Patent Publication Number: US-11386070-B2

Title: Method and system for secure data replication data integrity verification

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is related to U.S. patent application Ser. No. 14/102,043 entitled “ENCRYPTED VIRTUAL MACHINES IN A CLOUD” filed on Dec. 10, 2013 the teachings of which are hereby incorporated herein by reference in their entirety. 
     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 virtual machine replication. 
     BACKGROUND 
     Computer data is vital to today&#39;s organizations, and a significant part of protection against disasters is focused on data protection. As solid-state memory has advanced to the point where cost of memory has become a relatively insignificant factor, organizations can afford to operate with systems that store and process terabytes of data. 
     Conventional data protection systems include tape backup drives, for storing organizational production site data on a periodic basis. 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&#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. Data replication systems generally operate either at the application level, at the file system level, at the hypervisor 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. 
     One challenge to continuous data protection is the ability of a backup site to keep pace with the data transactions of a production site, without slowing down the production site. The overhead of journaling inherently requires several data transactions at the backup site for each data transaction at the production site. As such, when data transactions occur at a high rate at the production site, the backup site may not be able to finish backing up one data transaction before the next production site data transaction occurs. If the production site is not forced to slow down, then necessarily a backlog of un-logged data transactions may build up at the backup site. Without being able to satisfactorily adapt dynamically to changing data transaction rates, a continuous data protection system chokes and eventually forces the production site to shut down. 
     SUMMARY 
     Example embodiments of the present invention relate to a method, a system, and a computer program product for verifying the integrity of replicated virtual machine data. The method includes reading data from a production volume at a production site. A representation of the data may be stored at a replica site with the integrity of the data stored in the replica volume being verified according to the representation of the data. 
    
    
     
       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 block diagram of a data protection system according to an example embodiment of the present invention; 
         FIG. 2  is a block diagram of a write transaction for a journal according to an example embodiment of the present invention; 
         FIG. 3  is a block diagram of a secure data replication system storing data according to an example embodiment of the present invention; 
         FIG. 4  is a block diagram of a secure data replication system verifying integrity of data according to an example embodiment of the present invention; 
         FIGS. 5A-5D  are flow diagrams illustrating methods for storing and verifying integrity of data according to respective example embodiments of the present invention; 
         FIG. 6  is a block diagram of an apparatus according to example embodiment of the present invention; and 
         FIG. 7  is an illustration of computer program code according to an example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     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; 
     BLOCK VIRTUALIZATION—may be a layer which take back end storage volumes and, by slicing, concatenating and striping, creates a new set of volumes that serve as base volumes or devices in the virtualization layer; 
     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; 
     CONTINUOUS DATA PROTECTION (CDP)—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 may be in the same storage array as the production volume; 
     CONTINUOUS REMOTE REPLICATION (CRR)—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 at a site remote to the production volume and on a separate storage array; 
     DATA PROTECTION APPLIANCE (DPA)—a computer or a cluster of computers (i.e., a physical device), or a set of processes (i.e., a virtual device or a combination of virtual and physical devices), 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; 
     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; 
     DISTRIBUTED MIRROR—may be a mirror of a volume across a distance, either metro- or geo-, which is accessible at all sites; 
     FAIL ALL MODE—may be a mode of a volume in the splitter where all write and read I/Os intercepted by the splitter are failed to the host, but other SCSI command, like read capacity, are served; 
     GLOBAL FAIL ALL MODE—may be a mode of a volume in the virtual layer where all write and read I/Os to the virtual layer are failed to the host, but other SCSI commands, like read capacity, are served; 
     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; 
     LOGGED ACCESS—may be an access method provided by the appliance and the splitter in which the appliance rolls the volumes of the consistency group to the point in time the user requested and let the host access the volumes in a copy on first write base; 
     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 or one or more virtual disks or virtual LUNs which may correspond to one or more virtual machines; 
     MANAGEMENT AND DEPLOYMENT TOOLS—provide the means to deploy, control, and manage DPAs through virtual environment management tools; 
     MARKING ON SPLITTER—may be a mode in a splitter where intercepted I/Os are not split to an appliance and the storage, but rather changes (meta data) are tracked in a list and/or a bitmap and I/Os are sent immediately down the I/O stack; 
     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 (i.e., physical or virtual) where one or more host computers run data processing applications that write data to a storage system and read data from the storage system; 
     REPLICATION PROTECTION APPLIANCE (RPA)—another name for DPA; 
