Abstract:
In one aspect, a method includes pausing write I/Os for a second virtual machine running at a second site, generating a snapshot of a first virtual machine running at a first site after pausing the write I/Os for the second virtual machine, generating a bookmark for the second virtual machine, resuming the write I/Os for the second virtual machine after generating the bookmark, rolling a journal for the second virtual machine to a point-in-time of the bookmark and generating, at the first site, a second snapshot of the second virtual volume for the point-in-time of the bookmark using the journal.

Description:
BACKGROUND 
     Computer data is vital to today&#39;s organizations and a significant part of protection against disasters is focused on data protection. As solid-state memory has advanced to the point where cost of memory has become a relatively insignificant factor, organizations can afford to operate with systems that store and process terabytes of data. 
     Conventional data protection systems include tape backup drives, for storing organizational production site data on a periodic basis. Another conventional data protection system uses data replication, by creating a copy of production site data of an organization on a secondary backup storage system, and updating the backup with changes. The backup storage system may be situated in the same physical location as the production storage system, or in a physically remote location. Data replication systems generally operate either at the application level, at the file system level, or at the data block level. 
     SUMMARY 
     In one aspect, a method includes pausing write I/Os for a second virtual machine running at a second site, generating a snapshot of a first virtual machine running at a first site after pausing the write I/Os for the second virtual machine, generating a bookmark for the second virtual machine, resuming the write I/Os for the second virtual machine after generating the bookmark, rolling a journal for the second virtual machine to a point-in-time of the bookmark and generating, at the first site, a second snapshot of the second virtual volume for the point-in-time of the bookmark using the journal. 
     In another aspect, an apparatus includes electronic hardware circuitry configured to pause write I/Os for a second virtual machine running at a second site, generate a snapshot of a first virtual machine running at a first site after pausing the write I/Os for the second virtual machine, generate a bookmark for the second virtual machine, resume the write I/Os for the second virtual machine after generating the bookmark, roll a journal for the second virtual machine to a point-in-time of the bookmark and generate, at the first site, a second snapshot of the second virtual volume for the point-in-time of the bookmark using the journal. 
     In a further aspect, an article includes a non-transitory computer-readable medium that stores computer-executable instructions. The instructions cause a machine to pause write I/Os for a second virtual machine running at a second site, generate a snapshot of a first virtual machine running at a first site after pausing the write I/Os for the second virtual machine, generate a bookmark for the second virtual machine, resume the write I/Os for the second virtual machine after generating the bookmark, roll a journal for the second virtual machine to a point-in-time of the bookmark and generate, at the first site, a second snapshot of the second virtual volume for the point-in-time of the bookmark using the journal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example of a data protection system. 
         FIG. 2  is an illustration of an example of a journal history of write transactions for a storage system. 
         FIG. 3  is a block diagram of an example of the data protection system used in a virtual environment. 
         FIG. 4  is a simplified block diagram of an example of a data replication system that includes an additional virtual machine at the replication site. 
         FIG. 5  is a flowchart of an example of a process to maintain a consistent point-in-time at the production site. 
         FIG. 6  is a simplified block diagram of an example of a computer on which any of the process of  FIG. 5  may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     The following definitions may be useful in understanding the specification and claims. 
     BACKUP SITE—a facility where replicated production site data is stored; the backup site may be located in a remote site or at the same location as the production site; 
     BOOKMARK—a bookmark is metadata information stored in a replication journal which indicates a point in time. 
