Patent Publication Number: US-9846698-B1

Title: Maintaining point-in-time granularity for backup snapshots

Description:
A portion of the disclosure of this patent document may contain command formats and other computer language listings, all of which are subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     TECHNICAL FIELD 
     This invention relates generally to data backups, and more particularly to continuous data replication on deduplicated storage. 
     CROSS REFERENCE TO RELATED FILINGS 
     This application is related to co-pending U.S. patent application Ser. No. 14/108,021 for MAINTAINING BACKUP SNAPSHOT ON DEDUPLICATED STORAGE USING CONTINUOUS REPLICATION, U.S. patent application Ser. No. 14/108,032 for POINT-IN-TIME RECOVERY ON DEDUPLICATED STORAGE, U.S. patent application Ser. No. 14/108,002 for INITIALIZING BACKUP SNAPSHOTS ON DEDUPLICATED STORAGE, U.S. patent application Ser. No. 14/108,060 for MAINTAINING BACKUP SNAPSHOTS USING CONTINUOUS REPLICATION FOR MULTIPLE SOURCES, and U.S. patent application Ser. No. 14/108,072 for RECOVERING CORRUPT STORAGE SYSTEMS, all filed Dec. 16, 2013, and concurrently herewith and hereby incorporated by reference for all purposes. 
     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, and METHODS AND APPARATUS FOR OPTIMAL JOURNALING FOR CONTINUOUS DATA REPLICATION and with U.S. Pat. No. 8,332,687, which are hereby incorporated by reference. 
     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 backup drives for storing organizational production site data on a periodic basis. Such systems suffer from several drawbacks. First, they may require a system shutdown during backup since the data being backed up cannot be used during the backup operation. Second, they limit the points in time to which the production site can recover. For example, if data is backed up on a daily basis, there may be several hours of lost data in the event of a disaster. Third, the data recovery process itself may take a long time. 
     Another conventional data protection system uses data replication, by creating a copy of the organization&#39;s production site data 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Objects, features, and advantages of embodiments disclosed herein may be better understood by referring to the following description in conjunction with the accompanying drawings. The drawings are not meant to limit the scope of the claims included herewith. For clarity, not every element may be labeled in every figure. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments, principles, and concepts. Thus, features and advantages of the present disclosure will become more apparent from the following detailed description of exemplary embodiments thereof taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a simplified illustration of a data protection system, in accordance with an embodiment of the present disclosure; 
         FIG. 2  is a simplified illustration of a write transaction for a journal, in accordance with an embodiment of the present disclosure; 
         FIG. 3  is a system for initializing a backup snapshot, consistent with an embodiment of the present disclosure; 
         FIG. 4  is a flow chart for initializing a backup snapshot consistent with an embodiment of the present disclosure; 
         FIG. 5  is a system for initializing a backup snapshot, consistent with an embodiment of the present disclosure; 
         FIG. 6  is a system for synthesizing new backup snapshots, consistent with an embodiment of the present disclosure; 
         FIG. 7  is a flow chart for synthesizing new backup snapshots, consistent with an embodiment of the present disclosure; 
         FIG. 7A  is a flowchart for creating a synthesis plan consistent with an embodiment of the present disclosure; 
         FIG. 8  is a system for point-in-time recovery, consistent with an embodiment of the present disclosure; 
         FIG. 9  is a flowchart for point-in-time recovery consistent with an embodiment of the present disclosure; 
         FIG. 10  depicts data protection windows providing backup granularity, consistent with an embodiment of the present disclosure; 
         FIG. 11  depicts a flowchart for applying data protection windows, consistent with an embodiment of the present disclosure; 
         FIG. 12  depicts a system for mounting a recovery snapshot, consistent with an embodiment of the present disclosure; 
         FIG. 13  depicts a flowchart for mounting a recovery snapshot, consistent with an embodiment of the present disclosure; 
         FIG. 13A  depicts a flowchart for recovering data for a corrupt source data system consistent with an embodiment of the present disclosure; 
         FIG. 14  depicts a system for receiving application IO&#39;s from multiple data protection appliances, consistent with an embodiment of the present disclosure; 
         FIG. 15  depicts a flowchart for receiving application IO&#39;s from multiple data sources, consistent with an embodiment of the present disclosure; and 
         FIG. 16  depicts a general purpose computer system, consistent with an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. While the invention is described in conjunction with such embodiment(s), it should be understood that the invention is not limited to any one embodiment. On the contrary, the scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications, and equivalents. For the purpose of example, numerous specific details are set forth in the following description in order to provide a thorough understanding of the present invention. These details are provided for the purpose of example, and the present invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the present invention is not unnecessarily obscured. 
     It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium such as a computer readable storage medium or a computer network wherein computer program instructions are sent over optical or electronic communication links. Applications may take the form of software executing on a general purpose computer or be hardwired or hard coded in hardware. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention. 
     An embodiment of the invention will be described with reference to a data storage system in the form of a storage system configured to store files, but it should be understood that the principles of the invention are not limited to this configuration. Rather, they are applicable to any system capable of storing and handling various types of objects, in analog, digital, or other form. Although terms such as document, file, object, etc. may be used by way of example, the principles of the invention are not limited to any particular form of representing and storing data or other information; rather, they are equally applicable to any object capable of representing information. 
     Systems, processes, and methods are discussed herein for enabling continuous data backups to deduplicated storage. In some embodiments, an initial backup snapshot of a source storage system may be created on the deduplicated storage using a data protection appliance. As changes are made to the source storage system, the IO&#39;s may be continuously communicated to the deduplicated storage for backup and protection. 
     In some embodiments, the deduplicated storage and/or data protection appliance may maintain journals, including data journals and metadata journals, for synthesizing new backup snapshots and/or recovering files. The journals may include DO and UNDO information compiled from IO&#39;s communicated from the data protection appliance to the deduplicated storage. These IO&#39;s may be applied to a backup snapshot to restore the snapshot to a previous point-in-time, or may be used to synthesize a new snapshot. 
     In an embodiment, data protection windows may be defined based on policy or user preference. The data protection windows may be used to maintain snapshots and/or journals for designated periods of time. For example, short-term windows may maintain both snapshots and journals for any point-in-time recovery (assuming the point-in-time falls within the short-term window). Mid-term windows, in contrast, may delete journals but maintain all the snapshots created during a period, and long-term windows may delete all the journals and select snapshots. Defining different protection windows may allow point-in-time recovery for files accessed recently, while also providing reduced storage consumption for long-term backups. 
     The systems discussed herein may additionally allow backup snapshots to be synthesized on deduplicated storage from IO&#39;s provided from multiple data protection appliance. For example, two data protection appliances may protect a single SAN. Each of those data protection agents may report IO&#39;s to the deduplicated storage, and a single backup snapshot may be synthesized from the journals maintaining those IO&#39;s 
     The following non-limiting definitions may be helpful in understanding the specification and claims: 
     BACKUP SITE—may be 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; a backup site may be a virtual or physical site. 
     CDP—Continuous Data Protection, may refer to a full replica of a volume or a set of volumes along with a journal which allows any point in time access, the CDP copy is at the same site, and maybe the same storage array of the production site. 
     DATA PROTECTION APPLIANCE (“DPA”)—may be a computer or a cluster of computers, or a set of processes that serve as a data protection appliance, 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. The DPA may be a physical device, a virtual device running, or may be a combination of a virtual and physical device. 
     HOST—may be 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; a host may be a virtual machine. 
     HOST DEVICE—may be an internal interface in a host to a logical storage unit. 
     IMAGE—may be a copy of a logical storage unit at a specific point-in-time. 
     INITIATOR—may be a node in a SAN that issues I/O requests. 
     JOURNAL—may be a record of write transactions issued to a storage system. A journal may be used to maintain a duplicate storage system, and to rollback the duplicate storage system to a previous point-in-time. 
     LOGICAL UNIT—may be a logical entity provided by a storage system for accessing data from the storage system. 
     LUN—may be a logical unit number for identifying a logical unit. May also refer to one or more virtual disks or virtual LUNs, which may correspond to one or more Virtual Machines. As used herein, LUN and LU may be used interchangeably to refer to a LU. 
     PHYSICAL STORAGE UNIT—may be a physical entity, such as a disk or an array of disks, for storing data in storage locations that can be accessed by address. 
     PRODUCTION SITE—may be 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; may be a virtual or physical site. 
     RPA—may be replication protection appliance, and is another name for DPA. An RPA may be a virtual DPA or a physical DPA. 
     SAN—may be 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—may be 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. Source side may be a virtual or physical site. 
