Abstract:
In one aspect, a method includes receiving a request to access a virtual volume snapshot, preparing to bind the virtual volume snapshot, intercepting a command to prepare bind of the virtual volume snapshot, rolling back to a point in time corresponding to the requested virtual volume snapshot and generating a virtual volume snapshot in a storage array.

Description:
BACKGROUND 
     Computer data is vital to today&#39;s organizations and a significant part of protection against disasters is focused on data protection. As solid-state memory has advanced to the point where cost of memory has become a relatively insignificant factor, organizations can afford to operate with systems that store and process terabytes of data. 
     Conventional data protection systems include tape backup drives, for storing organizational production site data on a periodic basis. Another conventional data protection system uses data replication, by creating a copy of production site data of an organization on a secondary backup storage system, and updating the backup with changes. The backup storage system may be situated in the same physical location as the production storage system, or in a physically remote location. Data replication systems generally operate either at the application level, at the file system level, or at the data block level. 
     SUMMARY 
     In one aspect, a method includes receiving a request to access a virtual volume snapshot, preparing to bind the virtual volume snapshot, intercepting a command to prepare bind of the virtual volume snapshot, rolling back to a point in time corresponding to the requested virtual volume snapshot and generating a virtual volume snapshot in a storage array. 
     In another aspect, an article includes a non-transitory machine-readable medium that stores executable instructions. The instructions cause a machine to receive a request to access a virtual volume snapshot, prepare to bind the virtual volume snapshot, intercept a command to prepare bind of the virtual volume snapshot, roll back to a point in time corresponding to the requested virtual volume snapshot and generate a virtual volume snapshot in a storage array. 
     In a further aspect, an apparatus circuitry configured to receive a request to access a virtual volume snapshot, prepare to bind the virtual volume snapshot, intercept a command to prepare bind of the virtual volume snapshot, roll back to a point in time corresponding to the requested virtual volume snapshot and generate a virtual volume snapshot in a storage array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example of a data protection system. 
         FIG. 2  is an illustration of an example of a journal history of write transactions for a storage system. 
         FIG. 3  is a diagram of a virtual storage environment. 
         FIG. 4  is a flowchart of an example of a process to generate a virtual volume snapshot. 
         FIG. 5  is a flowchart of an example of a process to access a virtual volume snapshot. 
         FIG. 6  is a flowchart of an example of a process to unbind a virtual volume snapshot. 
         FIG. 7  is a computer on which any of the processes of  FIGS. 4 to 6  may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     Virtual volumes are a new storage abstraction to store virtual machines (VM). Virtual volumes allow for millions of snapshots to be generated. Described herein are techniques to allow a user to generate snapshots and to allow the user to access those snapshots that the user wants to access. 
     The following definitions may be useful in understanding the specification and claims. 
     BACKUP SITE—a facility where replicated production site data is stored; the backup site may be located in a remote site or at the same location as the production site; 
     DATA PROTECTION APPLIANCE (DPA)—a computer or a cluster of computers responsible for data protection services including inter alia data replication of a storage system, and journaling of I/O requests issued by a host computer to the storage system; 
     HOST—at least one computer or networks of computers that runs at least one data processing application that issues I/O requests to one or more storage systems; a host is an initiator with a SAN; 
     HOST DEVICE—an internal interface in a host, to a logical storage unit; 
     IMAGE—a copy of a logical storage unit at a specific point in time; 
     INITIATOR—a node in a SAN that issues I/O requests; 
     I/O REQUEST—an input/output request which may be a read I/O request (read request) or a write I/O request (write request), also referred to as an I/O; 
     JOURNAL—a record of write transactions issued to a storage system; used to maintain a duplicate storage system, and to roll back the duplicate storage system to a previous point in time; 
     LOGICAL UNIT—a logical entity provided by a storage system for accessing data from the storage system. The logical disk may be a physical logical unit or a virtual logical unit; 
     LUN—a logical unit number for identifying a logical unit; 
     PHYSICAL LOGICAL UNIT—a physical entity, such as a disk or an array of disks, for storing data in storage locations that can be accessed by address; 
     PRODUCTION SITE—a facility where one or more host computers run data processing applications that write data to a storage system and read data from the storage system; 
     REMOTE ACKNOWLEDGEMENTS—an acknowledgement from remote DPA to the local DPA that data arrived at the remote DPA (either to the appliance or the journal) 
     SPLITTER ACKNOWLEDGEMENT—an acknowledgement from a DPA to the protection agent that data has been received at the DPA; this may be achieved by SCSI status cmd. 
