailed illustrations of example processes for continuous data verification in synchronous replication for other embodiments implementing at least some of the above-described operations will be described below, including the example process presented in the flow diagrams ofFIGS. 4A and 4B.

It should be noted that the above-noted functionality for continuous data verification in synchronous replication described with reference to source storage system102S relative to target storage system102T can additionally or alternatively be implemented in target storage system102T relative to source storage system102S. The storage systems102in some embodiments therefore both implement substantially the same functionality for continuous data verification in synchronous replication via their respective instances of data verification logic114. Accordingly, designation of one of the storage systems102as the “source” and the other as the “target” can be reversed in other embodiments.

One or both of the storage systems102are illustratively implemented as respective distributed storage systems, also referred to herein as clustered storage systems, in which each such storage system comprises a plurality of storage nodes each comprising a set of processing modules configured to communicate with corresponding sets of processing modules on other ones of the storage nodes. The sets of processing modules of the storage nodes of the source storage system collectively comprise at least a portion of the storage controller108S or storage controller108T of the respective source storage system102S or target storage system102T. For example, in some embodiments the sets of processing modules of the storage nodes collectively comprise a distributed storage controller of the distributed storage system.

The source and target storage systems102in some embodiments comprise respective content addressable storage systems in which logical addresses of data pages are mapped to physical addresses of the data pages using respective content-based signatures that are generated from those data pages, as will now be described in more detail with reference to the illustrative embodiments ofFIGS. 2 and 3.

FIG. 2shows an example of a distributed content addressable storage (CAS) system205that illustratively represents a particular implementation of one of the source and target storage systems ofFIG. 1in some embodiments, and accordingly is assumed to be coupled to the other one of the storage systems102and to one or more host devices of a computer system within information processing system100. The other storage system illustratively comprises another instance of CAS system205.

The CAS system205comprises a plurality of storage devices206and an associated storage controller208. The storage devices206store data of a plurality of storage volumes. The storage volumes illustratively comprise respective LUNs or other types of logical storage volumes. The stored data comprises metadata pages220and user data pages222, both described in more detail elsewhere herein. The storage devices206and storage controller208are distributed across multiple storage nodes215. The CAS system205can include additional components, such as a write cache and a write cache journal, each also illustratively distributed across the storage nodes215of the CAS system205.

The CAS system205is illustratively implemented as a distributed storage system, also referred to herein as a clustered storage system, in which each of at least a subset of the storage nodes215comprises a set of processing modules configured to communicate with corresponding sets of processing modules on other ones of the storage nodes215. The sets of processing modules of the storage nodes of the CAS system205collectively comprise at least a portion of the storage controller208of the CAS system205. For example, in some embodiments the sets of processing modules of the storage nodes collectively comprise a distributed storage controller of the CAS system205. A “distributed storage system” as that term is broadly used herein is intended to encompass any storage system that, like the CAS system205, is distributed across multiple storage nodes.

Although it is assumed that both the source storage system102S and the target storage system102T are content addressable storage systems in some embodiments, other types of storage systems can be used for one or both of the source storage system102S and the target storage system102T in other embodiments. For example, it is possible that at least one of the storage systems102in an illustrative embodiment need not be a content addressable storage system and need not include an ability to generate content-based signatures. In an embodiment of this type, the signature generation functionality can be implemented in a host device.

The storage controller208in the present embodiment is configured to implement functionality for continuous data verification in synchronous replication of the type previously described in conjunction withFIG. 1. For example, the CAS system205illustratively participates as a source storage system in a replication process with a target storage system that is implemented as another instance of the CAS system205.

The storage controller208includes distributed modules212and214, which are configured to operate in a manner similar to that described above for respective corresponding replication control logic112and data verification logic114of the storage controllers108of system100. Module212is more particularly referred to as distributed replication control logic, and illustratively comprises multiple replication control logic instances on respective ones of the storage nodes215, with the multiple replication control logic instances comprising at least a portion of a replication engine configured to perform process operations associated with synchronous replication. Module214more particularly comprises distributed data verification logic with different instances thereof also being implemented on respective ones of the storage nodes215. Each of the storage nodes215of the CAS system205is assumed to be implemented using at least one processing device comprising a processor coupled to a memory.

In the CAS system205, logical addresses of data pages are mapped to physical addresses of the data pages using respective content-based signatures that are generated from those data pages. The data pages illustratively include user data pages222. Metadata pages220are typically handled in a different manner, as will be described.

The term “page” as used in this and other contexts herein is intended to be broadly construed so as to encompass any of a wide variety of different types of blocks that may be utilized in a block storage device of a storage system. Different native page sizes are generally utilized in different storage systems of different types. For example, XtremIO™ X1 storage arrays utilize a native page size of 8 kilobytes (KB), while XtremIO™ X2 storage arrays utilize a native page size of 16 KB. Larger native page sizes of 64 KB and 128 KB are utilized in VMAX® V2 and VMAX® V3 storage arrays, respectively. The native page size generally refers to a typical page size at which the storage system ordinarily operates, although it is possible that some storage systems may support multiple distinct page sizes as a configurable parameter of the system. Each such page size of a given storage system may be considered a “native page size” of the storage system as that term is broadly used herein.