     DATA PROTECTION APPLIANCE (DPA)—a computer or a cluster of computers responsible for data protection services including inter alia data replication of a storage system, and journaling of I/O requests issued by a host computer to the storage system; 
     HOST—at least one computer or networks of computers that runs at least one data processing application that issues I/O requests to one or more storage systems; a host is an initiator with a SAN; 
     HOST DEVICE—an internal interface in a host, to a logical storage unit; 
     IMAGE—a copy of a logical storage unit at a specific point in time; 
     INITIATOR—a node in a SAN that issues I/O requests; 
     I/O REQUEST—an input/output request (sometimes referred to as an I/O), which may be a read I/O request (sometimes referred to as a read request or a read) or a write I/O request (sometimes referred to as a write request or a write); 
     JOURNAL—a record of write transactions issued to a storage system; used to maintain a duplicate storage system, and to roll back the duplicate storage system to a previous point in time; 
     LOGICAL UNIT—a logical entity provided by a storage system for accessing data from the storage system. The logical disk may be a physical logical unit or a virtual logical unit; 
     LUN—a logical unit number for identifying a logical unit; 
     PHYSICAL LOGICAL UNIT—a physical entity, such as a disk or an array of disks, for storing data in storage locations that can be accessed by address; 
     PRODUCTION SITE—a facility where one or more host computers run data processing applications that write data to a storage system and read data from the storage system; 
     REMOTE ACKNOWLEDGEMENTS—an acknowledgement from remote DPA to the local DPA that data arrived at the remote DPA (either to the appliance or the journal) 
     SPLITITER ACKNOWLEDGEMENT—an acknowledgement from a DPA to the protection agent (splitter) that data has been received at the DPA; this may be achieved by an SCSI status command. 
     SAN—a storage area network of nodes that send and receive an I/O and other requests, each node in the network being an initiator or a target, or both an initiator and a target; 
     SOURCE SIDE—a transmitter of data within a data replication workflow, during normal operation a production site is the source side; and during data recovery a backup site is the source side, sometimes called a primary side; 
     STORAGE SYSTEM—a SAN entity that provides multiple logical units for access by multiple SAN initiators 
     TARGET—a node in a SAN that replies to I/O requests; 
     TARGET SIDE—a receiver of data within a data replication workflow; during normal operation a back site is the target side, and during data recovery a production site is the target side, sometimes called a secondary side; 
     THIN PROVISIONING—thin provisioning involves the allocation of physical storage when it is needed rather than allocating the entire physical storage in the beginning. Thus, use of thin provisioning is known to improve storage utilization. 
     THIN LOGICAL UNIT—a thin logical unit is a logical unit that uses thin provisioning; 
     VIRTUAL LOGICAL UNIT—a virtual storage entity which is treated as a logical unit by virtual machines; 
     WAN—a wide area network that connects local networks and enables them to communicate with one another, such as the Internet. 
     A description of journaling and some techniques associated with journaling may be described in the patent titled “METHODS AND APPARATUS FOR OPTIMAL JOURNALING FOR CONTINUOUS DATA REPLICATION” and with U.S. Pat. No. 7,516,287, which is hereby incorporated by reference. 
     Referring to  FIG. 1 , a data protection system  100  includes two sites; Site I, which is a production site, and Site II, which is a backup site or replica site. Under normal operation the production site is the source side of system  100 , and the backup site is the target side of the system. The backup site is responsible for replicating production site data. Additionally, the backup site enables roll back of Site I data to an earlier pointing time, which may be used in the event of data corruption of a disaster, or alternatively in order to view or to access data from an earlier point in time. 
       FIG. 1  is an overview of a system for data replication of either physical or virtual logical units. Thus, one of ordinary skill in the art would appreciate that in a virtual environment a hypervisor, in one example, would consume logical units and generate a distributed file system on them such as VMFS creates files in the file system and expose the files as logical units to the virtual machines (each VMDK is seen as a SCSI device by virtual hosts). In another example, the hypervisor consumes a network based file system and exposes files in the NFS as SCSI devices to virtual hosts. 
     During normal operations, the direction of replicate data flow goes from source side to target side. It is possible, however, for a user to reverse the direction of replicate data flow, in which case Site I starts to behave as a target backup site, and Site II starts to behave as a source production site. Such change of replication direction is referred to as a “failover”. A failover may be performed in the event of a disaster at the production site, or for other reasons. In some data architectures, Site I or Site II behaves as a production site for a portion of stored data, and behaves simultaneously as a backup site for another portion of stored data. In some data architectures, a portion of stored data is replicated to a backup site, and another portion is not. 