     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 (i.e., physical or virtual) 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 (i.e., in both physical and virtual systems) 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 may be in the IO stack of a system and may be located in the hypervisor for virtual machines; 
     STORAGE MEDIUM—may refer to one or more storage mediums such as a hard drive, a combination of hard drives, flash storage, combinations of flash storage, combinations of hard drives, flash, and other storage devices, and other types and combinations of computer readable storage mediums including those yet to be conceived; a storage medium may also refer both physical and logical storage mediums and may include multiple level of virtual to physical mappings and may be or include an image or disk image; 
     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 (i.e., physical or virtual) 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; 
     VASA—vSphere storage Application programming interfaces (APIs) for storage Awareness; 
     VIRTUAL ACCESS—may be an access method provided by the appliance and the splitter in which the appliance exposes a virtual volume from a specific point in time to the host, the data for the virtual volume is partially stored on the remote copy and partially stored on the journal; 
     VIRTUAL VOLUME—may be a volume which is exposed to a host by a virtualization layer and may span across more than one site; 
     VMDK—a virtual machine disk file containing disk data in a VMFS (analog to a LUN in a block storage array); 
     VMFS—a virtual machine file system provide by VMware, Inc. for storing a virtual machine; 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 point in 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 &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. 
     Image Access 
     Herein, some information is provided for conventional continuous data protection systems having journaling and a replication splitter which may be used in one or more embodiments is provided. A replication may set refer to an association created between the source volume and the local and/or remote target volumes, and a consistency group contains one or more replication sets. A snapshot may be the difference between one consistent image of stored data and the next. The exact time for closing the snapshot may determined dynamically depending on replication policies and the journal of the consistency group. 
     In synchronous replication, each write may be a snapshot. When the snapshot is distributed to a replica, it may be stored in the journal volume, so that is it possible to revert to previous images by using the stored snapshots. As noted above, a splitter mirrors may write from an application server to LUNs being protected by the data protection appliance. When a write is requested from the application server it may be split and sent to the appliance using a host splitter/driver (residing in the I/O stack, below any file system and volume manager, and just above any multipath driver (such as EMC® PowerPath™), through an intelligent fabric switch, through array-based splitter, such as EMC CLARiiON®. 
     There may be a number of image access modes. Image access may be used to restore production from the disaster recovery site, and to roll back to a previous state of the data. Image access may be also to temporarily operate systems from a replicated copy while maintenance work is carried out on the production site and to fail over to the replica. When image access is enabled, host applications at the copy site may be able to access the replica. 
     In virtual access, the system may create the image selected in a separate virtual LUN within the data protection appliance. While performance may be constrained by the appliance, access to the point-in-time image may be nearly instantaneous. The image may be used in the same way as logged access (physical), noting that data changes are temporary and stored in the local journal. Generally, this type of image access is chosen because the user may not be sure which image, or point in time is needed. The user may access several images to conduct forensics and determine which replica is required. Note that in known systems, one cannot recover the production site from a virtual image since the virtual image is temporary. Generally, when analysis on the virtual image is completed, the choice is made to disable image access. 
     If it is determined the image should be maintained, then access may be changed to logged access using ‘roll to image.’ When disable image access is disabled, the virtual LUN and all writes to it may be discarded. 
     In an embodiment of virtual access with roll image in background, the system first creates the image in a virtual volume managed by the data protection appliance to provide rapid access to the image, the same as in virtual access. Simultaneously in background, the system may roll to the physical image. Once the system has completed this action, the virtual volume may be discarded, and the physical volume may take its place. At this point, the system continues to function as if logged image access was initially selected. The switch from virtual to physical may be transparent to the servers and applications and the user may not see any difference in access. Once this occurs, changes may be read from the physical volume instead of being performed by the appliance. If image access is disabled, the writes to the volume while image access was enabled may be rolled back (undone). Then distribution to storage may continue from the accessed image forward. 
     In some embodiments in physical logged access, the system rolls backward (or forward) to the selected snapshot (point in time). There may be a delay while the successive snapshots are applied to the replica image to create the selected image. The length of delay may depend on how far the selected snapshot is from the snapshot currently being distributed to storage. Once the access is enabled, hosts may read data directly from the volume and writes may be handled through the DPA. The host may read the undo data of the write and the appliance may store the undo data in a logged access journal. During logged access the distribution of snapshots from the journal to storage may be paused. When image access is disabled, writes to the volume while image access was enabled (tracked in the logged access journal) may be rolled back (undone). Then distribution to storage may continue from the accessed snapshot forward. 
