Abstract:
In one aspect, a method includes replicating a first volume in a data protection system by sending data from the first volume to a replica site, determining whether the replica site should transition from a journal mode to a non-journal mode or transition from the non-journal to the journal mode based on the performance of the data protection system, transitioning the replica site from the journal mode to the non-journal mode if the performance of the data protection system degrades based on a first performance threshold and transitioning the replica site from the non-journal mode to the journal mode if the performance of the data protection system improves based on a second performance threshold.

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 replicating a first volume in a data protection system by sending data from the first volume to a replica site, determining whether the replica site should transition from a journal mode to a non-journal mode or transition from the non-journal to the journal mode based on the performance of the data protection system, transitioning the replica site from the journal mode to the non-journal mode if the performance of the data protection system degrades based on a first performance threshold and transitioning the replica site from the non-journal mode to the journal mode if the performance of the data protection system improves based on a second performance threshold. 
     In another aspect, an apparatus includes electronic hardware circuitry configured to replicate a first volume in a data protection system by sending data from the first volume to a replica site, determine whether the replica site should transition from a journal mode to a non-journal mode or transition from the non-journal to the journal mode based on the performance of the data protection system, transition the replica site from the journal mode to the non-journal mode if the performance of the data protection system degrades based on a first performance threshold and transition the replica site from the non-journal mode to the journal mode if the performance of the data protection system improves based on a second performance threshold. 
     In a further aspect, an article includes a non-transitory computer-readable medium that stores computer-executable instructions. The instructions causing a machine to replicate a first volume in a data protection system by sending data from the first volume to a replica site, determine whether the replica site should transition from a journal mode to a non-journal mode or transition from the non-journal to the journal mode based on the performance of the data protection system, transition the replica site from the journal mode to the non-journal mode if the performance of the data protection system degrades based on a first performance threshold and transition the replica site from the non-journal mode to the journal mode if the performance of the data protection system improves based on a second performance threshold. 
    
    
     
       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 flowchart of an example of a process to transition from a journal mode to a non-journal mode. 
         FIG. 4  is a flowchart of an example of a process to transition from the non-journal mode to the journal mode. 
         FIG. 5  is a flowchart of an example of a process to move to the non-journal mode. 
         FIG. 6  is a flowchart of an example of a process to move to the journal mode. 
         FIG. 7  is a flowchart of an example of a process to access a point-in-time while taking snapshots. 
         FIG. 8  is a flowchart of an example of a process to perform replication in a snapshot shipping mode with and without journaling at the replica site. 
         FIG. 9A  is a block diagram that depicts a journal mode and a non-journal mode in a continuous data protection mode. 
         FIG. 9B  is a block diagram that depicts a journal mode and a non-journal mode in a snapshot mode. 
         FIG. 10  is a flowchart of an example of a process to restore a snapshot from a journal. 
         FIG. 11  is a computer on which any of the processes of  FIGS. 4 to 8 and 10  may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein is a data replication system that may dynamically change from a journaling mode to a non-journaling mode and visa-versa for a data replication system that performs snapshots or continuous data protection, for example. 
     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; 
     BOOKMARK—a bookmark is metadata information stored in a replication journal which indicates a point in time. 
     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 (sometimes referred to as an I/O), which may be a read I/O request (sometimes referred to as a read request or a read) or a write I/O request (sometimes referred to as a write request or a write); 
     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; 
     JOURNAL MODE—a journal is used on the replica site; 
     NON-JOURNAL MODE—a journal is not used on the replica site; 
     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 (splitter) that data has been received at the DPA; this may be achieved by an SCSI status command. 
     SAN—a storage area network of nodes that send and receive an 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, sometimes called a primary 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, sometimes called a secondary 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. 
     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 or replica 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 (sometimes referred to as a 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. 
     The data replication system  100  in  FIG. 1  may be further modified to dynamically change. For example, the data replication system  100  may dynamically change from performing journaling (a journaling mode) to non-journaling (a non-journaling mode) and visa-versa for a data replication system that performs snapshots or continuous data protection, for example. 
     Referring to  FIG. 3 , an example of a process to transition from a journal mode to a non-journal mode is a process  300 . Process  300  sends data to the replica site ( 306 ). For example, I/O data is sent to the replica storage. 