     The production site and the backup site may be remote from one another, or they may both be situated at a common site, local to one another. Local data protection has the advantage of minimizing data lag between target and source, and remote data protection has the advantage is being robust in the event that a disaster occurs at the source side. 
     The source and target sides communicate via a wide area network (WAN)  128 , although other types of networks may be used. 
     Each side of system  100  includes three major components coupled via a storage area network (SAN); namely, (i) a storage system, (ii) a host computer, and (iii) a data protection appliance (DPA). Specifically with reference to  FIG. 1 , the source side SAN includes a source host computer  104 , a source storage system  108 , and a source DPA  112 . Similarly, the target side SAN includes a target host computer  116 , a target storage system  120 , and a target DPA  124 . As well, the protection agent (sometimes referred to as a splitter) may run on the host, or on the storage, or in the network or at a hypervisor level, and that DPAs are optional and DPA code may run on the storage array too, or the DPA  124  may run as a virtual machine. 
     Generally, a SAN includes one or more devices, referred to as “nodes”. A node in a SAN may be an “initiator” or a “target”, or both. An initiator node is a device that is able to initiate requests to one or more other devices; and a target node is a device that is able to reply to requests, such as SCSI commands, sent by an initiator node. A SAN may also include network switches, such as fiber channel switches. The communication links between each host computer and its corresponding storage system may be any appropriate medium suitable for data transfer, such as fiber communication channel links. 
     The host communicates with its corresponding storage system using small computer system interface (SCSI) commands. 
     System  100  includes source storage system  108  and target storage system  120 . Each storage system includes physical storage units for storing data, such as disks or arrays of disks. Typically, storage systems  108  and  120  are target nodes. In order to enable initiators to send requests to storage system  108 , storage system  108  exposes one or more logical units (LU) to which commands are issued. Thus, storage systems  108  and  120  are SAN entities that provide multiple logical units for access by multiple SAN initiators. 
     Logical units are a logical entity provided by a storage system, for accessing data stored in the storage system. The logical unit may be a physical logical unit or a virtual logical unit. A logical unit is identified by a unique logical unit number (LUN). Storage system  108  exposes a logical unit  136 , designated as LU A, and storage system  120  exposes a logical unit  156 , designated as LU B. 
     LU B is used for replicating LU A. As such, LU B is generated as a copy of LU A. In one embodiment, LU B is configured so that its size is identical to the size of LU A. Thus, for LU A, storage system  120  serves as a backup for source side storage system  108 . Alternatively, as mentioned hereinabove, some logical units of storage system  120  may be used to back up logical units of storage system  108 , and other logical units of storage system  120  may be used for other purposes. Moreover, there is symmetric replication whereby some logical units of storage system  108  are used for replicating logical units of storage system  120 , and other logical units of storage system  120  are used for replicating other logical units of storage system  108 . 
     System  100  includes a source side host computer  104  and a target side host computer  116 . A host computer may be one computer, or a plurality of computers, or a network of distributed computers, each computer may include inter alia a conventional CPU, volatile and non-volatile memory, a data bus, an I/O interface, a display interface and a network interface. Generally a host computer runs at least one data processing application, such as a database application and an e-mail server. 
     Generally, an operating system of a host computer creates a host device for each logical unit exposed by a storage system in the host computer SAN. A host device is a logical entity in a host computer, through which a host computer may access a logical unit. Host device  104  identifies LU A and generates a corresponding host device  140 , designated as Device A, through which it can access LU A. Similarly, host computer  116  identifies LU B and generates a corresponding device  160 , designated as Device B. 