     Disable image access may mean changes to the replica may be discarded or thrown away. It may not matter what type of access was initiated, that is, logged or another type, or whether the image chosen was the latest or an image back in time. Disable image access effectively says the work done at the disaster recovery site is no longer needed. 
     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, 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 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, all of which are assigned to EMC Corporation of Hopkinton, Mass. and are hereby incorporated by reference in their entirety. 
     Secure Data Replication Data Integrity Verification 
     Encryption of virtual machines for secure replication is described in copending U.S. patent application Ser. No. 14/102,043 entitled “ENCRYPTED VIRTUAL MACHINES IN A CLOUD” filed on Dec. 10, 2013 the teachings of which are hereby incorporated herein by reference in their entirety. 
     Conventional solutions for recovering encrypted virtual machines at a backup site involve storing keys at the backup site. Typically, these solutions require that the keys exist at the backup site at all times, since the encryption solution is not usually integrated with the replication solution. Usually a solution will include encrypting the replication data with a first key, sending the encrypted data to the replica site, decrypting the encrypted data at the replica site, and then writing the data at the replica site to a protected storage which encrypts the data again using a second key. Generally, this introduces a vulnerability since the second key to the storage has to be present at the replica site throughout the replication process, and not only at recovery times. 
     Replication to the public cloud has many security challenges, mainly around security of the data. For example, traditional methods do not allow data verification with continuous replication. Although some traditional methods may allow adding checks with the data, these traditional methods require additional storage space in the volume for storing these checks and cause performance and alignment issues. Example embodiments of the present invention overcome these and other deficiencies by providing a method and a system for verifying the integrity of replicated virtual machine data to verify that cloud data was not tampered with. 
       FIGS. 3 and 4  are block diagrams of a secure data replication system  300 ,  400  for storing and verifying integrity of data according to respective example embodiments of the present invention at respective times.  FIGS. 5A-5D  are flow diagrams illustrating methods for storing and verifying integrity of data according to respective example embodiments of the present invention.  FIGS. 3, 4, and 5A-5D  may be described in conjunction. 
     As illustrated in  FIGS. 3 and 4 , the system  300 ,  400  includes a production site  305 - 1 ,  405 - 1  and a replica site  305 - 2 ,  405 - 2 . The production site  305 - 1 ,  405 - 1  may include a hypervisor  310 - 1 ,  410 - 1  operating a virtual data protection appliance (VDPA)  320 ,  420 , a splitter  325 ,  425  operating in the hypervisor kernel  315 ,  415 , and one or more user virtual machines (VMs)  330 ,  430 . The production site  305 - 1 ,  405 - 1  also may include trusted storage (e.g., a production volume)  335 ,  435  for storing user data, and a key manager  340 - 1 ,  440 - 1  for issuing a key  345 ,  445  to the VDPA  320 ,  420  for encryption of user data  350 . The replica site  305 - 2 ,  405 - 2  also may include a hypervisor  310 - 2 ,  410 - 2  operating a VDPA  355 ,  455  and one or more VMs  360 ,  460  and a key manager  340 - 2 ,  440 - 2  for issuing a key (not shown) to the replica site VDPA  355 ,  455 . The replica site  305 - 2 ,  405 - 2  also may include a journal  365 ,  465  and storage (e.g., a replica volume)  370 ,  470  for storing encrypted user data. 
     When production site VM  330  is configured for secured replication, a replica VM  360  is generated at replica site  305 - 2  with the same configuration of VM  330  on the production site  305 - 1 , and a key  345  is generated for VM  330 . The key  345  is generated by a key manager  340 - 1 . The key  345  may be generated with a unique key ID which may be later used for retrieving the key  345  from the key manager  340 - 1 . In certain embodiments, Key IDs may be stored at a replication site journal  365 , which may be for persistency, and in the memory of VDPA  355 . Key IDs also may be stored at a production site journal (not shown) on trusted storage  335 . Key IDs also may be stored on the production site VDPA  320 . The encrypting VDPA  320  also may store a volatile copy of the key  345  for encryption. While the embodiment illustrated in  FIGS. 3 and 4  shows a single virtual machine on the production site and the replication site, in certain embodiments, the production site and replication site hypervisors may have multiple virtual machines. In some embodiments, the key manager may likewise store multiple keys corresponding to multiple virtual machines. 