A given “page” as the term is broadly used herein should therefore not be viewed as being limited to any particular range of fixed sizes. In some embodiments, a page size of 8 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 data pages of the CAS system205.

Also, 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 content-based signatures utilized in some embodiments illustratively comprise respective hash digests of respective data pages of a storage volume. A given one of the hash digests is generated in illustrative embodiments by applying a secure hashing algorithm to content of a corresponding one of the data pages of the storage volume. For example, a given hash digest can be generated by application of a hash function such as the well-known Secure Hashing Algorithm1(SHA1) to the content of its corresponding data 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.

As indicated above, the storage controller208in this embodiment is implemented as a distributed storage controller that comprises sets of processing modules distributed over the storage nodes215. The storage controller208is therefore an example of what is more generally referred to herein as a distributed storage controller.

It is assumed in some embodiments that the processing modules of the storage controller208are interconnected in a full mesh network, such that a process of one of the processing modules can communicate with processes of any of the other processing modules. Commands issued by the processes can include, for example, remote procedure calls (RPCs) directed to other ones of the processes.

The sets of processing modules of the storage controller208illustratively comprise control modules208C, data modules208D, routing modules208R and at least one management module208M. Again, these and possibly other processing modules of the storage controller208are illustratively interconnected with one another in the full mesh network, such that each of the modules can communicate with each of the other modules, although other types of networks and different module interconnection arrangements can be used in other embodiments.

The management module208M of the distributed storage controller in this embodiment may more particularly comprise a system-wide management module, also referred to herein as a system manager. Other embodiments can include multiple instances of the management module208M implemented on different ones of the storage nodes215. It is therefore assumed that the storage controller208comprises one or more management modules208M.

A wide variety of alternative configurations of nodes and processing modules are possible in other embodiments. Also, the term “storage node” as used herein is intended to be broadly construed, and may comprise a node that implements storage control functionality but does not necessarily incorporate storage devices.

The processing modules of the storage controller208as disclosed herein utilize metadata structures that include logical layer and physical layer mapping tables to be 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. The logical layer and physical layer mapping tables in this embodiment illustratively include the following:

1. An address-to-hash (“A2H”) table. The A2H table illustratively 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 one of the hash handles, and possibly one or more additional fields. In some embodiments, the A2H table is assumed to comprise full hash digests in place of or in addition to hash handles. Other configurations are possible, and the term “address-to-hash table” as used herein is therefore intended to be broadly construed.

2. A hash-to-data (“H2D”) table. The H2D table illustratively 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. Again, full hash digests can be used in place of or in addition to hash handles.

3. A hash metadata (“HMD”) table. The HMD table illustratively 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 illustratively comprises at least a portion of the same information that is found in the H2D table. Accordingly, in other embodiments, those two tables can be combined into a single table, illustratively referred to as an H2D table, an HMD table or another type of physical layer mapping table providing a mapping between hash values, such as hash handles or hash digests, and corresponding physical addresses of data pages.

4. A physical layer based (“PLB”) table. The PLB table 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. Hash digests can be used in place of in addition to hash handles in some embodiments.

Again, the logical layer and physical layer mapping tables referred to above are examples only, and can be varied in other embodiments. For example, other types of hash-to-physical (“H2P”) mapping tables may be used in addition to or in place of the above-noted H2D, HMD and/or PLB tables.

In some embodiments, certain ones of the above-described mapping tables are maintained by particular modules of storage controller208. For example, the mapping tables maintained by the control modules208C illustratively comprise at least one A2H table and possibly also at least one H2D table. The A2H tables are utilized to store address-to-hash mapping information and the H2D tables are utilized to store hash-to-data mapping information, in support of mapping of logical addresses for respective pages to corresponding physical addresses for those pages via respective hashes or other types of content-based signatures, as described in further detail elsewhere herein.

The control modules208C may further comprise additional components such as respective messaging interfaces that are utilized by the control modules208C to process routing-to-control messages received from the routing modules208R, and to generate control-to-routing messages for transmission to the routing modules208R. Such messaging interfaces can also be configured to process instructions and other messages received from the management module208M and to generate messages for transmission to the management module208M.

The data modules208D comprise respective control interfaces. These control interfaces support communication between the data modules208D and the control modules208C. Also included in the data modules are respective SSD interfaces. These SSD interfaces support communications with corresponding ones of the storage devices206of the CAS system205.

The above-described processing module arrangements are presented by way of example only, and can be varied in other embodiments.

In some embodiments, a given data path of the CAS system205comprises a particular one of the routing modules208R, a particular one of the control modules208C and a particular one of the data modules208D, each configured to handle different stages of the data path. For example, a given IO request can comprise a read request or a write request received in the particular control module from the particular routing module. The particular control module processes the received IO request to determine the particular data module that has access to the one or more data pages targeted by that IO request.

Communication links may be established between the various processing modules of the storage controller208using well-known communication protocols such as TCP/IP 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 modules208R.