     SNAPSHOT—a snapshot may refer to an image or differential representations of an image, i.e. the snapshot may have pointers to the original volume, and may point to log volumes for changed locations. Snapshots may be combined into a snapshot array, which may represent different images over a time period. 
     SPLITTER/PROTECTION AGENT—may be an agent running either on a production host a switch or a storage array which can intercept IO and split them to a DPA and to the storage array, fail IO redirect IO or do any other manipulation to the IO; the splitter or protection agent may be used in both physical and virtual systems. The splitter may be in the IO stack of a system and may be located in the hypervisor for virtual machines. May be referred to herein as an Open Replicator Splitter (ORS). 
     STORAGE SYSTEM—may be a SAN entity that provides multiple logical units for access by multiple SAN initiators. 
     STREAMING—may mean transmitting data in real time, from a source to a destination, as the data is read or created. 
     SYNTHESIZE—may mean creating a new file using pointers from existing files, without actually copying the referenced data. For example, a new file representing a volume at a points-in-time may be created using pointers to a file representing a previous point-in-time, as well pointers to journal representing changes to the volume 
     TARGET—may be a node in a SAN that replies to I/O requests. 
     TARGET SIDE—may be 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; may be a virtual or physical site. 
     VIRTUAL VOLUME—may be a volume which is exposed to host by a virtualization layer, the virtual volume may be spanned across more than one site and or volumes. 
     VIRTUAL RPA (vRPA)/VIRTUAL DPA (vDPA)—may be an DPA running in a VM. 
     WAN—may be a wide area network that connects local networks and enables them to communicate with one another, such as the Internet. 
     Overview of a Backup System Using a Journaling Process 
     Reference is now made to  FIG. 1 , which is a simplified illustration of a data protection system  100 , in accordance with an embodiment of the present invention. Shown in  FIG. 1  are two sites; Site I, which is a production site, on the right, and Site II, which is a backup site, on the left. 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 rollback 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. 
     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 are also adaptable for use with the present invention. 
     In accordance with an embodiment of the present invention, 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 . 
     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. 
     In an embodiment of the present invention, 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. A logical unit is identified by a unique logical unit number (LUN). In an embodiment of the present invention, 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. 
     In an embodiment of the present invention, 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, in certain embodiments of the present invention, 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. In an embodiment of the present invention, 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 an embodiment of the present invention, 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 hereinbelow, when acting as a target side DPA, a DPA may also enable rollback 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. 
     In accordance with an embodiment of the present invention, 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. 
     In accordance with an embodiment of the present invention, 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. In accordance with an embodiment of the present invention, 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.   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. In an embodiment of the present invention, protection agents communicate with their respective DPAs by sending SCSI commands over fiber channel. 
     In an embodiment of the present invention, 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. Additionally or alternatively, the protection agent may be installed as part of the storage array IO stack. In some embodiments the DPA may be installed as a virtual appliance or as a set of processes inside the storage array. 
     What follows is a detailed description of system behavior under normal production mode, and under recovery mode. 
     In accordance with an embodiment of the present invention, 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 requests. A replicated SCSI I/O request is sent to DPA  112 . After receiving an acknowledgement from DPA  124 , protection agent  144  then sends the SCSI I/O request to LU A. Only after receiving a second acknowledgement from storage system  108  may host computer  104  initiate another I/O request. 
     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, in accordance with an embodiment of the present invention, 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. 
     In accordance with an embodiment of the present invention, 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  (i) enters write transactions received by DPA  124  from DPA  112  into the journal, by writing them into the journal LU, (ii) applies the journal transactions to LU B, and (iii) updates the journal entries in the journal LU with undo information and removes already-applied transactions from the journal. As described below, with reference to  FIGS. 2 and 3A-3D , journal entries include four streams, two of which are written when write transaction are entered into the journal, and two of which are written when write transaction are applied and removed from the journal. 
     Reference is now made to  FIG. 2 , which is a simplified illustration of a write transaction  200  for a journal, in accordance with an embodiment of the present invention. 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 rollback 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 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. 
     By recording old data, a journal entry can be used to “undo” a write transaction. To undo a transaction, old data is read from the UNDO stream in a reverse order, from the most recent data to the oldest data, for writing into addresses within LU B. Prior to writing the UNDO data into these addresses, the newer data residing in such addresses is recorded in the DO stream. 
     The journal LU is partitioned into segments with a pre-defined size, such as 1 MB segments, with each segment identified by a counter. The collection of such segments forms a segment pool for the four journaling streams described hereinabove. Each such stream is structured as an ordered list of segments, into which the stream data is written, and includes two pointers—a beginning pointer that points to the first segment in the list and an end pointer that points to the last segment in the list. 
     According to a write direction for each stream, write transaction data is appended to the stream either at the end, for a forward direction, or at the beginning, for a backward direction. As each write transaction is received by DPA  124 , its size is checked to determine if it can fit within available segments. If not, then one or more segments are chosen from the segment pool and appended to the stream&#39;s ordered list of segments. 
     Thereafter the DO data is written into the DO stream, and the pointer to the appropriate first or last segment is updated. Freeing of segments in the ordered list is performed by simply changing the beginning or the end pointer. Freed segments are returned to the segment pool for re-use. 
     A journal may be made of any number of streams including less than or more than 5 streams. Often, based on the speed of the journaling and whether the back-up is synchronous or a synchronous a fewer or greater number of streams may be used. 
     Initializing a Backup Snapshot on Deduplicated Storage 
       FIG. 3 ,  FIG. 4 , and  FIG. 5  depict systems and processes for initializing a backup snapshot on deduplicated storage consistent with an embodiment of the present disclosure. Before deduplicated storage can provide continuous backup protection, it may be necessary to create an initial backup snapshot of the source storage system. This initial backup snapshot may represent the earliest point-in-time backup that may be restored. As changes are made to the source storage system, journal files and/or new backups may be updated and/or synthesized to provide continuous protection. In some embodiments, the initial backup snapshot may be created by streaming IO&#39;s from a storage system scan to a data protection appliance, or by taking an initial snapshot of the storage system and transmitting the entire snapshot to deduplicated storage. 
       FIG. 3  depicts a system for creating an initial backup snapshot by scanning a source storage system and streaming IO&#39;s to the deduplicated storage. Data protection application  300  may comprise journal processor  302 , and may be in communication with deduplicated storage  304 . In an embodiment, deduplicated storage  304  may be target side storage residing at a backup site. Data protection appliance  300  may be similar to data protection appliance  112  and/or  124 , and may be responsible for streaming IO&#39;s to deduplicated storage  304 . 
     In an embodiment, a source storage system may be scanned and individual offsets may be streamed to data protection appliance  300 . The offsets streamed from the scanned system may be referred to as initialization IO&#39;s, and may be streamed sequentially to data protection appliance  300 . For example, the scanned system may comprise offsets 0, 1, 2, and 3, comprising data A, B, C, and D. The initial scan may start at the beginning of the system, and transmit offset 0, followed by offset 1, et seq. 
     As data protection appliance  300  receives the initialization IO&#39;s, journal processor  302  may identify the offset data and metadata, and may stream the IO&#39;s to metadata journal  306  and/or data journal  308  residing on deduplicated storage  304 . Data journal  308  may comprise data stored within an offset, and metadata  306  may include metadata associated with that offset. Metadata could include, for example, an offset identifier, size, write time, and device ID. These journals may then be used to synthesize a backup snapshot on deduplicated storage  304 , as discussed below. 
     In some embodiments, a scanned storage system may operate in a live environment. As a result, applications may be writing to the storage concurrently with the scan process. If an application writes to a location that has already been streamed, the journal files and ultimately the synthesized snapshot may be out of date. To address this issue, application IO&#39;s may be streamed concurrently with the initialization IO&#39;s if the application IO&#39;s are to an offset that has already been scanned. For example, consider Table 1: 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                 Offset/Time 
                 t0 
                 t1 
                 t2 
                 t3 
               