     SAN—a storage area network of nodes that send and receive I/O and other requests, each node in the network being an initiator or a target, or both an initiator and a target; 
     SOURCE SIDE—a transmitter of data within a data replication workflow, during normal operation a production site is the source side; and during data recovery a backup site is the source side; 
     STORAGE SYSTEM—a SAN entity that provides multiple logical units for access by multiple SAN initiators 
     TARGET—a node in a SAN that replies to I/O requests; 
     TARGET SIDE—a receiver of data within a data replication workflow; during normal operation a back site is the target side, and during data recovery a production site is the target side; 
     VIRTUAL LOGICAL UNIT—a virtual storage entity which is treated as a logical unit by virtual machines; 
     WAN—a wide area network that connects local networks and enables them to communicate with one another, such as the Internet. 
     A description of journaling and some techniques associated with journaling may be described in the patent titled “METHODS AND APPARATUS FOR OPTIMAL JOURNALING FOR CONTINUOUS DATA REPLICATION” and with U.S. Pat. No. 7,516,287, which is hereby incorporated by reference. 
     An Example of a Replication System 
     Referring to  FIG. 1 , a data protection system  100  includes two sites; Site I, which is a production site, and Site II, which is a backup site. Under normal operation the production site is the source side of system  100 , and the backup site is the target side of the system. The backup site is responsible for replicating production site data. Additionally, the backup site enables roll back of Site I data to an earlier pointing time, which may be used in the event of data corruption of a disaster, or alternatively in order to view or to access data from an earlier point in time. 
       FIG. 1  is an overview of a system for data replication of either physical or virtual logical units. Thus, one of ordinary skill in the art would appreciate that in a virtual environment a hypervisor, in one example, would consume logical units and generate a distributed file system on them such as VMFS creates files in the file system and expose the files as logical units to the virtual machines (each VMDK is seen as a SCSI device by virtual hosts). In another example, the hypervisor consumes a network based file system and exposes files in the NFS as SCSI devices to virtual hosts. 
     During normal operations, the direction of replicate data flow goes from source side to target side. It is possible, however, for a user to reverse the direction of replicate data flow, in which case Site I starts to behave as a target backup site, and Site II starts to behave as a source production site. Such change of replication direction is referred to as a “failover”. A failover may be performed in the event of a disaster at the production site, or for other reasons. In some data architectures, Site I or Site II behaves as a production site for a portion of stored data, and behaves simultaneously as a backup site for another portion of stored data. In some data architectures, a portion of stored data is replicated to a backup site, and another portion is not. 
     The production site and the backup site may be remote from one another, or they may both be situated at a common site, local to one another. Local data protection has the advantage of minimizing data lag between target and source, and remote data protection has the advantage is being robust in the event that a disaster occurs at the source side. 
     The source and target sides communicate via a wide area network (WAN)  128 , although other types of networks may be used. 
     Each side of system  100  includes three major components coupled via a storage area network (SAN); namely, (i) a storage system, (ii) a host computer, and (iii) a data protection appliance (DPA). Specifically with reference to  FIG. 1 , the source side SAN includes a source host computer  104 , a source storage system  108 , and a source DPA  112 . Similarly, the target side SAN includes a target host computer  116 , a target storage system  120 , and a target DPA  124 . As well, the protection agent (splitter) may run on the host, or on the storage, or in the network or at a hypervisor level, and that DPAs are optional and DPA code may run on the storage array too, or the DPA  124  may run as a virtual machine. 