     It should be understood that, as described in copending U.S. patent application Ser. No. 14/102,043 entitled “ENCRYPTED VIRTUAL MACHINES IN A CLOUD” filed on Dec. 10, 2013, the five step journaling process described above may be performed between the production site  305 - 1  and the replica site  305 - 2  for I/O operations being performed at the production site  305 - 1  and the user data is encrypted, thus not allowing the plain data to arrive to the replica site. The encrypted data  350 ″ may be sent from the production site VDPA  320  to the replica site VDPA  355  to the replica site storage  370 . However, in such embodiments, the user cannot know if the image in the cloud was tampered with (i.e., if someone replaced the encrypted data in the replica site  305 - 2  (e.g., the “cloud”) with other data; thus, if the user tries to recover data, the image will be corrupt). 
     As described above, data protection products, such as EMC RecoverPoint® by EMC Corporation of Hopkinton, Mass. may manage a journal. Example embodiments of the present invention add integrity verifications in the journal for the stream of replicated data arriving at the replica site. As will be described in greater detail below, the system may add periodic hash values for sections of the volume at the production site in the journal. In other words, every period of time the system may add within the journal a hash value of a portion of the production site storage, thus allowing the system to verify the portion is indeed correct (i.e., if the portion was not touched again since the hash value was changed). 
     In certain embodiments, when a user wants to recover a volume from a point in time, the system will look for hash values in the journal around the relevant point in time and, for each hash value which should not have changed, a checking protocol will run to verify that the encrypted data indeed matches the actual data. In other embodiments, for special points in time, the system may include a full list of all the hashes (e.g., points in time which are daily application consistent backup). Therefore, a user recovering for such points in time will have assurance that the recovered data is identical to the backed up data. In yet other embodiments, the integrity verification may be a random testing that certain blocks did not change, thus giving partial assurance. 
     As illustrated in  FIGS. 3 and 5A , the production site VDPA  320  may read data (e.g., plain, unencrypted data) from the trusted storage production volume  335  at the production site  305 - 1  ( 505 ) and store a representation (e.g., hash) of the plain data  350 ′ at the replica site  305 - 2  ( 510 ). It should be understood that the encrypted user data  350 ″ may be sent at a different time as described above with reference to the five step journaling process. As will be described below, the replica site VDPA  355  then may later verify the integrity of the encrypted data  350 ″ stored in the storage  370  of the replica site  305 - 2  ( 550 ). 
     As illustrated in  FIGS. 3 and 5B , to store the representation of the data  350 ′ at the replica site  305 - 2  ( 510 ), the production site VDPA  320  may generate the representation of the data  350 ′ (i.e., a hash or other similar value) ( 515 ) and forward the representation of the data  350 ′ to the replica site  305 - 2 . The replica site VDPA  355  then may receive the representation of the data  350 ′ and store the representation of the data  350 ′ to the journal  365  ( 540 ). It should be understood that, in a preferred embodiment, hashes are performed on chunks of data (e.g., 1 MB). In certain embodiments, performing hashes on a per I/O basis during the replication process would be problematic as the data would need to be reread in order to recalculate the hash. 
     In certain embodiments, as illustrated in  FIGS. 3 and 5C , to generate the representation of the data  350 ′ ( 515 ), the production site VDPA  320  may read plain (i.e., unencrypted) data  350  from the trusted storage  335  ( 520 ) and then generate the representation of the data  350 ′ as a representation of the plain data by performing an operation (e.g., hash) on the plain data ( 530 ) to generate the representation of the data  350 ′. The representation of the data  350 ′ then may be sent from the production site VDPA  320  to the replica site VDPA  355  using the five step journaling process described above, with, in certain embodiments, the representation of the data  350 ′ being written to the journal  365  at the replica site  305 - 2  ( 545 ). It should be understood that, in certain embodiments, the key  345  may be a key that is not sent to the replica site  305 - 2  except for failover situations. 