               
                   
               
             
            
               
                 0 
                 A 
                   
                   
                 A′ 
               
               
                 1 
                 B 
                 B′ 
                   
                   
               
               
                 2 
                 C 
                   
                   
                   
               
               
                 3 
                 D 
                   
                 D′ 
               
               
                   
               
            
           
         
       
     
     Table 1 depicts four different offsets, denoted as 0, 1, 2, and 3, and four times, t 0 , t 1 , t 2 , and t 3 . Letters A, B, C, and D may represent the data stored at the offsets. Time t 0  may represent the offsets as they exist when the scan begins. These offsets may be streamed to data protection appliance  300  sequentially from 0 to 3. At time t 1 , however, the data at offset 1 is modified by an application from B to B′. Similarly, at t 2  the data at offset 3 changes from D to D′, and at t 3  the data at offset 0 changes from A to A′. If the scan transmits the data at offset 1 before t 1 , B′ may be missed since the change occurred after offset 1 was scanned and B was transmitted. Similarly, if the scan has not reached offset 3 before t 2 , only D′ will be transmitted since D no longer exists. It may therefore be beneficial to transmit application IO&#39;s to data protection appliance  300  if those IO&#39;s write to an offset that has already been scanned. If the offset has not been scanned, it may not be necessary to transmit the application IO&#39;s because the change will be transmitted when the scan reaches that offset. 
     Turning back to  FIG. 3  and with continued reference to Table 1, offset metadata journal entries  310  and offset data journal entries  312  depict the state of metadata journal  306  and data journal  308  after the initial scan is complete. While there are only four offsets on the scanned storage system, there are six entries in the journal because the data in offset 0 and 1 was modified by an application after they were scanned. They each therefore have two entries: B and B′. Segment D was modified after the scan began, but before it was reached. Segment D therefore only has one entry: D′. 
     Metadata journal entries  310  and data journal entries  312  may include all of the data necessary to synthesize a backup snapshot of the scanned storage system. Data journal entries  312  may contain the actual data from the storage system: A, B, B′ C, A′ and D′. Note that data D is not in the data journal  308  since it was modified on the storage system before its offset was scanned and transmitted. Metadata journal entries  310  may include metadata about the offsets. For example, metadata journal entries  310  may include an offset identifier, offset length, and write time, and volume/device ID. In the present example, metadata journal entries may include the entries shown in Table 2:
         0. Vol A, offset=0; size=8 kb; time=t 0     1. Vol A, offset=8 kb; size=8 kb; time=t 0     2. Vol A, offset=8 kb; size=8 kb; time=t 1     3. Vol A, offset=16 kb; size=8 kb; time=t 0     4. Vol A, offset=0; size=8 kb; time=t 3     5. Vol A, offset=24 kb; size=8 kb; time=t 2         