In some embodiments, at least portions of the functionality for continuous data verification in synchronous replication in the CAS system are distributed over at least the control modules208C and data modules208D of storage controller208. Numerous other arrangements are possible. For example, portions of the functionality can be implemented in the one or more management modules208, or using other types and arrangements of modules within or outside of the storage controller208.

As indicated previously, the storage devices206are configured to store metadata pages220and user data pages222, and may also store additional information not explicitly shown such as, for example, one or more system checkpoints and/or snapshots of storage volumes, and one or more write journals such as the write cache journal. The metadata pages220and the user data pages222in some embodiments are illustratively stored in respective designated metadata and user data areas of the storage devices206. Accordingly, metadata pages220and user data pages222may be viewed as corresponding to respective designated metadata and user data areas of the storage devices206.

As noted above, 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 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 pages220and the user data pages222.

The user data pages222are part of a plurality of logical storage volumes 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 CAS system205. Each such logical storage volume may comprise particular ones of the above-noted user data pages222of the user data area. The user data stored in the user data pages222can include any type of user data that may be utilized in the system100. The term “user data” herein is therefore also intended to be broadly construed.

A given storage volume for which content-based signatures are generated, illustratively by signature generators implemented in respective ones of the control modules208R and/or elsewhere in the storage nodes215, can comprise a set of one or more LUNs, each including multiple ones of the user data pages222stored in storage devices206.

The CAS system205in the embodiment ofFIG. 2is configured to generate hash metadata providing a mapping between content-based digests of respective ones of the user data pages222and 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 pages222. The hash metadata generated by the CAS system205is illustratively stored as metadata pages220in the metadata area. The generation and storage of the hash metadata is assumed to be performed under the control of the storage controller208.

Each of the metadata pages220characterizes a plurality of the user data pages222. For example, in a given set of n user data pages representing a portion of the user data pages222, each of the user data pages is characterized by a volume 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 devices206.

Each of the metadata pages220in 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 pages220in an illustrative embodiment comprises metadata pages having respective signatures. 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 volume 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 CAS system205is illustratively distributed among the control modules208C.

The functionality for continuous data verification in synchronous replication in the CAS system205in this embodiment is assumed to be distributed across multiple distributed processing modules, including at least a subset of the processing modules208C,208D,208R and208M of the storage controller208.

For example, the management module208M of the storage controller208may include a data verification logic instance that engages corresponding data verification logic instances in all of the control modules208C in order to support continuous data verification in synchronous replication in the CAS system205.

In some embodiments, each of the user data pages222has a fixed size such as, for example, 8 KB, and its content-based signature is a 20-byte signature generated using the SHA1 secure hashing algorithm. Also, each page has a volume identifier and an offset, and so is characterized by <lun_id, offset, signature>.

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.

The storage controller208in 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 control modules208C. For example, if there are 1024 slices distributed evenly across the control modules208C, and there are a total of 16 control modules in a given implementation, each of the control modules “owns” 1024/16=64 slices. In such arrangements, different ones of the slices are assigned to different ones of the control modules208C such that control of the slices within the storage controller208of the CAS system205is substantially evenly distributed over the control modules208C of the storage controller208.

The data modules208D allow 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 <lun_id, offset, signature> for respective ones of a plurality of the user data pages222. Such metadata pages220are illustratively generated by the control modules208C but are accessed using the data modules208D 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 volume identifier and offset information of that metadata page.

If a user wants to read a user data page having a particular volume 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 data module. This metadata page will include the <lun_id, offset, signature> 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 data module.

Write requests processed in the CAS system205each illustratively comprise one or more IO operations directing that at least one data item of the CAS system205be written to in a particular manner. A given write request is illustratively received in the CAS system205from one of the host devices101over network104. In some embodiments, a write request is received in the storage controller208of the CAS system205, and directed from one processing module to another processing module of the storage controller208. For example, a received write request may be directed from a routing module208R of the storage controller208to a particular control module208C of the storage controller208. Other arrangements for receiving and processing write requests from one or more of the host devices101can be used.

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 some embodiments, the control modules208C, data modules208D and routing modules208R of the storage nodes215communicate with one another over a high-speed internal network such as an InfiniBand network. The control modules208C, data modules208D and routing modules208R coordinate with one another to accomplish various IO processing tasks, as described elsewhere herein.

The write requests from the host devices identify particular data pages to be written in the CAS system205by their corresponding logical addresses each illustratively comprising a volume identifier 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 CAS system205illustratively utilizes a two-level mapping process to map logical block addresses to physical block addresses. In some embodiments, the first level of mapping uses an A2H table and the second level of mapping uses an HMD table, with the A2H and HMD tables corresponding to respective logical and physical layers of the content-based signature mapping within the CAS system205. The HMD table or a given portion thereof in some embodiments disclosed herein is more particularly referred to as an H2D table or H2P table, although it is to be understood that these and other mapping tables or other metadata structures referred to herein can be varied in other embodiments.

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 devices206. This is also referred to as physical layer mapping.

Examples of these and other metadata structures utilized in illustrative embodiments were described elsewhere herein. These particular examples illustratively include respective A2H, H2D, HMD and PLB tables. In some embodiments, the A2H and H2D tables are utilized primarily by the control modules208C, while the HMD and PLB tables are utilized primarily by the data modules208D.

