# Patent

## Title
Leveraging array snapshots for immediate continuous data protection

## Abstract

One challenge to continuous data protection (CDP) is the resources required for storage. Typically, when setting up replication, a production volume and replica are set up and initialized to copy data from the production volume to the replica. However, instead of taking a clone of the volume, there are significant advantages to taking an array-based snapshot of the volume and leveraging capabilities inherent to the snapshot to improve data protections systems using CDP. To leverage the benefits of snapshots in a CDP system, as described below, example embodiments of the present invention use multiple snapshots. Bookmarks are created to indicate the point in time each snapshot was create so that a newer snapshot may be used for replication as data is applied to the replica.

## Background
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 application relates to data replication.
BACKGROUND
Computer data is vital to today's organizations, and a significant part of protection against disasters is focused on data protection. As solid-state memory has advanced to the point where cost of memory has become a relatively insignificant factor, organizations can afford to operate with systems that store and process terabytes of data.
Conventional data protection systems include tape backup drives, for storing organizational production site data on a periodic basis. Such systems suffer from several drawbacks. First, they 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 takes a long time.
Another conventional data protection system uses data replication, by creating a copy of the organization'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. Data replication systems generally operate either at the application level, at the file system level, or at the data block level.
Current data protection systems try to provide continuous data protection, which enable the organization to roll back to any specified point in time within a recent history. Continuous data protection systems aim to satisfy two conflicting objectives, as best as possible; namely, (i) minimize the down time, in which the organization production site data is unavailable, during a recovery, and (ii) enable recovery as close as possible to any specified point in time within a recent history.
Continuous data protection typically uses a technology referred to as “journaling,” whereby a log is kept of changes made to the backup storage. During a recovery, the journal entries serve as successive “undo” information, enabling rollback of the backup storage to previous points in time. Journaling was first implemented in database systems, and was later extended to broader data protection.
SUMMARY
Example embodiments of the present invention provide a method, an apparatus and a computer-program product for leveraging snapshots for immediate continuous data protection (CDP) in a replication service comprising a production site including a splitter, a production volume and a replication appliance. For example, the method includes creating a replica of the production volume at a replication site and configuring the replication service to replicate from the production volume to the replica at the replication site. Replication is then performed using changes tracked at the production site.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and further advantages of the present invention may be better under stood by referring to the following description taken into 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 invention;
FIG. 2 is a simplified illustration of a write transaction for a journal, in accordance with an embodiment of the present invention;
FIG. 3 is a block diagram illustrating a production site according to an example embodiment of the present invention;
FIGS. 4 and 5A-5D are flow diagrams illustrating example methods for leveraging snapshots for immediate continuous data protection (CDP) in a replication service according to an example embodiment of the present invention;
FIG. 6 is a timeline for leveraging snapshots for immediate continuous data protection (CDP) in a replication service according to an example embodiment of the present invention;
FIG. 7 is a block diagram of an example embodiment apparatus according to the present invention; and
FIG. 8 is an illustration of an example embodiment of the present invention as embodied in program code.