     Table 2&#39;s metadata entries may correspond to the states shown in Table 1. The offset at location 0 may be offset 0, the offset at 8 kb may be offset 1, the offset at 16 kb may be offset 2, and the offset at 24 kb may be offset 3. The subscript of each journal entries  310  also identifies the offset associated with that metadata entry. 
     Deduplicated storage may use metadata journal  306  and data journal  308  to synthesize initial backup snapshot  314 . First, metadata journal  306  may be queried to identify the most recent data associated with each offset. Next, the data may be retrieved from journal data file  308  and synthesized into backup snapshot  314 . In some embodiments, synthesizing the backup snapshot may comprise creating and/or copying pointers rather than copying entire data blocks. This could be, for example, using a product such as EMC® DataDomain® Boost™ 
     For example, once the initial scan is complete, data journal  308  includes data A, B, B′, C, A′, and D′. A′ and B′ are the result of application IO&#39;s occurring during the scan process, and therefore represent the present state of offsets 0 and 1. To create backup snapshot  314 , deduplicated storage may therefore retrieve A′, B′, C, and D′ from the data journal  308  and synthesize them together. 
     Once initial backup snapshot  314  is synthesized, journal entries  310  and  312  may no longer be needed. In an embodiment, they may be removed from deduplicated storage  304  in order to conserve space. Alternatively, they may remain in the journals. 
     Turning now to  FIG. 4 , a process for creating an initial backup snapshot is discussed. At block  400 , sequential initialization IO&#39;s are received from a scanned storage volume. These IO&#39;s could be, for example, received at a data protection appliance, such as data protection appliance  300 . In some embodiments, the initialization IO&#39;s are read from the scanned storage volume by the data protection appliance. 
     At block  402 , the initialization IO&#39;s are streamed to a deduplicated storage. In an embodiment, the deduplicated storage may be substantially similar to deduplicated storage  304 . In some embodiments, the initialization IO&#39;s are streamed to a data journal using a data stream, and to a metadata journal using a metadata stream. Each stream may be a file in the deduplicated storage. Additionally or alternatively, writes to the journal files may be done though the DDBoost API or any other API. 
     At block  404 , the initialization IO&#39;s may be written to a journal on the deduplicated storage. This journal may be, for example, similar to metadata journal  306  and/or data journal  308 . In an embodiment, these journals may be in the same journal files. Alternatively, these may be separate files on the deduplicated storage system. 
     At block  406 , application IO&#39;s comprising writes to offsets on the scanned storage volume may be received. These application IO&#39;s may also be received at a data protection appliance, such as data protection appliance  300 . 
     At  408 , an offset associated with a specific application IO is identified, and at  410  it is determined whether the offset has already been streamed to the deduplicated storage. This determination could be made on data protection appliance  300  using journal processor  302 . If the offset has already been streamed, it must have already been scanned and included in an initialization  10 . If the offset has not been streamed, the storage volume scan may not have reached the offset on the storage volume. 
     At block  412 , the application IO is streamed to the deduplicated storage if its offset was included in a previously streamed initialization  10 . In an embodiment, the application IO is only streamed when its offset was included a previously streamed initialization  10 . Streaming the application IO when its offset was included in a previous initialization IO ensures that writes to the scanned volume are not missed during the initialization processes. In some embodiments, the application IO&#39;s are streamed to a data journal using a data stream, and to a metadata journal using a metadata stream. 
     In an embodiment, application IO&#39;s are not streamed if they comprise writes to an offset that has not yet been scanned and streamed in an initialization IO. This is because the data created by the write will be included in the initialization IO once the scan reaches that offset. This may reduce traffic between the data protection appliance and the deduplicated storage, and may reduce the workload on the deduplicated because the data will only be processed once. 
     At block  414 , the application IO is written to the journal. This journal may be the same journal as the initialization IO&#39;s, or it may be a separate journal. In an embodiment, the journal is data journal  308  and/or metadata journal  306 . 
     Finally, at  416 , a backup snapshot is synthesized from the initialization  10 &#39;s and the application IO&#39;s. This snapshot may be substantially similar to snapshot  314 . In an embodiment, the snapshot is synthesized by creating data pointers in a new file on the deduplicated storage. Additionally or alternatively, the pointers may be copied from the data journal. These pointers may point to the data referenced and/or contained in the journal. Synthesizing the snapshot using pointers may improve performance, as the data may not need to be replicated. 
       FIG. 5  depicts an additional or alternative embodiment for initializing a backup snapshot. The system shown in  FIG. 5  may include data protection appliance  500 , journal processor  502 , and deduplicated storage  504 . These elements may be substantially similar to those discussed in reference to  FIG. 3 . 
     Data protection appliance  500  may take a snapshot of a storage system and transmit that snapshot to deduplicated storage  504  for storage as a file. In an embodiment, this is different than streaming initialization IO&#39;s and synthesizing a snapshot from journal files. Rather than creating the snapshot on the deduplicated storage, the backup snapshot is created using the data protection appliance and transmitted to deduplicated storage to be stored as backup snapshot  514 . 
     In an embodiment, journal processor  502  may stream application IO&#39;s to deduplicated storage, and those application IO&#39;s may be stored in metadata journal  506  and data journal  508 . Like the journals of  FIG. 3 , metadata journal  506  may include metadata journal entries  510 , and data journal  508  may include data journal entries  512 . These journals may be used to synthesize a second backup snapshot or enable point-in-time recovery, as discussed below. 
     The systems and processes discussed in reference to  FIG. 3 ,  FIG. 4 , and  FIG. 5  enable a system to create an initial backup snapshot. Once the initial snapshot is created, additional processes may enable continuous data protection and point-in-time recovery. These processes will now be discussed. 
     Maintaining Backup Snapshots with Continuous Data Replication 
     With reference now to  FIG. 6 , a system and process for maintaining backups using continuous data replication is discussed. As datasets increase in size, backing them up to remote or local backup devices becomes increasingly costly and complex. Additionally, traditional backup processes may not allow point-in-time recovery since the backups occur on a periodic, rather than continuous, basis. The methods and systems discussed herein provide continuous backup protection as writes are made to a source device, thereby reducing backup cost and complexity, and may allowing point-in-time recovery for backed up files. 
     The system of  FIG. 6  includes data protection appliance  600 , journal processor  602 , and deduplicated storage  604 . These elements may be substantially similar to those discussed in reference to  FIG. 3 . Deduplicated storage  604  may include backup snapshot  614 , metadata journal file  606 , and data journal file  608 . In an embodiment, backup snapshot file  614  is synthesized in a manner substantially similar to backup snapshot  314 , and may be created using metadata journal entries  610  and data journal entries  612 . 
     As users, applications, and other processes access and use the source storage system, data on that system may change and/or new data may be created. As a result, initial backup snapshot  614  may become stale. If the source storage system should fail, there is a chance that any new or modified data may be lost. To address this concern, data protection appliance  600  may receive and stream application IO&#39;s to deduplicated storage system  604  on a continuous basis, even after initial backup snapshot  614  is synthesized. Streaming the application IO&#39;s allows the backups on deduplicated storage  604  to remain up-to-date, without needing to perform additional backups of large datasets. This may reduce network traffic, reduce workloads, and conserve space on deduplicated storage  604 . 
     For example, new metadata entries  611  and new data journal entries  613  represent IO&#39;s made after initial backup snapshot  614  was synthesized. These entries may be written to metadata journal  606  and data journal  608 , as shown in  FIG. 6 , or they may be written to separate journal files. In  FIG. 6 , data A′ and C were modified on the source storage device, and the journal entries therefore comprise A″ and C′. 