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 CAS system205. 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 CAS system205correspond to respective physical blocks of a physical layer of the CAS system205. 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 CAS system205. 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 modules208C,208D,208R and208M as shown in theFIG. 2embodiment is presented by way of example only. Numerous alternative arrangements of processing modules of a distributed storage controller may be used to implement continuous data verification in synchronous replication in a distributed CAS system or other type of distributed storage system in other embodiments.

Additional examples of content addressable storage functionality that may be implemented in some embodiments by control modules208C, data modules208D, routing modules208R and management module(s)208M of storage controller208can 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 distributed CAS system or other type of distributed storage system can be used in other embodiments.

As indicated above, the CAS system205illustratively comprises storage nodes215interconnected in a mesh network, with each such storage node comprising a set of processing modules configured communicate with corresponding sets of processing modules on other ones of the storage nodes. A given such set of processing modules comprises at least a routing module, a control module and a data module, with the sets of processing modules of the storage nodes215of the CAS system205collectively comprising at least a portion of the storage controller208of the CAS system205.

The storage nodes215and their respective sets of processing modules are managed by a system manager, illustratively implemented as a management module208M within the set of processing modules on at least one of the storage nodes215. Each storage node215illustratively comprises a CPU or other type of processor, a memory, a network interface card (NIC) or other type of network interface, and a subset of the storage devices206, possibly arranged as part of a disk array enclosure (DAE) of the storage node. These and other references to “disks” herein are intended to refer generally to storage devices, including SSDs, and should therefore not be viewed as limited in any way to spinning magnetic media.

An example of the operation of the CAS system205in processing IO operations will now be described with reference toFIG. 3, which shows the relationship between routing, control and data modules of one possible distributed implementation of CAS system205in an illustrative embodiment. More particularly,FIG. 3illustrates a portion300of the CAS system205, showing a routing module208R-x, a control module208C-y and a data module208D-z in a distributed implementation of the storage controller208. The routing module208R-x, the control module208C-y and the data module208D-z are also denoted in this embodiment as an R-module, a C-module and a D-module, respectively.

These modules are respective processing modules of the storage controller208, and are potentially located on different ones of the storage nodes215of the CAS system205. For example, each of the storage nodes215of the CAS system205illustratively comprises at least one R-module, at least one C-module and at least one D-module, although many other storage node configurations are possible. In the present embodiment, the routing module208R-x, the control module208C-y and the data module208D-z are assumed to be on respective different storage nodes x, y and z of the CAS system205. The storage nodes x, y and z represent respective particular ones of the storage nodes215. The storage node z that implements the D-module208D-z comprises a subset of the storage devices206of the CAS system205, with the subset of storage devices206on storage node z being denoted as storage devices206-z. Each of the other storage nodes215of the CAS system205similarly has a different subset of the storage devices206associated therewith.

It is assumed in this example that the CAS system205manages data using a fixed-size page granularity (e.g., 4 KB, 8 KB or 16 KB), also referred to herein as the native page size of the CAS system205. A unique hash digest is computed for each of the data pages by a content-based signature generator, illustratively using SHA1 or another secure hashing algorithm of the type described elsewhere herein.

In the CAS system205, routing modules208R such as R-module208R-x illustratively include a storage command parser as shown, such as a SCSI command parser, although other command parsers for other storage protocols can be used in other embodiments. The routing modules208R receive IO requests from one or more of the host devices101, parse the corresponding storage commands and route them to the appropriate control modules208C, which may be located on different storage nodes215, illustratively using an address-to-control (“A2C”) table. The A2C table maps different portions of a logical address space of the CAS system205across different ones of the control modules208C. A given IO request can be sent by the corresponding one of the host devices101to any of the routing modules208R of the CAS system205.

The control modules208C such as control module208C-y receive the IO requests from the routing modules208R, and use mapping tables such as the above-described A2H and H2D tables to identify the appropriate data modules208D that store the corresponding data pages in the distributed CAS system205. This illustratively includes performing a logical address to hash mapping as shown in the figure.

In processing read requests, the C-module208C-y retrieves from the A2H table the hash digests of the corresponding requested pages, and sends read requests to the appropriate data modules208D based on the H2D table.

In processing write requests, the C-module208C-y illustratively computes the hash digests of the data pages based on the write data, sends write requests to the corresponding data modules208D as determined from the H2D table, and updates the A2H table.

The data modules208D such as D-module208D-z are responsible for the physical storage of the data pages, and use mapping tables such as the above-described HMD and PLB tables and/or other types of H2P tables to determine the physical location of a given data page in the subset of storage devices206associated with that data module, using a hash digest, hash handle or other content-based signature supplied by a control module. This illustratively includes performing a hash to physical location mapping as shown in the figure. Such a hash to physical location mapping can utilize an H2P table of the type described elsewhere herein, illustratively comprising at least portions of the above-noted HMD and PLB tables. The data modules208D in some embodiments additionally store a copy or “mirror” of such metadata in a memory of the respective corresponding storage nodes215, in order to optimize performance by reducing accesses to the associated storage devices206during system operation.