     Generally, a SAN includes one or more devices, referred to as “nodes”. A node in a SAN may be an “initiator” or a “target”, or both. An initiator node is a device that is able to initiate requests to one or more other devices; and a target node is a device that is able to reply to requests, such as SCSI commands, sent by an initiator node. A SAN may also include network switches, such as fiber channel switches. The communication links between each host computer and its corresponding storage system may be any appropriate medium suitable for data transfer, such as fiber communication channel links. 
     The host communicates with its corresponding storage system using small computer system interface (SCSI) commands. 
     System  100  includes source storage system  108  and target storage system  120 . Each storage system includes physical storage units for storing data, such as disks or arrays of disks. Typically, storage systems  108  and  120  are target nodes. In order to enable initiators to send requests to storage system  108 , storage system  108  exposes one or more logical units (LU) to which commands are issued. Thus, storage systems  108  and  120  are SAN entities that provide multiple logical units for access by multiple SAN initiators. 
     Logical units are a logical entity provided by a storage system, for accessing data stored in the storage system. The logical unit may be a physical logical unit or a virtual logical unit. A logical unit is identified by a unique logical unit number (LUN). Storage system  108  exposes a logical unit  136 , designated as LU A, and storage system  120  exposes a logical unit  156 , designated as LU B. 
     LU B is used for replicating LU A. As such, LU B is generated as a copy of LU A. In one embodiment, LU B is configured so that its size is identical to the size of LU A. Thus for LU A, storage system  120  serves as a backup for source side storage system  108 . Alternatively, as mentioned hereinabove, some logical units of storage system  120  may be used to back up logical units of storage system  108 , and other logical units of storage system  120  may be used for other purposes. Moreover, there is symmetric replication whereby some logical units of storage system  108  are used for replicating logical units of storage system  120 , and other logical units of storage system  120  are used for replicating other logical units of storage system  108 . 
     System  100  includes a source side host computer  104  and a target side host computer  116 . A host computer may be one computer, or a plurality of computers, or a network of distributed computers, each computer may include inter alia a conventional CPU, volatile and non-volatile memory, a data bus, an I/O interface, a display interface and a network interface. Generally a host computer runs at least one data processing application, such as a database application and an e-mail server. 
     Generally, an operating system of a host computer creates a host device for each logical unit exposed by a storage system in the host computer SAN. A host device is a logical entity in a host computer, through which a host computer may access a logical unit. Host device  104  identifies LU A and generates a corresponding host device  140 , designated as Device A, through which it can access LU A. Similarly, host computer  116  identifies LU B and generates a corresponding device  160 , designated as Device B. 
     In the course of continuous operation, host computer  104  is a SAN initiator that issues I/O requests (write/read operations) through host device  140  to LU A using, for example, SCSI commands. Such requests are generally transmitted to LU A with an address that includes a specific device identifier, an offset within the device, and a data size. Offsets are generally aligned to 512 byte blocks. The average size of a write operation issued by host computer  104  may be, for example, 10 kilobytes (KB); i.e., 20 blocks. For an I/O rate of 50 megabytes (MB) per second, this corresponds to approximately 5,000 write transactions per second. 
     System  100  includes two data protection appliances, a source side DPA  112  and a target side DPA  124 . A DPA performs various data protection services, such as data replication of a storage system, and journaling of I/O requests issued by a host computer to source side storage system data. As explained in detail herein, when acting as a target side DPA, a DPA may also enable roll back of data to an earlier point in time, and processing of rolled back data at the target site. Each DPA  112  and  124  is a computer that includes inter alia one or more conventional CPUs and internal memory. 
     For additional safety precaution, each DPA is a cluster of such computers. Use of a cluster ensures that if a DPA computer is down, then the DPA functionality switches over to another computer. The DPA computers within a DPA cluster communicate with one another using at least one communication link suitable for data transfer via fiber channel or IP based protocols, or such other transfer protocol. One computer from the DPA cluster serves as the DPA leader. The DPA cluster leader coordinates between the computers in the cluster, and may also perform other tasks that require coordination between the computers, such as load balancing. 