     Periodically, new backup snapshots may be synthesized from a previous backup snapshot and new journal entries. For example, second backup snapshot  616  may be synthesized from initial backup snapshot  614 , new metadata journal entries  611 , and new data journal entries  613 . Second backup snapshot  616  may be used to restore source storage system up to the point-in-time the last journal entry was received. In other words, backup snapshot  616  represents a backup of the source storage system at a later timestamp than initial backup snapshot  614 . 
     In an embodiment, synthesizing second backup journal entry  616  may be substantially similar to synthesizing the initial backup snapshot  614 . Rather than synthesizing all of the data from data journal  608 , however, unchanged data may be synthesized from initial backup snapshot  614 . In an embodiment, this synthesis may comprise copying and/or creating a data pointer. For example, in  FIG. 6  the solid arrows between initial backup snapshot  614  and second backup snapshot  616  represent unchanged data that is common between the two. In this case, only B′ and D′ remain unchanged. The dashed arrows represent new or changed data that needs to be synthesized into second backup snapshot  616 . In  FIG. 6 , A′ is changed to A″, C is change to C′. Synthesizing the data into second backup snapshot  616  therefore results in A″, B′, C′, D′. 
     Additionally or alternatively, second backup snapshot  616  may be synthesized entirely from journal entries. Rather than synthesizing unchanged data from initial backup  614 , deduplicated storage  604  may retrieve the unchanged data from data journal entries  612 . For example, B′ and D′ may be synthesized from data journal entries  612  rather than from initial backup snapshot  614 . 
     Additional backup snapshots, such as second backup snapshot  616 , may be created periodically or on demand. For example, a user policy may specify that new snapshots should be created every week. Additionally or alternatively, a user may be preparing to perform some risky operations on the source storage system, and may demand that a snapshot be created in case something goes wrong. These policies may be maintained and applied using data protection appliance  600 , deduplicated storage  604 , and/or an external system. 
     Turning now to  FIG. 7 , a process for maintaining backup snapshots using continuous data replication is discussed. At block  700 , an initial snapshot of a source storage system may be created. This initial snapshot may be substantially similar to initial backup snapshot  614 , and may be created using the processes discussed in reference to  FIG. 3 ,  FIG. 4 , and  FIG. 5 . Additionally or alternatively, the initial snapshot may be any previously created snapshot. For example, the initial snapshot may be similar to second backup snapshot  616 , and may be used in conjunction with journal files to create a third backup snapshot. 
     At block  702 , application IO&#39;s comprising writes to the source storage system may be received. These writes may update existing data or create new data. In an embodiment, the application IO&#39;s may be received by a data protection appliance, such as data protection appliance  600 . 
     At  704 , the application IO&#39;s may be written to a journal file. This journal file may be substantially similar to metadata journal file  606  and/or data journal file  608 . In an embodiment, the application IO&#39;s may be written to one or more existing journals. Alternatively, application IO&#39;s arriving after a snapshot is synthesized may be written to their own unique journals. This may be beneficial, for example, when maintaining different levels of backup granularity, as discussed below. 
     In some embodiments, the application IO&#39;s are sequentially written to the journal as they are received. For example, if application IO&#39;s arrive in order B, C, A, their corresponding entries in the journal will also be B, C, A. 
     At block  706 , a second snapshot may be synthesized from the initial backup snapshot and the journal. The second snapshot may be substantially similar to second backup snapshot  616 , and the synthesis process may be similar to that depicted by the solid and dashed lines. In some embodiments, the second snapshot may be synthesized entirely from journal files rather than use the initial backup snapshot. 
     During and/or after the synthesis process, additional application IO&#39;s may be received at block  708 . These application IO&#39;s could be used, for example, to create the third backup snapshot in the future, and may be processed in a manner similar to all the other application IO&#39;s discussed herein. 
     Finally, at block  710  the additional application IO&#39;s may be written to a journal file. They may be written to the same journal as the previous IO&#39;s, or they may be written to a new journal file. 
       FIG. 7A  depicts a process for synthesizing snapshots used for continuous data replication. At block  712 , a metadata journal may be read. This metadata journal could be, for example, metadata journal file  606 . In an embodiment, the metadata journal may be read using a journal processor on a data protection appliance. Additionally or alternatively, the read operation may be local to the deduplicated storage device. 
     At block  714 , the latest IO&#39;s for each offset may be identified. For example, metadata journal file  606  comprises journal entries  610  and  611 . The latest entry for offset 0 is A″, 1 is B′, 2 is C′, and 3 is D′. In some embodiments, journal entries  610  and  611  may be written to different journals. In such an embodiment, the only IO&#39;s identified would be A″ and C′ since we are synthesizing a snapshot from initial backup snapshot  614 . 
     At  716 , a synthesis plan may be created. This plan may identify where each IO should be synthesized from. For example, the synthesis plan may only identify A″ and C′ for synthesis from data journal  608 . The B′ and D′, in contrast, may be obtained from initial backup snapshot  614  since they have not changed. 
     Finally, at block  718 , the backup snapshot may be synthesized. This backup snapshot could be, for example, substantially similar to backup snapshot  616 . 
     The system and processes discussed herein may enable additional backup snapshots to be synthesized from journal entries and existing snapshots. In some embodiments, the journal entries may be application IO&#39;s which are continuously streamed to a data protection appliance. While these snapshots may provide additional data protection, they may only allow data that exists in the snapshots to be recovered. Combining snapshots and journal files may, however, allow any point-in-time recovery. 
     Backup Point-in-Time Recovery from Deduplicated Storage 
     When datasets are backed-up on a periodic rather than continuous basis, data recovery may only be available for specific time intervals. For example, if a dataset is backed up at the end of every business day, the only data that is available for recovery is the data as it exists at the end of the day. Continuous backups, however, may allow recovery of data at any, or nearly any, point-in-time. By transmitting application IO&#39;s to a backup location as they occur, an interim snapshot may be synthesized between scheduled snapshots and data may be recovered. 
       FIG. 8  and  FIG. 9  depict a system and process for synthesizing an interim snapshot for point-in-time recovery. In an embodiment, the system may include data protection appliance  800 , journal processor  802 , and deduplicated storage  804 . The system may also include metadata journal file  806 , comprising metadata journal entries  810  and  811 , and data journal file  808 , comprising data journal entries  812  and  813 . 
     Data protection appliance  800  may receive application IO&#39;s as they are made to a source storage system. In an embodiment, journal processor  802  may write those IO&#39;s to metadata journal file  806  and data journal file  808 . Initialization journal entries  810  and  812  may be used to synthesize initial backup snapshot  814 . Metadata entries  811  and data journal file entries  813  may be application IO&#39;s made to the source storage volume after or while initial backup snapshot  814  was synthesized. These elements may be substantially similar to those discussed in reference to  FIG. 6 . 
     In an embodiment, metadata journal entries  811  and data journal entries  813  may be used to synthesize interim snapshot  816 . Interim snapshot  816  may then be used as a source for point-in-time recovery. For example, application IO&#39;s A″ and C′ may be streamed to deduplicated storage as they are made to the source storage system. A user may then decide they wish recover data from the point-in-time immediately after application IO A″ was streamed. When the user&#39;s request arrives, the most recent snapshot may be initial backup snapshot  814 , which does not contain A″ or C′. To respond to the user&#39;s request, deduplicated storage  804  may synthesize interim snapshot  816 . This snapshot may include unchanged data from initial backup snapshot  814 , as shown by the solid black arrows, and application IO A″ synthesized from data journal file  808 , as shown by the dashed arrow. Note that interim snapshot  816  does not include C′. This is because the user requested data recovery at a point-in-time before C′ may made. 