DETAILED DESCRIPTION
Journaling uses significant amounts of the volume on which the journal is stored. Conventionally, replication systems dynamically add storage for a journal, as described in U.S. Pat. No. 7,840,536, entitled “METHODS AND APPARATUS FOR DYNAMIC JOURNAL EXPANSIONASSIGNED,” issued on Nov. 23, 2010 and assigned to EMC Corporation of Hopkinton, Mass., which is hereby incorporated by reference in its entirety. However, such conventional journaling mechanisms lack efficient methods to decrease the size of the journal.
Accordingly, example embodiments of the present invention leverage the punch command and thin LUNs in order to create a fully dynamic journal which can shrink and grow on demand. The journal may be assigned a maximum allowed size, as well as a journal protection window defining a period for which the journal should retain journal information. A set of currently used blocks will be used and, if protection window is large enough, the oldest data blocks from an undo stream will be punched out from the stream. The blocks may be large journal blocks of a direct LUN (DLUN). DLUNs are LUNs which thin having large volume chunks (e.g., 128 MB or 1 GB). Accordingly, the punch command punches large slices out of the LUN; however, the chunks are big enough that sequential IOs remain sequential, thereby increasing performance. A manager may monitor how much free space there is in the storage pool and regulate the journal growth if the pool is nearly empty. Additionally, the journal capacity may also depend on the actual usage of the replicated LUNs if the replicated LUNs are thin (i.e., maintain the journal at a particular percentage of the used storage capacity).
The following definitions are employed throughout the specification and claims.
BACKUP SITE—a facility where replicated production site data is stored; the backup site may be located in a remote site or at the same location as the production site;
CLONE—a clone may be a copy or clone of the image or images, drive or drives of a first location at a second location;
DELTA MARKING STREAM—may mean the tracking of the delta between the production and replication site, which may contain the meta data of changed locations, the delta marking stream may be kept persistently on the journal at the production site of the replication, based on the delta marking data the DPA knows which locations are different between the production and the replica and transfers them to the replica to make both sites identical;
DPA—a computer or a cluster of computers 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;
HOST—at least one computer or networks of computers that runs at least one data processing application that issues I/O requests to one or more storage systems; a host is an initiator with a SAN;
HOST DEVICE—an internal interface in a host, to a logical storage unit;
IMAGE—a copy of a logical storage unit at a specific point in time;
INITIATOR—a node in a SAN that issues I/O requests;
JOURNAL—a record of write transactions issued to a storage system; used to maintain a duplicate storage system, and to rollback the duplicate storage system to a previous point in time;
LOGICAL UNIT—a logical entity provided by a storage system for accessing data from the storage system;
LUN—a logical unit number for identifying a logical unit;
PHYSICAL STORAGE UNIT—a physical entity, such as a disk or an array of disks, for storing data in storage locations that can be accessed by address;
PRODUCTION SITE—a facility where one or more host computers run data processing applications that write data to a storage system and read data from the storage system;
SAN—a storage area network of nodes that send and receive I/O and other requests, each node in the network being an initiator or a target, or both an initiator and a target;
SOURCE SIDE—a transmitter of data within a data replication workflow, during normal operation a production site is the source side; and during data recovery a backup site is the source side;
SNAPSHOT—a Snapshot may refer to 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;
STORAGE SYSTEM—a SAN entity that provides multiple logical units for access by multiple SAN initiators;
TARGET—a node in a SAN that replies to I/O requests;
TARGET SIDE—a receiver of data within a data replication workflow; during normal operation a back site is the target side, and during data recovery a production site is the target side; and
WAN—a wide area network that connects local networks and enables them to communicate with one another, such as the Internet.
Description of Embodiments Using a Five State Journaling Process
FIG. 1 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; and
        Delay a SCSI command by not returning an acknowledgement to the respective host computer.
    
    