     In certain embodiments, the production site VDPA  320  may generate the representation of the data  350 ′ on a periodic basis or, in other embodiments, based on a volume at the production site  305 - 2 . Example embodiments of the present invention are equally as applicable to CDP (i.e., hashes calculated periodically) and snapshots (i.e., hash calculated for a full snapshot). In other embodiments, when a bookmark is created, a snapshot also may be created on the production site storage  335 , with a hash for the snapshot being created and sent to the replica site journal  365 . When a user wants to access this point in time, a full hash of the PIT is available for verification. 
     In other words, example embodiments of the present invention may include an integrity verification engine that runs at the production site  305 - 1  and periodically reads plain data  350  from the production volume  335  and creates a hash value  350 ′ of the plain data  350 . The hash  350 ′ then may be stored in the replica site journal  365  (e.g., periodically, for a chunk of data, or for a point in time). Therefore, if someone has access to the replica site storage  370  (i.e., a vulnerability) but not the replica site VDPA  355  they will not be able to tamper with the data. 
     As illustrated in  FIGS. 4 and 5D , the replica site VDPA  455  may verify the integrity of data stored in a replica volume storage  470  ( 550 ), such as prior to allowing access to the replica volume  470 . For example, as illustrated in  FIG. 5D , if a user requests access to a point in time image at the replica site  405 - 2 , the key  445  for accessing the data may be transferred from the production site  405 - 1  to the replica site  405 - 2 . The replica site VDPA  455  then may start verifying integrity of the encrypted data  450 ″ at the replica site ( 555 ). In a preferred embodiment, the replica site VDPA  455  looks for a hash of a data portion which was not overwritten in the journal  475 . The replica site VDPA  455  then may read encrypted data  450 ″ from the replica site storage  470  and decrypt it according to the key  445  transferred from the production site  405 - 1  to the replica site  405 - 2 . The replica site VDPA  455  then may take a hash of the decrypted data (not shown) and compare it to the hash  450 ′ of the plain data retrieved from the journal  475 . If the newly calculated hash (not shown) does not match the hash  450 ′ retrieved from the journal  475 , the encrypted data  450 ″ may be marked as tampered with. 
     Example embodiments of the present invention may perform verification at various points, including during image access mode, during replication, and when accessing a point in time. 
     In certain embodiments, the replica site VDPA  455  may verify encrypted data integrity when in image access mode. For example, the replica site VDPA  455  may receive a selection of an image at the replica site and examine one or more representations of data (i.e., hashes) stored previously in the journal  475  at the replica site  405 - 2  for locations in the volume  470  not overwritten since the representation of data was generated. Therefore, when the user VM  460  accesses an image, the VDPA  455  may look for hash values for the portions of data stored previously on the journal for the locations that were not overwritten since the hash value was taken. The VDPA  455  then may verify that the data was not tampered with by calculating the hash value for data  450  in the encrypted storage according to the key  445 . 
     In other embodiments, the replica site VDPA  455  may verify encrypted data integrity while performing replication. In other words, every time the VDPA  455  reads a hash  450 ′ from the journal  475 , the VDPA  455  may verify that the hash  450 ′ corresponds to the encrypted data  450 ″ on the replica site storage  470  and that the encrypted data  450 ″ was not tampered with again by reading the encrypted data  450 ″ from the replica site storage  470 , decrypting it using the key  445  transferred from the production site  405 - 1  to the replica site  405 - 2 , and calculating a hash of the decrypted data (not shown) and comparing the newly calculated hash (not shown) to the hash  450 ′ retrieved from the journal  475 . 
     In yet other embodiments, for a specific point in time, the VDPA  455  may receive a selection of a point in time (e.g., from a user) at the replica site  405 - 2 . The VDPA  455  then may examine the journal  475  at the replica site  405 - 2  prior to the point in time for one or more representations of data (e.g., hashes)  450 ′. 
     It should be understood that, in certain embodiments, the VDPA  455  may provide access to the encrypted data  450  according to the verification of the integrity of the data by unencrypting the data according to the private key (e.g., key  345  of  FIG. 3 ). 
       FIG. 6  is a block diagram of an example embodiment apparatus  605  according to the present invention. The apparatus  605  may be part of a system  600  and includes memory  610  storing program logic  615 , a processor  620  for executing a process  625 , and a communications I/O interface  630 , connected via a bus  635 . 
     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. 6 , 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. 7  shows program logic  755  embodied on a computer-readable medium  760  as shown, and wherein the logic  755  is encoded in computer-executable code configured for carrying out the methods of this invention, thereby forming a computer program product  700 . 
     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. 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.