     In an embodiment, the data from interim snapshot  816  may be transmitted back to the source storage system and recovered. Additionally or alternatively, it may be exposed to a host as LUN, as discussed in reference to  FIG. 12  and  FIG. 13 . Interim snapshot  816  may be deleted after recovery, or may be retained. In an embodiment, if interim snapshot  816  is created at a point-in-time sufficiently close to a scheduled synthesis time, the scheduled synthesis may be cancelled and interim snapshot  816  may be treated as second backup snapshot  616 . 
     Turning now to  FIG. 9 , a flow chart depicting a process for point-in-time recovery is discussed. At block  900 , a request to recover some data is received. This request could be, for example, received at data protection appliance  800  and/or deduplicated data storage  804 . In an embodiment, the request may specify a file representing a LUN to recover, or it may be a request to recover an entire system. Additionally or alternatively, the request may specify a point-in-time for the recovery. The point-in-time may be a date, event, or any other mechanism to identify a specific time. In some embodiments, the point-in-time may be between snapshots. 
     At  902 , a snapshot nearest the point-in-time may be identified. The snapshot could be, for example, initial backup snapshot  814 . 
     At block  906 , a recovery snapshot may be synthesized. This recovery snapshot could be, for example, substantially similar to interim snapshot  816 . If the recovery snapshot is synthesized using a snapshot from an earlier point-in-time, IO&#39;s stored in a journal file may be applied to synthesize the recovery snapshot. 
     Finally, at block  908  the recovery snapshot may be provided in response to the request. For example, the recovery snapshot may be exposed as a LUN and mounted on a host computer, or exposed as a network file system share. Additionally or alternatively, the recovery snapshot may be transmitted back to the source storage system. In an embodiment, only a portion of the snapshot, such as a specific file, may be provided. 
     Combining backup snapshots, journals, and continuous data replication may provide point-in-time recovery capabilities. As more data is written to and/or modified on a source storage system, however, the number of journals and snapshots may increase. In some embodiments, data protection windows may be used to manage this data growth. 
     Maintaining Backup Granularity on Deduplicated Storage 
     As the number of snapshots and journals on the deduplicated storage grows, more space may be required. Deleting snapshots and journals may result in important information being lost, and adding to space to the deduplicated storage may be expensive. To address these concerns, backup windows and policies may be defined. Backup windows may be defined intervals designating which snapshot and journals should be stored, and for how long. 
       FIG. 10  and  FIG. 11  depict a system and process for defining backup granularity using data protection windows.  FIG. 10  shows seventeen snapshot files, labeled Si through S 17 , stored on a deduplicated storage device. These snapshots may be created and maintained in a manner substantially similar to that discussed above. The deduplicated storage device may also include six journal files, labeled J 1  through J 6 , which may be used to synthesize new snapshots or perform point-in-time recovery. 
       FIG. 10  also includes three data protection windows: short-term protection window  1000 , mid-term protection window  1002 , and long term protection window  1004 . Each of these protection windows may have an associated policy specifying actions to take on any snapshot and/or journal file within the protection window. For example, one policy may be “delete all journals within this protection window.” While the embodiments discussed herein address deletion and/or retention policies, any other policy which may be applied to the journals and/or snapshots is consistent with this disclosure. 
     Short-term protection window  1000  may be defined to protect both snapshots and journal files allowing for point-in-time recovery. This window may be particularly beneficial for snapshots that were created recently and/or were created on demand by a user. On demand creation may signify that the snapshot is more important than a scheduled snapshot because a user must go out of their way to create it. Further, it may be more likely that a user needs to recover data which was modified or created recently. 
     Mid-term protection window  1002  may include only snapshot files. As time progresses and journal files move from short-term protection window  1000  into mid-term protection window  1002 , they may be deleted. While deleting journal files may prevent most point-in-time recovery, the snapshots may be maintained in mid-term protection window. As a result, some level of point-in-time recovery is preserved. Specifically, any data contained in one of the maintained snapshots may be recovered. Mid-term protection window therefore balances storage needs with recovery needs. 
     As snapshots move from mid-term protection  1002  window into long-term protection window  1004 , certain snapshots may be deleted. Point-in-time recovery may be less important for long-term backups because of their age. The deleted snapshots may be chosen based on a policy, such as size or a user assigned priority. Additionally or alternatively, they may be arbitrarily chosen (for example, only retaining every fifth snapshot). 
     In an embodiment, data protection windows may be defined and maintained using a data protection appliance, a deduplicated storage device, and/or an external system. For example, if the data protection window is defined using a deduplicated storage device, that device may delete the journals and/or snapshots as the move from one data protection window into another. In some embodiments, the data protection windows may change dynamically based on available space on the deduplicated storage device. For example, if there is a large amount of available space the short-term protection window may be very large, and/or the mid-term and long-term protection windows may not exist. Similarly, if there is not much available space the long-term protection window may be very long. 
     Turning now to  FIG. 11 , a method for defining and applying data protection windows is discussed. At  1100 , a plurality of backup snapshots may be initialized on a deduplicated storage. These snapshots may be initialized or created using any of the systems or methods discussed herein. 
     At  1102 , a plurality of journals may be initialized on the deduplicated storage device. These journals may be created from application IO&#39;s received from source side storage, and may provide point-in-time recovery. In an embodiment, any of the processes discussed herein for creating and/or maintaining journals may be used. 
     At  1104 , a first backup window comprising a first time range may be defined. The time range may specify the amount of time snapshots and/or journals remain inside the backup window. In an embodiment, the time range is dynamically based on the available storage on a deduplicated storage device. 
     At block  1106 , the first backup window may be applied to the backup snapshots and journals. In an embodiment, the first backup window may be defined to maintain all of the snapshots and journals it covers. This may permit point-in-time recovery of data from those files. In an embodiment, the first backup window is substantially similar to the short-term backup window discussed above. 
     At block  1108 , second backup window comprising a second time range may be defined. This second backup window may comprise a second time range similar to the first window&#39;s time range. In an embodiment, the second backup window only applies to journals, and may be substantially similar to mid-term backup widow  1002 . 
     At  1110 , the second backup window may be applied to the journals. This may occur as the journals move from the first backup window into the second backup window. For example, as time progresses the journal files may move outside the time range defined in the first protection window. This may be, for example, because the time range is shorter than the journals age. The journal may move into the time range specified in the second backup window, and that backup window may be applied. 
     At block  1111 , as journals move into the second time range the may be deleted. In an embodiment, this may mean deleting every journal inside the second time range. 
     Additionally or alternatively, only select journals may be deleted. For example, the data protection window may have an associated policy specifying that only every second journal should be deleted. 
     At  1112 , a third backup window comprising a third time range may be defined. This third backup window may be substantially similar to long-term window discussed above. In an embodiment, the third time range may be a catch-all, meaning it applies to any journal and/or backup snapshot not covered by the first backup window or the second backup window. 
     At  1114 , the third backup window may be applied to any remaining snapshots and/or journals. In an embodiment, the journals have all been deleted and the third backup window is only applied to snapshot files. 
     Finally, at block  1116 , a number of snap shots inside the third time range may be deleted. The number of snapshots to delete may be based on a policy, such as only retain every tenth snapshot, in some embodiments only a daily snapshot may be kept, or any other policy may be applied (for instance keep ten daily snapshots and then ten weekly snapshots and then ten monthly snapshots, etc). 