A given one of the host devices101illustratively sends an IO request to a particular one of the routing modules208R, possibly using random selection or another type of algorithm such as round robin to select a particular routing module for a particular IO request. Such selection can be implemented as part of a path selection algorithm performed by a multi-path input-output (MPIO) driver of the host device, in order to select a particular path comprising an initiator-target pair for delivery of the IO request to the CAS system205. The initiator illustratively comprises a particular host bus adaptor (HBA) of the given host device, and the target illustratively comprises a particular port of the CAS system205.

The particular features described above in conjunction withFIGS. 2 and 3should not be construed as limiting in any way, and a wide variety of other distributed implementations of source storage system102S and target storage system102T are possible.

Additional details regarding example processes for continuous data verification in system100will now be described. It is assumed for these processes that each of the source storage system102S and the target storage system102T comprises a corresponding instance of the CAS system205ofFIG. 2, each with its control, data and routing modules operating in the manner illustrated inFIG. 3.

As indicated previously, in conjunction with synchronous replication, various malfunctions such as link loss, storage module restart or code issues can cause the source and target storage systems102S and102T to lose their synchronization, and thereafter the target storage system102T becomes inconsistent with the source storage system102S, due to host writes being applied to data on the source but not on the target. Often, such problems are found only after the target data needs to be used, for example, after a disaster has destroyed the source storage system. By then it is of course too late to fix any problem.

Illustrative embodiments herein provide techniques for discovering and correcting such inconsistencies between source and target data at an early stage, and without adversely impacting storage system performance. Such embodiments advantageously avoid data inconsistency problems that might otherwise arise due to loss of synchronization between source and target storage systems, by allowing inconsistent data to be detected and corrected in a particularly efficient manner, illustratively in real time.

In synchronous replication, it is generally desirable for the source storage system102S and the target storage system102T to have identical data at all times. Therefore, it is important to detect and correct discrepancies between source and target data on a substantially continuous basis, possibly in real time. However, during an initial synchronization or a resynchronization after recovery from a link loss or a consistency group “trip” event, the source and target will likely have different data. During these times, data verification may be suspended since there is no expectation that the source and target data is the same. Under such conditions, the target typically maintains a static consistent older copy of the data in case recovery is needed.

Once synchronization of source and target is complete, there is an expectation that the source and target data are the same. However, every host write has to be written to one side before it is written to the other. In some systems, the host write is first written to the source, and then replicated to the target. In others, the host write is first written to the target, then replicated to the source. Either way, the host write cannot be written simultaneously to both sides at the same time. This means that in practice, unless host writes have stopped for at least a few seconds, it is only expected that the source and target are identical for pages that are not involved in an active IO process, or in other words, are not involved in “in-flight” IO operations.

Illustrative embodiments herein account for this potential short-term inconsistency by utilizing an address lock on both sides for any data pages being compared. Since IO processing generally requires a lock at least on one side, this guarantees that any page being compared is not part of an active IO process.

In addition, such embodiments are fast and effective by ensuring that actual data is transmitted only responsive to a detected inconsistency, with the above-noted address locks in place on both sides. This is achieved in some embodiments by using a “hash of hashes” or other type of additional signature generated from hash signatures, also referred to herein as simply “hashes,” that already exist in an A2H table or other similar metadata structure, such that those hash signatures do not have to be computed as part of the continuous data verification, and instead only the additional signature is computed from the existing hash signatures as part of the continuous data verification.

Furthermore, illustrative embodiments herein do not compare source and target hash signatures individually on a page-by-page basis, but instead group multiple pages into a set and generate the above-noted additional signature for the set, such that comparing the additional signatures effectively compares sets of multiple pages at a time.

Moreover, some embodiments are configured to accommodate situations in which a logical storage volume is mostly empty, by effectively skipping sets of zero or non-existent pages. For example, a 10 TB volume of data can have only 100 GB of real user data, and is therefore only 1% full. It would be wasteful to compare 9.9 TB of zero pages. A space-efficient A2H table can be used in which these pages do not exist, and therefore can be skipped. However, this still has to be done carefully. For example, if one were to use only the A2H entries on one side to detect and skip zero pages, such an approach would miss inconsistencies in which zero pages on that side are non-zero on the other side. Illustrative embodiments herein advantageously avoid such issues, and can effectively detect inconsistencies in which a zero page on one side is a non-zero page on the other side, by performing the continuous data verification in two different directions, as described in more detail below.

The example process in this embodiment comprises a “scan and correct” algorithm, and is assumed to operate in both directions, from source to target and from target to source, although any detected inconsistencies are corrected by the source sending the corresponding sets of data pages to the target. For a given instance of the scan and correct algorithm, one side is performed by what is referred to herein as the “local side” and the other side is performed by what is referred to herein as the “remote side.” The designation of one side as “local side” and the other side as “remote side” is arbitrary, in that in a given embodiment, each side illustratively sees itself as “local” relative to the other side which it sees as “remote.” This is distinct from the direction of the synchronous replication process, which is carried out from source to target as previously described.