A protection agent may handle different SCSI commands, differently, according to the type of the command. For example, a SCSI command inquiring about the size of a certain logical unit may be sent directly to that logical unit, while a SCSI write command may be split and sent first to a DPA associated with the agent. A protection agent may also change its behavior for handling SCSI commands, for example as a result of an instruction received from the DPA.
Specifically, the behavior of a protection agent for a certain host device generally corresponds to the behavior of its associated DPA with respect to the logical unit of the host device. When a DPA behaves as a source site DPA for a certain logical unit, then during normal course of operation, the associated protection agent splits I/O requests issued by a host computer to the host device corresponding to that logical unit. Similarly, when a DPA behaves as a target device for a certain logical unit, then during normal course of operation, the associated protection agent fails I/O requests issued by host computer to the host device corresponding to that logical unit.
Communication between protection agents and their respective DPAs may use any protocol suitable for data transfer within a SAN, such as fiber channel, or SCSI over fiber channel. The communication may be direct, or via a logical unit exposed by the DPA. 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.
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 will 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.
FIG. 2 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 & 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 & 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 & 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'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.
Delta Marking
A delta marker stream may contain the locations that may be different between the latest I/O data which arrived to the remote side (the current remote site) and the latest I/O data which arrived at the local side. In particular, the delta marking stream may include metadata of the differences between the source side and the target side. For example, every I/O reaching the data protection appliance for the source 112 may be written to the delta marking stream and data is freed from the delta marking stream when the data safely arrives at both the source volume of replication 108 and the remote journal 180 (e.g. DO stream). Specifically, during an initialization process no data may be freed from the delta marking stream; and only when the initialization process is completed and I/O data has arrived to both local storage and the remote journal data, may be I/O data from the delta marking stream freed. When the source and target are not synchronized, data may not be freed from the delta marking stream. The initialization process may start by merging delta marking streams of the target and the source so that the delta marking stream includes a list of all different locations between local and remote sites. For example, a delta marking stream at the target might have data too if a user has accessed an image at the target site.
The initialization process may create one virtual disk out of all the available user volumes. The virtual space may be divided into a selected number of portions depending upon the amount of data needed to be synchronized. A list of ‘dirty’ blocks may be read from the delta marker stream that is relevant to the area currently being synchronized to enable creation of a dirty location data structure. The system may begin synchronizing units of data, where a unit of data is a constant amount of dirty data, e.g., a data that needs to be synchronized.
The dirty location data structure may provide a list of dirty location until the amount of dirty location is equal to the unit size or until there is no data left. The system may begin a so-called ping pong process to synchronize the data. The process may transfer the differences between the production and replica site to the replica.
A discussion of mirroring may be found in U.S. Pat. No. 7,346,805, entitled “PROTECTION OF MIRRORED DATA,” issued on Mar. 18, 2008 and assigned to EMC Corporation of Hopkinton, Mass., which is hereby incorporated by reference in its entirety.
A discussion of journaling and some techniques associated with journaling may be found in U.S. Pat. No. 7,516,287, entitled “METHODS AND APPARATUS FOR OPTIMAL JOURNALING FOR CONTINUOUS DATA REPLICATION,” issued on Apr. 7, 2009 and assigned to EMC Corporation of Hopkinton, Mass., which is hereby incorporated by reference in its entirety.
A discussion of dynamically adding storage for a journal may be found in U.S. Pat. No. 7,840,536, entitled “METHODS AND APPARATUS FOR DYNAMIC JOURNAL EXPANSION,” issued on Nov. 23, 2010 and assigned to EMC Corporation of Hopkinton, Mass., which is hereby incorporated by reference in its entirety.
Leveraging Array Snapshots for Immediate Continuous Data Protection
One challenge to continuous data protection (CDP) is the resources required for storage. Typically, when setting up replication, a production volume and replica are set up and initialized to copy data from the production volume to the replica. For example, continuous data protection systems using RecoverPoint by EMC Corporation of Hopkinton, Mass. traditionally are created by attaching the source volume of the array to a splitter to split I/O commands sent to the storage volume to both the storage volume and a replication appliance to track the changes and apply the changes to a replica volume.
Traditionally, a user may configure a replication volume which may reside on a different storage array than the production volume, and the replication system copies data from the production volume to the replication volume (note, however, that in example embodiments of the present invention, the replication volume resides on the same storage array as the production volume). Once initialization ends, a five-phase distribution process is used to create CDP, as described in U.S. Pat. Nos. 7,627,612 to Ahal et al. entitled “METHODS AND APPARATUS FOR OPTIMAL JOURNALING FOR CONTINUOUS DATA REPLICATION” and 7,516,287 entitled “METHODS AND APPARATUS FOR OPTIMAL JOURNALING FOR CONTINUOUS DATA REPLICATION,” both to Ahal et al. and assigned to EMC Israel Development Center, Ltd.