     The data protection windows discussed herein may provide varying degree of backup granularity. More recent and/or important information may include both snapshots and journal files, which longer term backups may only comprise periodic backup snapshots. 
     Providing Snapshot Access and Retrieval 
     Data backups are only useful if they can be accessed and/or retrieved when needed. Accessing backup snapshots may allow a user or system to retrieve data needed for recovery. For a system-wide recovery, the entire backup snapshot may be retrieved. In some embodiments, however, it may be beneficial to allow a user to access the snapshot directly to retrieve discrete pieces of data. 
       FIGS. 12 and 13  depict a system and process for providing access to data snapshots. The system of  FIG. 12  may include data protection appliance  1200  comprising journal processor  1202 , deduplicated storage  1204 , and host  1222 . In an embodiment, deduplicated storage may comprise metadata journal  1206 , data journal  1208 , backup snapshot  1214 , and recovery snapshot  1216 . Recovery snapshot  1216  may be a synthesized snapshot for point-in-time recovery, as discussed in reference to  FIG. 8 . Metadata journal  1206  may include metadata journal entries  1210  and  1211 , and data journal  1208  may include data journal entries  1212  and  1213 . In an embodiment, these elements may be substantially similar to those discussed in reference to the figures above. 
     In some embodiments, a user may need to recover data from recovery snapshot  1216 . Transmitting the entire snapshot over a network, however, may be inefficient given the snapshot&#39;s size. For example, if recovery snapshot is several gigabytes, and a user wishes to recover a file on the snapshot that is only a few megabytes, transferring the entire recovery snapshot  1216  may consume an unnecessary amount of network bandwidth. 
     Rather than transmitting recovery snapshot  1216  over a network, deduplicated storage  1204  may create block LUN  1218  on top of the snapshot. LUN  1218  may be exposed and mounted on host  1222  as device  1220 . A user may access LUN  1218  through device  1220 , and could thereby browse the snapshot for the data they wish to recover. Once the data is located, it may be transmitted to the source storage system directly, through data protection appliance  1200 , or through host  1222 . 
     In some embodiments, the source storage system may be down for a prolonged period of time, but a user may still need to access their data. In such an embodiment, the user may mount LUN  1218  as device  1220 . Rather than merely recovering data, however, the user may make writes to the recovery snapshot  1216  through LUN  1218 . The writes may be tracked, for example using metadata journal  1206  and data journal  1208 . Once the source storage system is accessible again, a difference between the latest point in time of the volume at the storage array and the current data at recovery snapshot  1216  may be taken. The results of the difference may indicate which data has changed, and those changes may be communicated back to the source storage system. This may allow a user to continue to access and make changes to their data, even while the source storage system is down. 
     In an embodiment block LUN  1218  may not be created and mounted on host  1222 . Instead, the recovery snapshot may be exposed as a network file system (“NFS”) share. Host  1222  may accessed the NFS share, retrieve data, and/or perform write operations. 
     In some embodiments, recovery snapshot  1216  may comprise a virtual machine disk (“VMDK”) file. The VMDK file may be, for example, accessed directly by an NFS mount and the virtual machine may boot using the VMDK file. Host  1222  may interact with the VMDK, and may make changes in the process. The VMDK may be transferred to the source side storage when it is available. In some embodiments, the transfer may occur while the VMDK is running 
     Turning now to  FIG. 13 , a method for exposing a recovery snapshot as a logical unit is discussed. At  1300 , a recovery snapshot may be exposed as a LUN. This exposure could occur, for example, by creating a block LUN on top of the snapshot. In some embodiments, the snapshot resides on a deduplicated storage, and the block LUN is created on that storage. 
     At  1302 , the LUN may be mounted on a host device. Once mounted, the host device may access and interact with the LUN as if it were a local drive. For example, the host device may make read and/or write requests to the LUN. 
     At  1304 , the deduplicated storage may receive IO&#39;s for the LUN from the host device. These IO&#39;s could be, for example, read and/or write requests. 
     At  1306 , the IO&#39;s may be applied to the recovery snapshot to create a modified recovery snapshot. For example, the IO&#39;s could modify recovery snapshot  1216  to change data E to E′. 
     At block  1308 , a difference between the modified recovery snapshot and a backup snapshot may be identified. This difference could be identified, for example, using API&#39;s native to the deduplicated storage device. 
     Finally, at block  1310 , the difference may be transmitted to a recovery device. In some embodiments, the recovery device may be a source storage system. Additionally or alternatively, the difference may be transmitted in response to a request. For example, a user may access the system and request the difference between the modified recovery snapshot and a backup snapshot. 
     Turning now to  FIG. 13A , a method for recovering a logically corrupted source storage system is discussed. In an embodiment, a user may wish to recover the data on the corrupted system. The user may identify a point-in-time where the system is believed to not be corrupt, and synthesize a snapshot comprising data between that point-in-time and the latest point on the storage system. This snapshot may be applied to the system to recover the corrupt data. 
     At  1312 , a request may be received from a user to recover data. The request may comprise a point-in-time identifying a time when the user believes the data was not corrupt. In an embodiment, the data between the point-in-time and the present time must be recovered because it is corrupt on the source storage system. Since the application IO&#39;s are made to the deduplicated storage in real time, however, the data on that location may be correct. 
     At  1314 , a recovery snapshot may be synthesized for the point-in-time. This recovery snapshot could be, for example, synthesized in a manner substantially similar to the discussed above. 
     At  1316 , a difference may be identified between the recovery snapshot and the latest backup snapshot. The latest backup snapshot may be a snapshot for a point-in-time later than the recover snapshot. For example, if the recovery snapshot comprises A, B, C, D, and E, and the latest backup snapshot comprises A′, B, C′, D, and E, the difference may be A′ and C′. In an embodiment, the difference is identified on a deduplicated storage device and may use native APIs. 
     At  1318 , the difference may be added to a recovery list as entries in that recovery list. In an embodiment, the entries may be metadata associated with the difference. For example, the recovery list may comprise metadata entries for A′ and C′. 
     At block  1320 , metadata entries in the latest journal file may be identified. The metadata entries may comprise metadata that is not included in the latest synthesized backup. For example, they may comprise application IO&#39;s that arrived after the latest backup snapshot was synthesized. This identification process could occur, for example, at a journal processor. 
     At block  1322 , the metadata journal entries may be added to the recovery list. For example, if the entries comprise A″ and D′, the new recovery list may comprise A″, C′, and D′. 
     At block  1324 , additional metadata entries may be identified in a data protection appliance memory. In an embodiment, these entries may be for application IO&#39;s which have arrived at the data protection appliance, but have not yet been transmitted and/or written to the journal file. 
     At  1326 , the additional metadata entries may be added to the recovery list. For example, if the data protection appliance&#39;s memory includes B′, the recovery list may comprise A″, B′, and D′. 
     Finally, at  1318  data associated with the metadata entries may be identified and transmitted back to the source storage system for recovery. If the metadata comprises pointers, for example, the data pointed to by the pointers may be transmitted back to the source storage system. 
     Synthesizing a Backup Snapshot from Multiple Data Protection Appliances 
     For large, heavily used source storage systems, the volume of application and/or initialization IO&#39;s may be too great for a single data protection appliance to maintain. If a data protection appliance is overburdened, journal files may not be up-to-date and valuable data may be lost if the source storage system fails. Using multiple data protection appliances may alleviate this risk. 
       FIG. 14  and  FIG. 15  show a system and method for creating backup snapshots using multiple data protection appliances. The system depicted in  FIG. 15  includes data protection appliance  1400  and data protection appliance  1401 . In an embodiment, each of these data protection appliances is responsible for protecting specific portions of a LUN in a source storage system. For example, data protection appliance  1400  may be responsible for protecting a first chunk of a LUN in a common storage array, and data protection appliance  1401  may be responsible for protecting a second chunk in a LUN of the common storage array. If an application IO is made to the first chunk, it is received by data protection appliance  1400  and written to journal file  1406  using journal processor  1402 . Similarly, if an application IO is made to the second chunk it may be received by data protection appliance  1401  and written to journal file  1407  using journal processor  1403 . In this manner, all of the application IO&#39;s may be written to a journal using two data protection appliances. 