In obtaining address locks, the scan and correct algorithm avoids locking pages for a lengthy amount of time, and instead uses a locking approach without lock wait that is quick to give up lock and to retry in the presence of lock contention. Also, the scan and correct algorithm is illustratively configured to allow for pausing between sets of pages and after a completed instance of the data verification scan.

The scan and correct algorithm in the present embodiment comprises the following steps, performed in one direction by source as local side and target as remote side, while also simultaneously being performed in the opposite direction with target as local side and source as remote side:

1. Local side initializes a pointer P to the beginning of a logical storage volume, and sets a number N of pages per set to a small value such as 16. The pointer P is illustratively a logical address of a first page in a given set of N pages, and is initialized to the address of the first page of the logical storage volume.

2. Is the volume part of an active synchronous replication from source to target, not suspended due to a consistency group “trip” event, and has initial synchronization of source and target been achieved?a. Yes—continue.b. No—wait for a designated amount of time and then retry by going back to the start of step 2.

3. Are all pages in set of N pages starting at the page identified by pointer P zero pages or non-existent pages?a. Yes—increment pointer P by N and go back to the start of step 3.b. No—continue.

4. Local side locks N pages starting at page identified by pointer P without lock wait. Has lock succeeded on all pages?a. Yes—continue.b. No—unlock all pages, wait for a designated amount of time and then retry by going back to the start of step 4.

5. Local side reads N hash signatures from its A2H table starting with the page identified by pointer P. Any non-existent pages result in a default hash of a zero page.

6. Local side computes a hash H1 of the N hash signatures. The hash H1 is an example of what is more generally referred to herein as an “additional signature.”

7. Local side transmits H1 to remote side along with the pointer P and a volume identifier.

8. Remote side receives H1, pointer P and volume identifier, and locks N pages starting at the page identified by pointer P without lock wait. Has the lock succeeded on all pages?a. Yes—continue.b. No—unlock all pages and send retry code to other side.

9. Remote side reads N hash signatures from its A2H table starting at the page identified by pointer P.

10. Remote side computes a hash H2 of the N hash signatures.

11. Remote side compares H1 and H2.a. H1=H2: send back good status, indicating consistency has been successfully verified.b. H1 and H2 are different: send back bad status, indicating consistency has not been successfully verified, and if this side is the source of the synchronous replication, correct the inconsistency by resending the contents of the N pages to the other side.

12. Local side receives status:a. Good status—continue.b. Bad status—if this side is the source of the synchronous replication, correct the inconsistency by resending the contents of the N pages to the other side.c. Retry status—go back to the start of step 4.

13. Local side increases pointer P by set size N.

14. Is this the end of the volume?a. Yes—set pointer P to zero and wait for a designated amount of time before restarting verification.b. No—continue.

15. Local side pauses for a designated amount of time, to account for delay between verification commands.

16. Go back to start of step 2.

In the above scan and correct algorithm, the computation of a given additional signature as a “hash of hashes” H1 or H2 in respective step 6 or step 10 can be implemented in a number of different ways. For example, the N hash signatures for respective pages of a set of N pages can be stored in a buffer, and a cryptographic hash function such as SHA1 is then applied to the buffer contents to generate H1 or H2.

As another example, logical addresses and respective hash signatures can be stored in a buffer, ignoring any zero pages or non-existent pages. If the buffer is empty, as would be the case if the set of N pages is all zero pages, a predetermined signature such as 0 is used as the additional signature H1 or H2. Otherwise, the additional signature is computed as the cryptographic hash function of the buffer contents. An advantage of this example approach relative to the previous example is that it makes it very easy to skip sets of zero pages without having to do any computations.

It should be noted in this regard that these and other techniques used to compute H1, H2 or other additional signatures herein are selected so as to be sensitive to ordering of the component hash signatures, such that same hash signatures appearing in different orders will result in different additional signatures.

The example scan and correct algorithm described above advantageously provides highly efficient continuous verification of replicated data in conjunction with an ongoing synchronous replication of at least one logical storage volume. For example, it avoids transmitting data pages that already exist on the other side, while also skipping zero data pages and non-existent data pages, and utilizing previously-computed hash signatures that are present in an A2H table or other similar metadata structure.

Additional or alternative steps may be used in such a process in other embodiments. Also, the ordering of the steps can be varied, and different portions of the process can be performed at least in part in parallel with one another.

The above-described operations carried out in conjunction with a process for continuous data verification in synchronous replication involving the storage systems102are illustratively performed at least in part under the control of the replication engine comprising the multiple instances of replication control logic112, utilizing instances of data verification logic114.

The storage systems102in theFIG. 1embodiment are assumed to be implemented using at least one processing platform, with each such processing platform comprising one or more processing devices, and each such processing device comprising a processor coupled to a memory. Such processing devices can illustratively include particular arrangements of compute, storage and network resources.

The storage systems102may 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 systems102or on separate processing platforms.

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 withFIGS. 5 and 6.

Accordingly, different numbers, types and arrangements of system components such as host devices101, storage systems102, network104, storage devices106, storage controllers108, storage volumes110, replication control logic112and data verification logic114can be used in other embodiments.