FIG. 3 is a block diagram illustrating a production site 300 of a replication environment including a data protection appliance (DPA) 312, a splitter 310 at a storage array 308 storing user volumes 336, snapshots SS1, SS2 and a journal volume 384 storing bookmarks BM1, BM2. The block diagram of FIG. 3 is intended to be read in conjunction with the flow diagrams of FIG. 4 illustrating a first example method for using array-based clones in continuous data protection (CDP) and FIGS. 5A-5D and 6 illustrating a second example method for using array-based snapshots in CDP.
It should be noted that the DPA 312 does not have to be a physical device and may be a set of processes on the storage array 308 or virtual machines running inside the storage array 308 or at any external host 304. It also should be noted that, although the splitter 310 in FIG. 3 is illustrated as being resident in the storage array 308, the splitter 310 also may reside in the switch 348 or in the host 304.
A first example embodiment of the present invention which uses array-based clones in continuous data protection (CDP) to avoid the initial full synchronization required by copying data from the production volume to the replication volume using the replication appliance. As illustrated in the block diagram of FIG. 3 and the flow diagram of FIG. 4, the DPA 312 attaches the production volume 336 to the splitter as clean (i.e., the system does not set the volume entries as dirty in the delta marker stream so not all the locations will be synchronized when replication begins) (405). The DPA 312 then takes a clone 350 of the production volume 336 (410) and creates a consistency group replicating from the production volume 336 to the clone 350 (415). The DPA 312 then attaches the clone 350 to the splitter 310 as a clean volume (420). The DPA 314 may then start replication (425) because both the production and replication volumes were attached to the splitter as clean, and only locations which changed during the creation of the clone (i.e., snapshot) and the consistency group will need resynchronization. Accordingly, the consistency group will complete its initialization within a very short period of time. However, using clones still requires a full copy of the volume data at the beginning of replication, but synchronization occurs within the storage array 308 in the background. The clone is accessible immediately as is continuous replication before the initialization is complete.
A second example embodiment of the present invention for using array-based snapshots in continuous data protection (CDP) is illustrated in the block diagram of FIG. 3, the flow diagrams of FIGS. 5A-5D and the timeline of FIG. 6. The block diagram of FIG. 3 is intended to be read in conjunction with the flow diagrams of FIGS. 5A-5D illustrating example methods for using array-based snapshots in continuous data protection (CDP) and FIG. 6 illustrating a timelines for leveraging snapshots for immediate CDP in a replication service according to an example embodiment of the present invention. Instead of taking a clone of the volume, as described above with respect to FIGS. 3 and 4, there are significant advantages to taking an array-based snapshot SS1, SS2 of the volume 336 and leveraging capabilities inherent to the snapshot SS1, SS2 to improve data protections systems using CDP. By way of background, a snapshot includes a set of pointers containing changes to a data volume. It should be noted that this is a storage (i.e., array)—based snapshot and is not the same as the virtual snapshot maintained by the DPA.
Similar to clones described with reference to FIGS. 3 and 4, array-based snapshots may be configured for use in data protection systems using CDP. However, while writing to the snapshot, the number of differences (i.e., pointers) written to the snapshot continues to grow and eventually is comparable to a clone, both in size and contents. This presents a problem in a CDP system because the volume and the snapshot, including its changes, begin to look the same (i.e., the snapshot gets full and becomes similar to a clone). To leverage the benefits of snapshots in a CDP system, as described below, example embodiments of the present invention use multiple snapshots.
FIGS. 5A-5D are flow diagrams illustrating example methods for leveraging snapshots for immediate continuous data protection (CDP) in a replication service according to an example embodiment of the present invention. As illustrated in FIG. 3, the DPA 312 may attach the production volume 336 to the splitter 310 as clean and track write (i.e., I/O) commands arriving at the splitter 310, such as from a host 304 via a switch 348. The splitter 310 splits write I/Os to the DPA 312 and storage 308.
As illustrated in FIG. 5A, at time t0, a snapshot SS1 of the production volume (i.e., user volumes) 336 may be created (505). It should be noted that the number of snapshots taken in a preferred embodiment may be limited so that the CDP system does not take too many snapshots, thereby reducing system resource requirement, because snapshots can be very intensive of system resources.
Further, as will be described below in greater detail, respective snapshots of all volumes in a consistency group may be taken to provide a consistent snapshot across all volumes in the consistency group at the same time. These snapshots may be taken sequentially or concurrently. As illustrated in FIG. 3, there are a plurality of production volumes 336 (i.e., V1, V2, V3) in a consistency group. Thus, when taking the first snapshot SS1 of the production volume 336 at time t0, a first snapshot SS1 (i.e., V11, V21, V31) of all volumes 536 (i.e., V1, V2, V3) in the consistency group to provide a consistent snapshot across all volumes in the consistency group at the same time (i.e., t0). In another embodiment, the snapshots may be create sequentially between times to and t1.