     In the architecture illustrated in  FIG. 1 , DPA  112  and DPA  124  are standalone devices integrated within a SAN. Alternatively, each of DPA  112  and DPA  124  may be integrated into storage system  108  and storage system  120 , respectively, or integrated into host computer  104  and host computer  116 , respectively. Both DPAs communicate with their respective host computers through communication lines such as fiber channels using, for example, SCSI commands or any other protocol. 
     DPAs  112  and  124  are configured to act as initiators in the SAN; i.e., they can issue I/O requests using, for example, SCSI commands, to access logical units on their respective storage systems. DPA  112  and DPA  124  are also configured with the necessary functionality to act as targets; i.e., to reply to I/O requests, such as SCSI commands, issued by other initiators in the SAN, including inter alia their respective host computers  104  and  116 . Being target nodes, DPA  112  and DPA  124  may dynamically expose or remove one or more logical units. 
     As described hereinabove, Site I and Site II may each behave simultaneously as a production site and a backup site for different logical units. As such, DPA  112  and DPA  124  may each behave as a source DPA for some logical units, and as a target DPA for other logical units, at the same time. 
     Host computer  104  and host computer  116  include protection agents  144  and  164 , respectively. Protection agents  144  and  164  intercept SCSI commands issued by their respective host computers, via host devices to logical units that are accessible to the host computers. A data protection agent may act on an intercepted SCSI commands issued to a logical unit, in one of the following ways: send the SCSI commands to its intended logical unit; redirect the SCSI command to another logical unit; split the SCSI command by sending it first to the respective DPA; after the DPA returns an acknowledgement, send the SCSI command to its intended logical unit; fail a SCSI command by returning an error return code; and delay a SCSI command by not returning an acknowledgement to the respective host computer. 
     A protection agent may handle different SCSI commands, differently, according to the type of the command. For example, a SCSI command inquiring about the size of a certain logical unit may be sent directly to that logical unit, while a SCSI write command may be split and sent first to a DPA associated with the agent. A protection agent may also change its behavior for handling SCSI commands, for example as a result of an instruction received from the DPA. 
     Specifically, the behavior of a protection agent for a certain host device generally corresponds to the behavior of its associated DPA with respect to the logical unit of the host device. When a DPA behaves as a source site DPA for a certain logical unit, then during normal course of operation, the associated protection agent splits I/O requests issued by a host computer to the host device corresponding to that logical unit. Similarly, when a DPA behaves as a target device for a certain logical unit, then during normal course of operation, the associated protection agent fails I/O requests issued by host computer to the host device corresponding to that logical unit. 
     Communication between protection agents and their respective DPAs may use any protocol suitable for data transfer within a SAN, such as fiber channel, or SCSI over fiber channel. The communication may be direct, or via a logical unit exposed by the DPA. Protection agents communicate with their respective DPAs by sending SCSI commands over fiber channel. 
     Protection agents  144  and  164  are drivers located in their respective host computers  104  and  116 . Alternatively, a protection agent may also be located in a fiber channel switch, or in any other device situated in a data path between a host computer and a storage system or on the storage system itself. In a virtualized environment, the protection agent may run at the hypervisor layer or in a virtual machine providing a virtualization layer. 
     What follows is a detailed description of system behavior under normal production mode, and under recovery mode. 
     In production mode DPA  112  acts as a source site DPA for LU A. Thus, protection agent  144  is configured to act as a source side protection agent; i.e., as a splitter for host device A. Specifically, protection agent  144  replicates SCSI I/O write requests. A replicated SCSI I/O write request is sent to DPA  112 . After receiving an acknowledgement from DPA  124 , protection agent  144  then sends the SCSI I/O write request to LU A. After receiving a second acknowledgement from storage system  108  host computer  104  acknowledges that an I/O command complete. 
     When DPA  112  receives a replicated SCSI write request from data protection agent  144 , DPA  112  transmits certain I/O information characterizing the write request, packaged as a “write transaction”, over WAN  128  to DPA  124  on the target side, for journaling and for incorporation within target storage system  120 . 