It should be understood that the particular sets of modules and other components implemented in the system100as illustrated inFIGS. 1, 2 and 3are 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, functionality for continuous data verification in synchronous replication can be implemented in one or more host devices, or partially in a host device and partially in a storage system. Accordingly, 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.

As another example, it is possible in some embodiments that the source storage system and the target storage system can comprise 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 so as to encompass such possibilities.

The operation of the information processing system100will now be described in further detail with reference to the flow diagrams of the illustrative embodiment ofFIGS. 4A and 4B, which collectively implement another example process for continuous data verification in synchronous replication.

The flow diagrams ofFIGS. 4A and 4Bmore particularly show respective source storage system and target storage system portions of a process for continuous data verification in synchronous replication in an illustrative embodiment.FIG. 4Aincludes steps400through418, illustratively performed by source storage system102S, andFIG. 4Bincludes steps420through434, illustratively performed by target storage system102T. These portions of the continuous data verification process are suitable for use in system100but are more generally applicable to a wide variety of other types of information processing systems comprising first and second storage systems implementing functionality for continuous data verification in synchronous replication. The process to be described is assumed to be carried out between first and second storage systems that are configured to participate in a replication process that includes at least a synchronous replication mode, and possibly includes both asynchronous and synchronous replication modes with transitions occurring between the modes. The first and second storage systems are more particularly assumed to comprise respective distributed CAS systems of the type previously described in conjunction withFIGS. 2 and 3.

The first and second storage systems are referred to in the context ofFIGS. 4A and 4Bas respective “source” and “target” for simplicity. However, as indicated elsewhere herein, the source and target designations are arbitrary, and can be reversed in other embodiments. Moreover, in some embodiments, a continuous data verification process of the type described in conjunction withFIGS. 4A and 4Bis simultaneously performed in two different directions, namely, both from source to target and from target to source, although any detected inconsistencies in such arrangements are illustratively corrected by sending the corresponding sets of pages from source to target.

Referring initially toFIG. 4A, the following steps are performed by source storage system102S interacting with target storage system102T, illustratively via their respective instances of replication control logic112and data verification logic114.

In step400, the source initializes a pointer P to the start of a logical storage volume in an ongoing synchronous replication. The pointer P once initialized points to a first data page of the logical storage volume. For example, the pointer can comprise at least a portion of an LBA of the initial data page. A wide variety of other pointer arrangements can be used in other embodiments, and the term “pointer” as used herein is therefore intended to be broadly construed, and should not be viewed as being limited to an LBA or portion thereof.

In step401, a determination is made as to whether or not all of the pages in a set of N pages starting at the page identified by pointer P are zero pages or non-existent pages. Zero pages are illustratively pages that include only zero entries, while non-existent pages are illustratively pages that do not have any entries in the A2H table of the source. If all of the pages in the set of N pages are zero pages or non-existent pages, the process moves to step402, and otherwise the process moves to step404.

In step402, the pointer P is incremented by N, and the process returns to step401to test the next set of N pages for zero pages or non-existent pages.

In step404, which is reached when a set of N pages starting at the page identified by pointer P does not include all zero pages or all non-existent pages, the source acquires lock for that set of N pages.

In step406, the source retrieves hashes of the pages in the set from the A2H table of the source, and computes an additional signature as a function of the hashes. Although hashes are utilized in this embodiment, other embodiments herein can utilize other types of content-based signatures.

In step408, the source sends the additional signature, the pointer P and an identifier (ID) of the logical storage volume to the target. The ID of the logical storage volume is referred to herein as a “volume identifier.”

In step410, a determination is made as to whether or not the source has received a positive status indicator from the target to indicate that consistency of the set of N pages has been verified by the target. If a positive status indicator has not been received, the process moves to step412, and otherwise moves to step414.

In step412, the source sends the set of N pages to the target, in order to correct the detected inconsistency, and the process moves to step402to increment the pointer P by N and then begins processing the next set of N pages in step401.

In step414, which is reached if the positive status indicator has been received in the source from the target to indicate that the target has successfully verified the consistency of the set of N pages, a further determination is made as to whether or not all pages of the logical storage volume have been processed. If all pages in the logical storage volume have not been processed, the process moves to step402to increment the pointer P by N and then begins processing the next set of N pages in step401. Otherwise, the process moves to step416as indicated.

In step416, the current iteration of the data verification scan is complete, and the process moves to step418.

In step418, the source waits for a designated amount of time before starting the next iteration of the data verification scan for the logical volume, by returning to step400to reinitialize the pointer P to the start of the logical storage volume.

Turning now toFIG. 4B, the following steps are performed by target storage system102T interacting with source storage system102S, again illustratively via their respective instances of replication control logic112and data verification logic114.

In step420, the target receives the additional signature, pointer P and volume ID that the source sent in step408ofFIG. 4A.

In step422, the target acquires lock for the set of N pages starting at the page identified by the pointer P.

In step424, the target retrieves hashes of the pages in the set from the A2H table of the target, and computes an additional signature as a function of the hashes. Again, although hashes are utilized in this embodiment, other embodiments herein can utilize other types of content-based signatures.

In step426, the target compares its computed additional signature with the additional signature received from the source.