     Process  300  determines if the replica storage should remain in journaling mode ( 312 ). For example, process  300  determines if the performance of the data protection system  100  remains at performing at a performance level that supports journaling. In one particular example, process  300  determines if the storage bandwidth is still available to perform the journaling. In another particular example, response times are below a certain threshold. In a further example, the current storage throughput is not below a certain threshold. 
     Process  300  continues to send data to the replica site ( 306 ) if the replica storage should remain in the journal mode because its performance has not degraded based on a predetermined performance threshold. Process  300  moves to non-journal mode ( 316 ) if the replica storage should not remain in journal mode, if, for example, the replica storage system&#39;s response time is higher than a threshold or journal lag grows to be larger than a threshold, or required storage bandwidth is higher than a threshold. 
     Referring to  FIG. 4 , an example of a process to transition from a non-journal mode to a journal mode is a process  400 . Process  400  sends data to the replica site ( 406 ). For example, I/O data is sent to the replica storage. Process  400  determines if the replica storage should move to the journaling mode ( 412 ). For example, process  400  determines if the performance of the data protection system  100  is now performing at a performance level that supports journaling. In one particular example, process  400  determines if the storage bandwidth is now available to perform the journaling. In one particular example, the response times are now below a certain threshold or the I/O throughput is now below a certain threshold. 
     Process  400  continues to send data to the replica site ( 406 ) if the replica storage should remain in the non-journal mode. Process  400  moves to the journal mode ( 416 ) if the replica storage can move to the journal mode because its performance based one predetermined performance threshold. 
     Referring to  FIG. 5 , an example of a process to move to the non-journal mode is a process  500 . Process  500  generates a storage array snapshot of a replica volume at the replica site ( 502 ). 
     Process  500  generates a data structure of metadata in the DO METADATA Stream (sometimes called a redo log) ( 506 ). The data structure includes locations that were overwritten after moving to the non-journal node. For example, the data structure includes a list of the locations which are currently written in the DO stream and thus are currently different between the production volume and the snapshot taken. The data in the DO stream for these locations has to be applied to the replica volume before erasing the journal. The data structure is used so that if a new IO arrives to a location which is in the journal the data in the journal will not overwrite the newer data and will be discarded. 
     Process  500  writes new I/Os arriving from production volume directly to the replica volume ( 508 ). Process  500  reads I/Os from DO stream (redo log) ( 510 ) and determines if there is no data in the DO stream ( 512 ) (i.e., process  300  determines if all data from the DO stream was applied to the replica volume and thus the replica volume now has a consistent image of the data). If there is no data in the DO stream, the journal is erased and the process now moves to non-journal based replication (i.e., the new arriving I/Os are no longer tracked in the journal and the journal DO and UNDO streams are empty). 
     Process  500  periodically generates snapshots of the replica volume at certain points in time at the replica site ( 532 ). The replica volume is a replica of the production volume at the replica site, thus all the snapshots at the replica site are taken from the replica volume. 
     Process  500  maintains a list of the snapshots ( 536 ). The snapshots are storage array based and stored in the storage array. The list of snapshots may be stored in the journal and managed by the DPA. Process  500  erases snapshots from the storage array based on a policy ( 542 ). 
     If there is data in the DO stream (i.e., there is data in the journal which must be applied to the replica volume before the journal is erased), process  500  determines if the data read from the journal was overwritten by new data arriving from the production site ( 518 ). Since new I/Os from the production site are written in processing block  508  directly to the replica volume, the data which is currently still in the DO stream still needs to be applied to the replica volume to make it consistent with the production volume unless it was overwritten by newer data from the production site in processing block  508 . Thus overwritten data from the journal is discarded. 
     If the data is overwritten, process  500  discards the data ( 522 ) and repeats processing block  510  and reads I/Os from the DO stream. If the data is not overwritten, process  500  writes the data to the replica volume ( 526 ) and repeats processing block  510  by reading I/Os from the DO stream. 
     Referring to  FIG. 6 , an example of a process to move to the journal mode is a process  600 . Process  600  starts writing new data to the journal ( 602 ) and periodically generates a snapshot at the storage array ( 612 ). 
     Referring to  FIG. 7 , an example of a process to access a point-in-time while taking array based snapshots using journaling is a process  700 . Process  700  reverts to the array snapshot closest to the point-in-time ( 702 ) and rolls data from snapshot to the PIT ( 712 ). 
     Referring to  FIG. 8 , an example of a process to perform replication in a snapshot shipping mode with and without journaling at the replica site is a process  800 . 