     In the course of continuous operation, host computer  104  is a SAN initiator that issues I/O requests (write/read operations) through host device  140  to LU A using, for example, SCSI commands. Such requests are generally transmitted to LU A with an address that includes a specific device identifier, an offset within the device, and a data size. Offsets are generally aligned to 512 byte blocks. The average size of a write operation issued by host computer  104  may be, for example, 10 kilobytes (KB); i.e., 20 blocks. For an I/O rate of 50 megabytes (MB) per second, this corresponds to approximately 5,000 write transactions per second. 
     System  100  includes two data protection appliances, a source side DPA  112  and a target side DPA  124 . A DPA performs various data protection services, such as data replication of a storage system, and journaling of I/O requests issued by a host computer to source side storage system data. As explained in detail herein, when acting as a target side DPA, a DPA may also enable roll back of data to an earlier point in time, and processing of rolled back data at the target site. Each DPA  112  and  124  is a computer that includes inter alia one or more conventional CPUs and internal memory. 
     For additional safety precaution, each DPA is a cluster of such computers. Use of a cluster ensures that if a DPA computer is down, then the DPA functionality switches over to another computer. The DPA computers within a DPA cluster communicate with one another using at least one communication link suitable for data transfer via fiber channel or IP based protocols, or such other transfer protocol. One computer from the DPA cluster serves as the DPA leader. The DPA cluster leader coordinates between the computers in the cluster, and may also perform other tasks that require coordination between the computers, such as load balancing. 
     In the architecture illustrated in  FIG. 1 , DPA  112  and DPA  124  are standalone devices integrated within a SAN. Alternatively, each of DPA  112  and DPA  124  may be integrated into storage system  108  and storage system  120 , respectively, or integrated into host computer  104  and host computer  116 , respectively. Both DPAs communicate with their respective host computers through communication lines such as fiber channels using, for example, SCSI commands or any other protocol. 
     DPAs  112  and  124  are configured to act as initiators in the SAN; i.e., they can issue I/O requests using, for example, SCSI commands, to access logical units on their respective storage systems. DPA  112  and DPA  124  are also configured with the necessary functionality to act as targets; i.e., to reply to I/O requests, such as SCSI commands, issued by other initiators in the SAN, including inter alia their respective host computers  104  and  116 . Being target nodes, DPA  112  and DPA  124  may dynamically expose or remove one or more logical units. 
     As described hereinabove, Site I and Site II may each behave simultaneously as a production site and a backup site for different logical units. As such, DPA  112  and DPA  124  may each behave as a source DPA for some logical units, and as a target DPA for other logical units, at the same time. 
     Host computer  104  and host computer  116  include protection agents  144  and  164 , respectively. Protection agents  144  and  164  intercept SCSI commands issued by their respective host computers, via host devices to logical units that are accessible to the host computers. A data protection agent may act on an intercepted SCSI commands issued to a logical unit, in one of the following ways: send the SCSI commands to its intended logical unit; redirect the SCSI command to another logical unit, split the SCSI command by sending it first to the respective DPA; after the DPA returns an acknowledgement, send the SCSI command to its intended logical unit; fail a SCSI command by returning an error return code; and delay a SCSI command by not returning an acknowledgement to the respective host computer. 
     A protection agent may handle different SCSI commands, differently, according to the type of the command. For example, a SCSI command inquiring about the size of a certain logical unit may be sent directly to that logical unit, while a SCSI write command may be split and sent first to a DPA associated with the agent. A protection agent may also change its behavior for handling SCSI commands, for example as a result of an instruction received from the DPA. 
     Specifically, the behavior of a protection agent for a certain host device generally corresponds to the behavior of its associated DPA with respect to the logical unit of the host device. When a DPA behaves as a source site DPA for a certain logical unit, then during normal course of operation, the associated protection agent splits I/O requests issued by a host computer to the host device corresponding to that logical unit. Similarly, when a DPA behaves as a target device for a certain logical unit, then during normal course of operation, the associated protection agent fails I/O requests issued by host computer to the host device corresponding to that logical unit. 