The replication service then may be configured to replicate from the production volume 336 to the replica (i.e., snapshot) and attach the snapshot SS1 as clean to the splitter 310 (510). Replication then may be performed using changes tracked at the production site (515). For a plurality of production volumes in a consistency group, replication is performed to each production volume's respective snapshot (i.e., V1→V11, V2→V21, V3→V31). Once the initialization process described in the delta marking session above completes, all the snapshots are brought to the same point in time and provide a consistent point in time across all the volumes, even if the snapshots where created sequentially and not exactly at the same time.
The journal 384 allows rolling the volume data for transitioning to any point in time in the journal protection window, thus providing a protection window illustrated in the timeline of FIG. 6. If changes tracked in the do and undo stream data of the journal 384 are written to the first snapshot SS1, as described above with the snapshot SS1 behaving as the replica volume, the first snapshot SS1 continues to grow in size (i.e., becomes fuller) and eventually loses its differential qualities and becomes essentially identical to the volume 336. This is not ideal and stands counter to the intended purposes of snapshots because the pointers in the snapshot increasingly point to other data instead of to the volume, thereby eliminating the savings provided by the snapshot. Thus, it is advantageous to avoid this consequence and create new snapshots and erase old snapshots, thus leverage the inherent qualities of the snapshot to keep the differentials small.
As illustrated in FIG. 5B, the DPA 312 may create a first bookmark BM1 at time t1 (520) to mark in the journal 384 that the snapshot refresh process has begun. In a preferred embodiment, the first bookmark BM1 is created after the first snapshot SS1 is created, when the process for replacing the snapshot starts. A bookmark is a descriptor entered in the journal 384 describing a consistent point in time in journal-based replication. The DPA 312 then sends the first bookmark BM1 to the journal 384. After the first bookmark BM1 is created, a second snapshot SS2 is created at the storage array 308 at time t2. Once the snapshot creation process completes for the second snapshot SS2, a second bookmark BM2 is created at time t3. Note that the bookmarks BM1, BM2 are not part of the storage array 308 but rather are metadata stored in the journal 384. It should also be noted that the bookmarks BM1, BM2 are created a short period of time apart (e.g., on the order of a few seconds).
As illustrated in FIG. 5C, the DPA 312 continues updating the first snapshot SS1 with all changes tracked at the production volume 336. When the DPA 312 reads the first bookmark BM1, the DPA 312 begins updating both the first snapshot SS1 and the second snapshot SS2 with all the tracked changes. Accordingly, the DPA 312 reads data from do stream (535) and reads undo data from the first snapshot SS1. The DPA 312 then updates undo stream data with undo data read from SS1 and applies data read from the do stream to both the first snapshot SS1 and the second snapshot SS2 (540).
As illustrated in FIG. 5D, after reading the second bookmark SS2 from the do stream and applying all data before the second bookmark SS2 to both the first snapshot SS1 and the second snapshot SS2 (545), the first snapshot SS1 may be erased (550) and the second snapshot SS2 used as the new target replica for the production volume 336 (555). Accordingly, the DPA 312 may now read data from do stream, read undo data from the second snapshot SS2, update the undo stream with undo data and apply data read from the do stream to the second snapshot SS2 (560).
It should be understood that the methods illustrated in FIGS. 5B-5D may repeat periodically, for example every thirty minutes, thereby assuring the snapshot used as the target replication contains only changes from that period of time.
FIG. 7 is a block diagram of an example embodiment DPA 735 according to the present invention. The DPA includes memory 790 storing program logic 795, a processor 780, a communications interface 760, a replicator 781 and a manager 782.
The methods and apparatus of this invention may take the form, at least partially, of program code (i.e., instructions) embodied in tangible non-transitory media, such as floppy diskettes, CD-ROMs, hard drives, random access or read only-memory, or any other machine-readable storage medium. When the program code is loaded into and executed by a machine, such as the computer of FIG. 7, the machine becomes an apparatus for practicing the invention. When implemented on one or more general-purpose processors, the program code combines with such a processor to provide a unique apparatus that operates analogously to specific logic circuits. As such a general purpose digital machine can be transformed into a special purpose digital machine.
FIG. 8 shows program logic 855 embodied on a computer-readable medium 860 as shown, and wherein the logic is encoded in computer-executable code configured for carrying out this invention and thereby forming a computer program product 800.
The logic for carrying out the method may be embodied as part of the aforementioned system, which is useful for carrying out a method described with reference to embodiments shown in, for example, FIGS. 3, 4, 5A-5D and 6. For purposes of illustrating the present invention, the invention is described as embodied in a specific configuration and using special logical arrangements, but one skilled in the art will appreciate that the device is not limited to the specific configuration but rather only by the claims included with this specification.
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.
In reading the above description, persons skilled in the art will realize that there are many apparent variations that can be applied to the methods and systems described. In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific exemplary embodiments without departing from the broader spirit and scope of the invention as set forth in the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