     Process  800  periodically generates array snapshots of the production volume ( 820 ) and sends the differences between snapshots to the replica site ( 822 ). Process  800  determines if the replica is in a journal mode ( 828 ) and if the replica is in a journal mode, process  800  writes snapshot data to the replica DO stream ( 632 ) and periodically generates storage snapshots at the replica site when data of full snapshot is applied ( 834 ). 
     If the replica is not in a journal mode process  800  writes a snapshot to the replica volume ( 842 ) and generates a snapshot of the replica volume ( 846 ). Process  800  periodically erases old array snapshots if needed ( 848 ). 
       FIG. 9A  is a block diagram that depicts a journal mode and use of a non-journal mode in continuous data protection. For example, an array snapshot  912   a  is generated and after a period of time, a second array snapshot  912   a  is taken and after a period of time, a third array snapshot  912   c  is taken. Between generation of the array snapshot  912   a  and the array snapshot  912   b  a DO stream  916   a  is populated (i.e., the system is in the journaling mode). Between the array snapshot  912   a  and the array snapshot  912   b  any point-in-time may be accessible. However, between the array snapshot  912   b  and the array snapshot  912   c  there is no journaling and thus there is no accessibility between the array snapshots  912   b ,  912   c.    
       FIG. 9B  is a block diagram that depicts a journal mode and a non-journal mode using snapshots. For example, an array snapshot  922   a  is generated and after a period of time, a second array snapshot  922   b  is generated and after a period of time a third array snapshot  922   c  is generated. Between the array snapshot  922   a  and the array snapshot  922   b  a series of journal snapshots (e.g., a journal snapshot  932   a , a journal snapshot  932   b , and a journal snapshot  932   c ) are taken (i.e., there is a journal containing the differences between point in time  922   a  and  932   a , the differences from  932   a  to  932   b  and the differences from the journal  932   b  to the journal snapshot  932   c  (which is a point-in-time identical to the array snapshot  922   b )). Between the array snapshot  922   a  and the array snapshot  922   b  the point-in-time journal snapshots  932   a ,  932   b  are accessible by rolling the journal to the point-in-time  922   a . However, between the array snapshot  922   b  and the array snapshot  922   c  there are no journaling snapshots and thus there is no accessibility between the array snapshots  922   b ,  922   c.    
     Referring to  FIG. 10 , an example of a process to restore a snapshot from a journal is a process  1000 . Process  1000  restores array snapshot before the journal snapshot ( 1002 ) and rolls journal to PIT ( 1012 ). For example, if a user wants to roll back to a time during the journal snapshot  932   a , the array snapshot  922   a  is restored and the journal rolls to the time using the journal snapshot  932   a.    
     Referring to  FIG. 11 , in one example, a computer  1100  includes a processor  1102 , a volatile memory  1104 , a non-volatile memory  1106  (e.g., hard disk) and the user interface (UI)  1108  (e.g., a graphical user interface, a mouse, a keyboard, a display, touch screen and so forth). The non-volatile memory  1106  stores computer instructions  1112 , an operating system  1116  and data  1118 . In one example, the computer instructions  1112  are executed by the processor  1102  out of volatile memory  1104  to perform all or part of the processes described herein (e.g., processes  300 ,  400 ,  500 ,  600 ,  700 ,  800  and  1000 ). 
     The processes described herein (e.g., processes  300 ,  400 ,  500 ,  600 ,  700 ,  800  and  1000 ) are not limited to use with the hardware and software of  FIG. 11 ; 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 non-transitory machine-readable 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 non-transitory machine-readable storage medium such as, for example, a non-transitory computer-readable medium), 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 non-transitory machine-readable medium that is readable by a general or special purpose programmable computer for configuring and operating the computer when the non-transitory machine-readable medium is read by the computer to perform the processes described herein. For example, the processes described herein may also be implemented as a non-transitory 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  300 ,  400 ,  500 ,  600 ,  700 ,  800  and  1000  are not limited to the specific processing order of  FIGS. 4 to 8 and 10 , respectively. Rather, any of the processing blocks of  FIGS. 4 to 8 and 10  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  300 ,  400 ,  500 ,  600 ,  700 ,  800  and  1000 ) 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)). All or part of the system may be implemented using electronic hardware circuitry that include electronic devices such as, for example, at least one of a processor, a memory, a programmable logic device or a logic gate. 
     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.