In step428, a determination is made as to whether or not the computed and received additional signatures are the same. If the computed and received additional signatures are not the same, the process moves to step430, and otherwise moves to step434as indicated.

In step430, the target sends a negative status indicator to the source to indicate that consistency was not verified for the set of N pages. Receipt of this negative status indicator by the source triggers the source to send the N pages of the set in step412ofFIG. 4A. The process then moves to step432.

In step432, the target receives the set of N pages from the source, which the source sent in step412ofFIG. 4A, and the process then returns to step420to await the next additional signature, pointer value and volume ID from the source.

In step434, which is reached if the computed and additional signatures are the same, the target sends a positive status indicator to the source to indicate that consistency is verified for the set of N pages. This is the positive status indicator that once received by the source triggers movement from step410to step414inFIG. 4A. After the target sends the positive status indicator in step434, the process then returns to step420to await the next additional signature, pointer value and volume ID from the source.

The particular processing operations and other system functionality described in conjunction with the flow diagrams ofFIGS. 4A and 4Bare 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 for continuous data verification in synchronous replication. For example, as indicated above, 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 continuous data verification processes for respective different replication sessions or for different storage systems or portions thereof within a given information processing system.

For example, storage controllers such as storage controllers108of storage systems102that are configured to control performance of one or more steps ofFIGS. 4A and 4Bin their corresponding system100can 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 such as Docker containers or Linux containers (LXCs). The storage controllers108, 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 controllers108, 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.

Illustrative embodiments provide techniques for continuous data verification during synchronous replication of one or more logical storage volumes from a source storage system to a target storage system. Such techniques can provide a number of significant advantages relative to conventional arrangements.

For example, some embodiments disclosed herein advantageously avoid data inconsistency problems that might otherwise arise due to loss of synchronization between source and target storage systems, by allowing inconsistent data to be detected and corrected in a particularly efficient manner, illustratively in real time, and without any adverse impact to storage system performance.

As a more particular example, continuous data verification in some embodiments disclosed herein is applied to a sequence of multiple sets of data pages of a given storage volume in a process that is performed repeatedly throughout at least a portion of an ongoing synchronous replication of the logical storage volume. Numerous other continuous data verification arrangements are possible in accordance with the disclosed techniques.

Some embodiments advantageously avoid transmitting data pages that already exist on the other side, while also skipping zero data pages and non-existent data pages, and utilizing previously-computed hash signatures that are present in an A2H table or other similar metadata structure.

Functionality for continuous data verification in synchronous replication as disclosed herein can be implemented in at least one storage system, in at least one host device, or partially in one or more storage systems and partially in one or more host devices.

Illustrative embodiments of processing platforms utilized to implement host devices and storage systems with functionality for continuous data verification in synchronous replication will now be described in greater detail with reference toFIGS. 5 and 6. Although described in the context of system100, these platforms may also be used to implement at least portions of other information processing systems in other embodiments.

FIG. 5shows an example processing platform comprising cloud infrastructure500. The cloud infrastructure500comprises a combination of physical and virtual processing resources that may be utilized to implement at least a portion of the information processing system100. The cloud infrastructure500comprises multiple virtual machines (VMs) and/or container sets502-1,502-2, . . .502-L implemented using virtualization infrastructure504. The virtualization infrastructure504runs on physical infrastructure505, 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 infrastructure500further comprises sets of applications510-1,510-2, . . .510-L running on respective ones of the VMs/container sets502-1,502-2, . . .502-L under the control of the virtualization infrastructure504. The VMs/container sets502may 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 theFIG. 5embodiment, the VMs/container sets502comprise respective VMs implemented using virtualization infrastructure504that comprises at least one hypervisor. Such implementations can provide functionality for continuous data verification in synchronous replication 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 instances, data verification logic instances, and/or other components for supporting functionality for continuous data verification in synchronous replication in the system100.

In other implementations of theFIG. 5embodiment, the VMs/container sets502comprise respective containers implemented using virtualization infrastructure504that 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 functionality for continuous data verification in synchronous replication 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, data verification logic and/or other components for supporting functionality for continuous data verification in synchronous replication in the system100.

As is apparent from the above, one or more of the processing modules or other components of system100may 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 infrastructure500shown inFIG. 5may represent at least a portion of one processing platform. Another example of such a processing platform is processing platform600shown inFIG. 6.

The processing platform600in this embodiment comprises a portion of system100and includes a plurality of processing devices, denoted602-1,602-2,602-3, . . .602-K, which communicate with one another over a network604.

The processing device602-1in the processing platform600comprises a processor610coupled to a memory612.

The processor610may 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 memory612may comprise random access memory (RAM), read-only memory (ROM), flash memory or other types of memory, in any combination. The memory612and 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.

Also included in the processing device602-1is network interface circuitry614, which is used to interface the processing device with the network604and other system components, and may comprise conventional transceivers.

The other processing devices602of the processing platform600are assumed to be configured in a manner similar to that shown for processing device602-1in the figure.

For example, other processing platforms used to implement illustrative embodiments can comprise converged infrastructure such as VxRail™, VxRack™, VxRack™ FLEX, VxBlock™ or Vblock® converged infrastructure from Dell EMC.