     DPA  112  may send its write transactions to DPA  124  using a variety of modes of transmission, including inter alia (i) a synchronous mode, (ii) an asynchronous mode, and (iii) a snapshot mode. In synchronous mode, DPA  112  sends each write transaction to DPA  124 , receives back an acknowledgement from DPA  124 , and in turns sends an acknowledgement back to protection agent  144 . Protection agent  144  waits until receipt of such acknowledgement before sending the SCSI write request to LU A. 
     In asynchronous mode, DPA  112  sends an acknowledgement to protection agent  144  upon receipt of each I/O request, before receiving an acknowledgement back from DPA  124 . 
     In snapshot mode, DPA  112  receives several I/O requests and combines them into an aggregate “snapshot” of all write activity performed in the multiple I/O requests, and sends the snapshot to DPA  124 , for journaling and for incorporation in target storage system  120 . In snapshot mode DPA  112  also sends an acknowledgement to protection agent  144  upon receipt of each I/O request, before receiving an acknowledgement back from DPA  124 . 
     For the sake of clarity, the ensuing discussion assumes that information is transmitted at write-by-write granularity. 
     While in production mode, DPA  124  receives replicated data of LU A from DPA  112 , and performs journaling and writing to storage system  120 . When applying write operations to storage system  120 , DPA  124  acts as an initiator, and sends SCSI commands to LU B. 
     During a recovery mode, DPA  124  undoes the write transactions in the journal, so as to restore storage system  120  to the state it was at, at an earlier time. 
     As described hereinabove, LU B is used as a backup of LU A. As such, during normal production mode, while data written to LU A by host computer  104  is replicated from LU A to LU B, host computer  116  should not be sending I/O requests to LU B. To prevent such I/O requests from being sent, protection agent  164  acts as a target site protection agent for host Device B and fails I/O requests sent from host computer  116  to LU B through host Device B. 
     Target storage system  120  exposes a logical unit  176 , referred to as a “journal LU”, for maintaining a history of write transactions made to LU B, referred to as a “journal”. Alternatively, journal LU  176  may be striped over several logical units, or may reside within all of or a portion of another logical unit. DPA  124  includes a journal processor  180  for managing the journal. 
     Journal processor  180  functions generally to manage the journal entries of LU B. Specifically, journal processor  180  enters write transactions received by DPA  124  from DPA  112  into the journal, by writing them into the journal LU, reads the undo information for the transaction from LU B. updates the journal entries in the journal LU with undo information, applies the journal transactions to LU B, and removes already-applied transactions from the journal. 
     Referring to  FIG. 2 , which is an illustration of a write transaction  200  for a journal. The journal may be used to provide an adaptor for access to storage  120  at the state it was in at any specified point in time. Since the journal contains the “undo” information necessary to roll back storage system  120 , data that was stored in specific memory locations at the specified point in time may be obtained by undoing write transactions that occurred subsequent to such point in time. 
     Write transaction  200  generally includes the following fields: one or more identifiers; a time stamp, which is the date &amp; time at which the transaction was received by source side DPA  112 ; a write size, which is the size of the data block; a location in journal LU  176  where the data is entered; a location in LU B where the data is to be written; and the data itself. 
     Write transaction  200  is transmitted from source side DPA  112  to target side DPA  124 . As shown in  FIG. 2 , DPA  124  records the write transaction  200  in the journal that includes four streams. A first stream, referred to as a DO stream, includes new data for writing in LU B. A second stream, referred to as an DO METADATA stream, includes metadata for the write transaction, such as an identifier, a date &amp; time, a write size, a beginning address in LU B for writing the new data in, and a pointer to the offset in the DO stream where the corresponding data is located. Similarly, a third stream, referred to as an UNDO stream, includes old data that was overwritten in LU B; and a fourth stream, referred to as an UNDO METADATA, include an identifier, a date &amp; time, a write size, a beginning address in LU B where data was to be overwritten, and a pointer to the offset in the UNDO stream where the corresponding old data is located. 