     Communication between protection agents and their respective DPAs may use any protocol suitable for data transfer within a SAN, such as fiber channel, or SCSI over fiber channel. The communication may be direct, or via a logical unit exposed by the DPA. Protection agents communicate with their respective DPAs by sending SCSI commands over fiber channel. 
     Protection agents  144  and  164  are drivers located in their respective host computers  104  and  116 . Alternatively, a protection agent may also be located in a fiber channel switch, or in any other device situated in a data path between a host computer and a storage system or on the storage system itself. In a virtualized environment, the protection agent may run at the hypervisor layer or in a virtual machine providing a virtualization layer. 
     What follows is a detailed description of system behavior under normal production mode, and under recovery mode. 
     In production mode DPA  112  acts as a source site DPA for LU A. Thus, protection agent  144  is configured to act as a source side protection agent; i.e., as a splitter for host device A. Specifically, protection agent  144  replicates SCSI I/O write requests. A replicated SCSI I/O write request is sent to DPA  112 . After receiving an acknowledgement from DPA  124 , protection agent  144  then sends the SCSI I/O write request to LU A. After receiving a second acknowledgement from storage system  108  host computer  104  acknowledges that an I/O command complete. 
     When DPA  112  receives a replicated SCSI write request from data protection agent  144 , DPA  112  transmits certain I/O information characterizing the write request, packaged as a “write transaction”, over WAN  128  to DPA  124  on the target side, for journaling and for incorporation within target storage system  120 . 
     DPA  112  may send its write transactions to DPA  124  using a variety of modes of transmission, including inter alia (i) a synchronous mode, (ii) an asynchronous mode, and (iii) a snapshot mode. In synchronous mode, DPA  112  sends each write transaction to DPA  124 , receives back an acknowledgement from DPA  124 , and in turns sends an acknowledgement back to protection agent  144 . Protection agent  144  waits until receipt of such acknowledgement before sending the SCSI write request to LU A. 
     In asynchronous mode, DPA  112  sends an acknowledgement to protection agent  144  upon receipt of each I/O request, before receiving an acknowledgement back from DPA  124 . 
     In snapshot mode, DPA  112  receives several I/O requests and combines them into an aggregate “snapshot” of all write activity performed in the multiple I/O requests, and sends the snapshot to DPA  124 , for journaling and for incorporation in target storage system  120 . In snapshot mode DPA  112  also sends an acknowledgement to protection agent  144  upon receipt of each I/O request, before receiving an acknowledgement back from DPA  124 . 
     For the sake of clarity, the ensuing discussion assumes that information is transmitted at write-by-write granularity. 
     While in production mode, DPA  124  receives replicated data of LU A from DPA  112 , and performs journaling and writing to storage system  120 . When applying write operations to storage system  120 , DPA  124  acts as an initiator, and sends SCSI commands to LU B. 
     During a recovery mode, DPA  124  undoes the write transactions in the journal, so as to restore storage system  120  to the state it was at, at an earlier time. 
     As described hereinabove, LU B is used as a backup of LU A. As such, during normal production mode, while data written to LU A by host computer  104  is replicated from LU A to LU B, host computer  116  should not be sending  1 /O requests to LU B. To prevent such I/O requests from being sent, protection agent  164  acts as a target site protection agent for host Device B and fails I/O requests sent from host computer  116  to LU B through host Device B. 
     Target storage system  120  exposes a logical unit  176 , referred to as a “journal LU”, for maintaining a history of write transactions made to LU B, referred to as a “journal”. Alternatively, journal LU  176  may be striped over several logical units, or may reside within all of or a portion of another logical unit. DPA  124  includes a journal processor  180  for managing the journal. 
     Journal processor  180  functions generally to manage the journal entries of LU B. Specifically, journal processor  180  enters write transactions received by DPA  124  from DPA  112  into the journal, by writing them into the journal LU, reads the undo information for the transaction from LU B. updates the journal entries in the journal LU with undo information, applies the journal transactions to LU B, and removes already-applied transactions from the journal. 