## Claims

1. A method for leveraging snapshots for immediate continuous data protection (CDP) in a data replication environment comprising a production site comprising a host, a storage array, and a splitter, the method comprising:
creating a replica of a production volume at the production site;
configuring a replication service to replicate from the production volume to the replica of the production volume; and
performing replication to the replica of the production volume using changes tracked at the production site, comprising:
attaching the production volume to the splitter as clean;
attaching the replica to the splitter as clean;
tracking input/output (I/O) commands arriving at the splitter; and
starting replication to the replica of the production volume using the tracked I/O commands.

2. The method of claim 1 wherein the replica is a clone or a snapshot.

3. The method of claim 1 wherein the production volume is among a plurality of volumes in a consistency group and wherein creating a replica of a production volume comprises creating respective replicas of all volumes in the consistency group contemporaneously.

4. The method of claim 1 wherein the production volume is among a plurality of volumes in a consistency group and wherein creating a replica of the production volume comprises:
creating a respective replica of each volume in the consistency group sequentially; and
making the replicas consistent at the same point in time by applying changes from a journal to each replica.

5. The method of claim 1 wherein the replica of the production volume is a first snapshot, the method further comprising:
creating a first bookmark in a journal;
creating a second snapshot of the production volume; and
creating a second bookmark in the journal.

6. The method of claim 5 further comprising maintaining thin snapshots by updating the snapshot of the production volume with journal data.

7. The method of claim 5 wherein updating the snapshot of the production volume comprises:
reading the first bookmark from a do stream of the journal; and
applying replication data to the first snapshot and the second snapshot.