     In practice each of the four streams holds a plurality of write transaction data. As write transactions are received dynamically by target DPA  124 , they are recorded at the end of the DO stream and the end of the DO METADATA stream, prior to committing the transaction. During transaction application, when the various write transactions are applied to LU B, prior to writing the new DO data into addresses within the storage system, the older data currently located in such addresses is recorded into the UNDO stream. In some examples, the metadata stream (e.g., UNDO METADATA stream or the DO METADATA stream) and the data stream (e.g., UNDO stream or DO stream) may be kept in a single stream each (i.e., one UNDO data and UNDO METADATA stream and one DO data and DO METADATA stream) by interleaving the metadata into the data stream. 
     Referring to  FIG. 3 , the data protection system  100  can be modified to a continuous data protection (CDP)  300 . For example, the replication site (target side) and the production site (source side) are at the same site. In particular, the source and the target are the same machine. This allows a snapshot to be generated and stored locally. The CDP  300  includes a source-side virtual storage environment  302 . In this configuration, the host  104  is removed and replaced by a virtual machine  312 . The DPA  112  is replaced with a DPA  112 ′ which may either run as a virtual or physical machine. In one example, the DPA  112 ′ runs either in the virtual machine  312  or as set of processes in a storage array  308 . The source side data protection agent  144  is removed from the host  104  and replaced by a data protection agent  144 ′ at the storage array  308 . In other examples, the data protection agent  144 ′ is placed at a virtual server  306 . 
     In one example, the source side virtual storage environment  302  includes the virtual server  306  and the storage array  308 . The virtual server  306  includes the virtual machine  312 , which includes a virtual device  316 . In one example, the virtual server  306  is a VMWARE® ESX® server. 
     The storage array  308  includes the data protection agent  144 ′, a virtual volume API (Application Program Interface) provider  310 , a protocol endpoint  322 , a data virtual volume  324 , a metadata virtual volume  326 , and a key-value pair database for each virtual volume (e.g., a key-value pair database  336  for the metadata virtual volume  326  and a key-value pair database  338  for the data virtual volume  324 ). The data virtual volume  324  stores data associated with one virtual disk or virtual disk derivative (e.g., a snapshot). The storage array also includes a target data virtual volume  374  (a replica of the metadata virtual volume  324 ), a target metadata virtual volume  376  (a replica of the metadata virtual volume  326 ), a target key-value pair data base  386  (a replica of the key-value pair data base  336 ), a target key-value pair data base  388  (a replica of the key-value pair data base  338 ) and a journal  176 ′ (similar to the journal  176 ). 
     The virtual volume API provider  310  provides APIs to allow integration and use of components within the source side virtual storage environment  302 . For example it would allow a hypervisor (virtual server  306 ) to provision storage virtual volumes for virtual machines. The virtual volume API provider  310  may run in other locations than the storage array  308  such as on the virtual server  306  or in a virtual machine, which will be a different machine than virtual machine  312 , which is an application machine (e.g., when the data protection agent  144 ′ runs in a hypervisor level). In one example, the virtual volume API provider  310  is a VMWARE® vSphere Storage APIs—Storage Awareness (VASA) provider. 
     The virtual volume API provider  310  includes a data protection API agent  350 . The data protection API agent  350  is used to intercept any commands used to update the key-value pair databases  336 ,  338 . The data protection API agent  350  will notify the data protection agent  144 ′ (splitter) or the DPA  112 ′ on any change occurring to the key-value pair databases  336 ,  338 . 
     In one example, the virtual volumes  324 ,  326  may be exposed by a virtualization layer such as a virtual volume filter, and in this case the data protection agent  144 ′ runs in the virtualization layer and the virtual volume API provider  310  may run inside the virtualization layer or in a hypervisor. 