     While there are only two data protection appliances shown in  FIG. 14 , it should be appreciated that any number of data protection appliances is consistent with this disclosure. For example, the source storage system may be divided into five sections or chunks which are protected by five separate and distinct data protection appliances. 
     In some embodiments, there is no overlap between the chunks protected by different data protection appliances. For example, data protection appliance  1400  will not protect a chunk that is also protected by data protection appliance  1401 . Alternatively, there may be protection overlap for redundancy. 
     In some embodiments the chunks of the LUN may not consist of continuous parts. For instance, a first chunk may contain 1 MB portions which start at an offset which is 2N MB, while chunk 2 contains 1 MB portions starting an offset of 2(N+1)MB. 
     Journals  1406  and  1407  may be separate journal files accessible only by their associated data protection appliance. Alternatively, they may be the same journal file accessible by both data protection appliances. Additionally, journals  1406  and  1407  may include both a metadata journal file and a data journal file, as discussed above. The metadata journal and data journals are not shown in  FIG. 14  for ease of illustration, but they may still be part of journal file  1406  and  1407 . 
     Journals files  1406  and  1407  may be used by deduplicated storage  1404  to synthesize snapshots for each of the regions their respective data protection appliances protect. For example, suppose source storage system comprises data A, B, C, D, and E. Data protection appliance  1400  may be responsible for protecting A, B, and C, and data protection appliance  1401  may be responsible for protecting D and E. As a result, journal  1406  contains data and metadata for A, B, and C, but not D or E. Similarly, journal  1407  contains data and metadata for D and E, but not A, B, or C. 
     Since each journal only contains data associated with specific regions of the source storage system, neither journal alone can synthesize a complete backup snapshot. First snapshot  1408  may be synthesized using the data and metadata from journal file  1406 . Snapshot  1408  may include A′, B, and C′, but does not contain data for D or E as shown by the hash marks in the Figure. Similarly, second snapshot  1409  contains data D′ and E′, but no data for A, B, or C. Neither of these snapshots therefore represents a complete backup snapshot of the source storage system. 
     Backup snapshot  1410  may be synthesized by merging first snapshot  1408  and second snapshot  1409 . Backup snapshot  1410  may comprise a complete backup of the source storage system. In an embodiment, this synthesis may be accomplished by synthesis data from each snapshot  1408  and  1409  into snapshot  1410 . Alternatively, first snapshot  1408  may be merged into second snapshot  1409  to created backup snapshot  1410 . Once backup snapshot  1410  is synthesized, snapshots  1408  and  1409  may be deleted to conserve space, or retained for redundancy. 
     In some embodiments, first snapshot  1408  and second snapshot  1409  may never be created. Instead, backup snapshot  1410  may synthesize the necessary data directly from journal files  1406  and  1407 . 
     Turning now to  FIG. 15 , a flow chart for synthesizing a backup snapshot from data received from multiple data protection appliances is discussed. At  1500 , a first set of sequential IO&#39;s may be streamed from a data protection appliance to a deduplicated storage device. The IO&#39;s could be, for example, sequential IOs. In an embodiment the IO&#39;s may be for a specific section, chunk, or region of the source storage system. 
     At  1502 , a second set of sequential IO&#39;s may be streamed from a second data protection appliance to the deduplicated storage. In an embodiment, these IO&#39;s may be for a separate section of the source storage system than those streamed from the first data protection appliance. While the first set of sequential IO&#39;s and the second set of sequential IO&#39;s may be for different regions of a source storage system, such as a data array, they may both be made to the same source data system. In other words, the application IO&#39;s may be made to a common source, which is divided into chunks protected by separate data protection appliances. 
     At  1504 , a first snapshot may be synthesized from the first journal file. This snapshot could be, for example, snapshot  1408 . In an embodiment, the first snapshot only contains data included in the first journal. 
     At  1506 , a second snapshot may be synthesized from the second journal. This snapshot could be substantially similar to snapshot  1409 , and may only contain data from the second journal file. 
     Finally, at  1508  the first snapshot and the second snapshot may be merged to create a backup snapshot. This could be, for example, backup snapshot  1410 . 
     General Purpose Computer System 
       FIG. 16  depicts a computer system which may be used to implement different embodiments discussed herein. General purpose computer  1600  may include processor  1602 , memory  1604 , and system IO controller  1606 , all of which may be in communication over system bus  1608 . In an embodiment, processor  1602  may be a central processing unit (“CPU”) or accelerated processing unit (“APU”). Some embodiments may comprise multiple processors, or a processor with multiple cores. Processor  1602  and memory  1604  may together execute a computer process, such as the processes described herein. 
     System IO controller  1606  may be in communication with display  1610 , input device  1612 , non-transitory computer readable storage medium  1614 , and/or network  1616 . Display  1610  may be any computer display, such as a monitor, a smart phone screen, or wearable electronics and/or it may be an input device such as a touch screen. Input device  1612  may be a keyboard, mouse, track-pad, camera, microphone, or the like, and storage medium  1614  may comprise a hard drive, flash drive, solid state drive, magnetic tape, magnetic disk, optical disk, or any other computer readable and/or writable medium. Storage device  1614  may also reside inside general purpose computer  1600 , rather than outside as shown in  FIG. 1 . 
     Network  1616  may be any computer network, such as a local area network (“LAN”), wide area network (“WAN”) such as the internet, a corporate intranet, a metropolitan area network (“MAN”), a storage area network (“SAN”), a cellular network, a personal area network (PAN), or any combination thereof. Further, network  1616  may be either wired or wireless or any combination thereof, and may provide input to or receive output from IO controller  1606 . In an embodiment, network  1616  may be in communication with one or more network connected devices  1618 , such as another general purpose computer, smart phone, PDA, storage device, tablet computer, or any other device capable of connecting to a network. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Accordingly, the present implementations are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims. 
     For the sake of clarity, the processes and methods herein have been illustrated with a specific flow, but it should be understood that other sequences may be possible and that some may be performed in parallel, without departing from the spirit of the invention. Additionally, steps may be subdivided or combined. As disclosed herein, software written in accordance with the present invention may be stored in some form of computer-readable medium, such as memory or CD-ROM, or transmitted over a network, and executed by a processor. 
     All references cited herein are intended to be incorporated by reference. Although the present invention has been described above in terms of specific embodiments, it is anticipated that alterations and modifications to this invention will no doubt become apparent to those skilled in the art and may be practiced within the scope and equivalents of the appended claims. More than one computer may be used, such as by using multiple computers in a parallel or load-sharing arrangement or distributing tasks across multiple computers such that, as a whole, they perform the functions of the components identified herein; i.e. they take the place of a single computer. Various functions described above may be performed by a single process or groups of processes, on a single computer or distributed over several computers. Processes may invoke other processes to handle certain tasks. A single storage device may be used, or several may be used to take the place of a single storage device. The disclosed embodiments are illustrative and not restrictive, and the invention is not to be limited to the details given herein. There are many alternative ways of implementing the invention. It is therefore intended that the disclosure and following claims be interpreted as covering all such alterations and modifications as fall within the true spirit and scope of the invention.