8. The method of claim 7 wherein updating the snapshot of the production volume further comprises:
reading the second bookmark from the do stream of the journal;
deleting the first snapshot;
applying the replication data to the second snapshot; and
using the second snapshot as the replica copy.

9. An apparatus for leveraging snapshots for immediate continuous data protection (CDP), the apparatus comprising:
a replicator configured to create a replica of a production volume at a production site; and
a manager configured to configure a replication service to replicate from the production volume to the replica of the production volume at the production site, perform replication using changes tracked at the production site, attach the production volume to a splitter as clean, attach the replica to the splitter as clean, track input/output (I/O) commands arriving at the splitter and start replication to the replica of the production volume using the tracked I/O commands.

10. The apparatus of claim 9 wherein the replica is a clone or a snapshot.

11. The apparatus of claim 9 wherein the production volume is among a plurality of volumes in a consistency group and wherein the replicator is further configured to create respective replicas of all volumes in the consistency group contemporaneously.

12. The apparatus of claim 9 wherein the production volume is among a plurality of volumes in a consistency group and wherein the replicator is further configured to create respective replicas of each volume in the consistency group sequentially and make the replicas consistent at the same point in time by applying changes from a journal to each replica.

13. The apparatus of claim 9 wherein the replica of the production volume is a first snapshot and wherein the replicator is further configured to create a first bookmark in a journal, create a second snapshot of the production volume and create a second bookmark in the journal.

14. The apparatus of claim 13 further comprising a journal configured to provide a protection window and wherein the manager is further configured to maintain thin snapshots by updating the snapshot of the production volume with journal data.

15. The apparatus of claim 13 wherein the manager is further configured to read the first bookmark from a do stream of the journal and apply replication data to the first snapshot and the second snapshot.

16. The apparatus of claim 15 wherein the manager is further configured to read the second bookmark from the do stream of the journal, delete the first snapshot, apply replication data to the second snapshot and use the second snapshot as the replica copy.

17. A computer-program product including a non-transitory computer-readable storage medium encoded with computer-program code that, when executed on a processor of a computer, cause the computer to leverage snapshots for immediate continuous data protection (CDP), the computer-program code comprising:
computer-program code for creating a replica of a production volume at a production site;
computer-program code for configuring a replication service to replicate from the production volume to the replica of the production volume at the production site; and
computer-program code for performing replication using changes tracked at the production site, comprising:
computer-program code for attaching the production volume to a splitter as clean;
computer-program code for attaching the replica to the splitter as clean;
computer-program code for tracking input/output (I/O) commands arriving at the splitter; and
computer-program code for starting replication to the replica of the production volume using the tracked I/O commands.

18. The computer-program product of claim 17 wherein the replica is a clone or a snapshot.

19. The computer-program product of claim 17 wherein the production volume is among a plurality of volumes in a consistency group and wherein computer-program code for creating a replica of a production volume comprises computer-program code for creating respective clones of all volumes in the consistency group contemporaneously or creating a respective clone of each volume in the consistency group sequentially and computer-program code for making the clones consistent at the same point in time by applying changes from a journal to each clone.

20. The computer-program product of claim 17 wherein a journal provides a protection window and wherein the replica of the production volume is a first snapshot, the computer-program product further comprising:
computer-program code for creating a first bookmark in the journal;
computer-program code for creating a second snapshot of the production volume;
computer-program code for creating a second bookmark in the journal; and
computer-program code for maintaining thin snapshots by updating the snapshot of the production volume with journal data.

21. The computer-program product of claim 20 wherein computer-program code for updating the snapshot of the production volume comprises:
computer-program code for reading the first bookmark from a do stream of the journal;
computer-program code for applying replication data to the first snapshot and the second snapshot;
computer-program code for reading the second bookmark from the do stream of the journal;
computer-program code for deleting the first snapshot;
computer-program code for applying the replication data to the second snapshot; and
computer-program code for using the second snapshot as the replica copy.