     In one particular example, the data protection agent  144 ′ runs in the hypervisor kernel, and in this case a second virtual volume API provider layer may run outside the storage array  308  intercepting the API commands and sending them to both data the data protection API agent  350 , which will run in the second virtual volume API provider and to first virtual volume API provider  310  running inside storage array  308  (in this case, the data protection agent  350  will not run inside the virtual volume API provider  310  but in the layered second virtual volume provider outside the storage array  308 ). 
     The key-value pair databases  336 ,  338  each include information about their respective virtual volume and other metadata information about their respective virtual volume to allow recovery of the system (e.g., to discover which virtual machines are available) in case of a failure. 
     Normally, key-value pairs from the key-value pair database are not used in a normal operation; but rather, used to salvage virtual machines from shared storage when the virtual server (e.g., the virtual server  306 ) databases are corrupted. During recovery, a key-match query operation is performed to rediscover “lost” virtual machines and virtual disks (e.g., the virtual machine  312  with both its metadata virtual volume  326  and data virtual volume  324 ). 
     In one example, a key-value pair are well-known keys. In particular, the definition of certain keys (and hence the interpretation of their values) are publicly available. In another example, the key-value pairs are VMWARE®-specific keys. In a further example, the key-value pairs are storage vendor specific keys. In some examples, the key-value pairs are encoded as UTF-8; and a maximum length of a key is 64 bytes and a maximum length of a value is 8 KB. 
     In one example, each virtual device is associated with one protocol endpoint and one data virtual volume. In one example, the virtual volumes are VMWARE® virtual volumes. In other example, multiple virtual devices may be associated with the same protocol endpoint. 
     Referring to  FIG. 4 , an example of a process to generate a snapshot is a process  400 . Process  400  receives a request to generate a snapshot ( 402 ). For example, a user using a user interface (e.g., a user interface  708  ( FIG. 7 )) requests that a virtual volume snapshot be generated. 
     Process  400  prepares for a virtual volume snapshot ( 408 ). For example, an API command is called by the virtual volume API provider  310 . Executing the command returns a unique ID of the new virtual volume snapshot to be generated. Executing the command also returns virtual volume information on the virtual volume being snapshot such as key value pair metadata so the virtual server can update it for the snapshot. Executing the command further returns space statistics on the virtual volume that is snapshot. In one example, the command is a VMWARE® command: PrepareToSnapshotVirtualVolume. 
     Process  400  generates a virtual volume snapshot ( 416 ). For example, an API command is called by the virtual volume AP provider  310  to generate the snapshot of the virtual volume and the virtual volume snapshot is generated. In one example, the unique ID is attached to the generated virtual volume snapshot. In one example, the command is a VMWARE® command: SnapshotVirtualVolume. 
     Process  400  generates a bookmark in the journal. For example, a bookmark is generated in the journal  176 ′ ( FIG. 3 ). In one example, metadata associated with the generated virtual volume snapshot, such as the unique ID, is also kept with the bookmark. At this point no real snapshot of the virtual volume is generated at the storage array  308 , just a bookmark. 
     Referring to  FIG. 5 , an example of a process to access a snapshot is a process  500 . Process  500  receives a request from the user to access a virtual volume at a requested point in time (e.g., a request to access a specific snapshot generated by the user) ( 502 ). For example, the user using a user interface (e.g., the user interface  708  ( FIG. 7 )) requests access to a virtual volume snapshot. 
     Process  500  prepares to bind (i.e., to allow access to) the virtual volume snapshot requested ( 506 ). For example, an API command is called by the virtual volume AP provider  310  to bind the virtual volume snapshot. In one example, the command is a VMWARE® command: prepareBindVirtualVolume. 
     Process  500  intercepts the prepare bind command ( 512 ). For example, the data protection API agent  350  intercepts the prepare bind command. 
     Process  500  rolls back to the point in time relevant to the requested virtual volume snapshot ( 522 ). For example, the data protection API agent  350  sends a command to roll back to the relevant bookmark in the journal  176 ′. Process  500  generates a real virtual volume snapshot in the storage array  308  ( 528 ) and allows the user access to the real virtual volume snapshot stored at the storage array  308  ( 536 ). 
     Referring to  FIG. 6 , an example of a process to unbind a virtual volume snapshot is a process  600 . Process  600  receives a request from the user to unbind the virtual volume snapshot stored at the storage array  308  ( 602 ). For example, the user using a user interface (e.g., the user interface  708  ( FIG. 7 )) requests to unbind the virtual volume snapshot stored at the storage array. 
     Process  600  unbinds the virtual volume snapshot ( 608 ). For example, an API command is called by the virtual volume AP provider  310  to unbind the virtual volume snapshot. In one example, the command is a VMWARE® command: unbind Virtual Volume. 
     Process  600  determines if there have been any changes to the real virtual volume snapshot ( 616 ). If there have been no changes to the real virtual volume snapshot, process  600  discards the real virtual volume snapshot stored at the storage array  308  ( 622 ). If there have been changes to the real virtual volume snapshot, process  600  does nothing and keeps the real virtual volume snapshot on the storage array  308 . 
     In other examples, a user can configure the CDP system  300  to discard virtual volume snapshots after unbinding based on preferences. For example, if a virtual volume snapshot is greater than a predetermined file size, the virtual volume snapshot is discarded after unbinding. 
     Referring to  FIG. 7 , a computer  700  includes a processor  702 , a volatile memory  704 , a non-volatile memory  706  (e.g., hard disk) and a user interface (UI)  708  (e.g., a mouse, a keyboard, a display, touch screen and so forth). The non-volatile memory  706  stores computer instructions  712 , an operating system  716  and data  718 . In one example, the computer instructions  712  are executed by the processor  702  out of volatile memory  704  to perform all or part of the processes described herein (e.g., processes  400 ,  500 ,  600 ). 
     The processes described herein (e.g., processes  400 ,  500 ,  600 ) are not limited to use with the hardware and software of  FIG. 7 ; they may find applicability in any computing or processing environment and with any type of machine or set of machines that is capable of running a computer program. The processes described herein may be implemented in hardware, software, or a combination of the two. The processes described herein may be implemented in computer programs executed on programmable computers/machines that each includes a processor, a storage medium or other article of manufacture that is readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and one or more output devices. Program code may be applied to data entered using an input device to perform any of the processes described herein and to generate output information. 
     The system may be implemented, at least in part, via a computer program product, (e.g., in a machine-readable storage device), for execution by, or to control the operation of, data processing apparatus (e.g., a programmable processor, a computer, or multiple computers)). Each such program may be implemented in a high level procedural or object-oriented programming language to communicate with a computer system. However, the programs may be implemented in assembly or machine language. The language may be a compiled or an interpreted language and it may be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program may be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. A computer program may be stored on a storage medium or device (e.g., CD-ROM, hard disk, or magnetic diskette) that is readable by a general or special purpose programmable computer for configuring and operating the computer when the storage medium or device is read by the computer to perform the processes described herein. The processes described herein may also be implemented as a machine-readable storage medium, configured with a computer program, where upon execution, instructions in the computer program cause the computer to operate in accordance with the processes. A non-transitory machine-readable medium may include but is not limited to a hard drive, compact disc, flash memory, non-volatile memory, volatile memory, magnetic diskette and so forth but does not include a transitory signal per se. 
     The processes described herein are not limited to the specific examples described. For example, the processes  400 ,  500 ,  600  are not limited to the specific processing order of  FIGS. 4 to 6 , respectively. Rather, any of the processing blocks of  FIGS. 4 to 6  may be re-ordered, combined or removed, performed in parallel or in serial, as necessary, to achieve the results set forth above. 
     The processing blocks (for example, in the processes  400 ,  500 ,  600 ) associated with implementing the system may be performed by one or more programmable processors executing one or more computer programs to perform the functions of the system. All or part of the system may be implemented as, special purpose logic circuitry (e.g., an FPGA (field-programmable gate array) and/or an ASIC (application-specific integrated circuit)). 
     Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Other embodiments not specifically described herein are also within the scope of the following claims.