     Referring to  FIG. 2 , which is an illustration of a write transaction  200  for a journal. The journal may be used to provide an adaptor for access to storage  120  at the state it was in at any specified point in time. Since the journal contains the “undo” information necessary to roll back storage system  120 , data that was stored in specific memory locations at the specified point in time may be obtained by undoing write transactions that occurred subsequent to such point in time. 
     Write transaction  200  generally includes the following fields: one or more identifiers; a time stamp, which is the date &amp; time at which the transaction was received by source side DPA  112 ; a write size, which is the size of the data block; a location in journal LU  176  where the data is entered; a location in LU B where the data is to be written; and the data itself. 
     Write transaction  200  is transmitted from source side DPA  112  to target side DPA  124 . As shown in  FIG. 2 , DPA  124  records the write transaction  200  in the journal that includes four streams. A first stream, referred to as a DO stream, includes new data for writing in LU B. A second stream, referred to as an DO METADATA stream, includes metadata for the write transaction, such as an identifier, a date &amp; time, a write size, a beginning address in LU B for writing the new data in, and a pointer to the offset in the DO stream where the corresponding data is located. Similarly, a third stream, referred to as an UNDO stream, includes old data that was overwritten in LU B; and a fourth stream, referred to as an UNDO METADATA, include an identifier, a date &amp; time, a write size, a beginning address in LU B where data was to be overwritten, and a pointer to the offset in the UNDO stream where the corresponding old data is located. 
     In practice each of the four streams holds a plurality of write transaction data. As write transactions are received dynamically by target DPA  124 , they are recorded at the end of the DO stream and the end of the DO METADATA stream, prior to committing the transaction. During transaction application, when the various write transactions are applied to LU B, prior to writing the new DO data into addresses within the storage system, the older data currently located in such addresses is recorded into the UNDO stream. In some examples, the metadata stream (e.g., UNDO METADATA stream or the DO METADATA stream) and the data stream (e.g., UNDO stream or DO stream) may be kept in a single stream each (i.e., one UNDO data and UNDO METADATA stream and one DO data and DO METADATA stream) by interleaving the metadata into the data stream. 
     Referring to  FIG. 3 , an example of the data protection system used in a virtual environment is a data protection system  300 . The system  300  includes virtual machine hosts (hypervisors) (e.g., a virtual machine hosts (hypervisors)  302   a - 302   c ), storage arrays (a storage array  306   a ,  306   b ) and a wide area network (WAN)  304 . The virtual machine hosts (hypervisors)  302   a ,  302   b  and the storage array  306   a  are on the productions site and the virtual machine hosts (hypervisors)  302   c  and the storage array  306   b  are on the replication site. In one example, the virtual machine monitors  302   a - 302   c  is a MICROSOFT® HYPER-V®. In another example, the virtual machine monitors  302   a - 302   c  is a VMWARE® virtualization. 
     The virtual machine monitor  302   a  includes virtual machines (e.g., virtual machines  308   a ,  308   b ), a splitter  314   a  and virtual machine hard drive (VHD) (e.g., VHD  316   a ) and the virtual machine hosts (hypervisors)  302   b  includes a virtual data protection appliance (DPA)  312   a , virtual machines (e.g., virtual machines  308   c ,  308   d ), a splitter  314   a , VHDs (e.g., VHD  316   b ) and a journal virtual disk  320   a . The VHDs  316   a ,  316   b  and the journal  320   a  are part of a file system  310   a . The splitters  314   a ,  314   b  intercept I/Os arriving to the virtual hard drives, VHDs  316   a ,  316   b , respectively. In one example, the file system  310   a  is a clustered shared volume file system (Microsoft® CSVFS) or a VMWARE® file system (VMFS). In another example the virtual disks are VMWARE® VVOLS or virtual volumes. 
     The virtual machine host (hypervisor)  302   c  includes a data protection appliance  312   a , a splitter  314   a , a virtual disk  316   c  and a journal  320   b . The VHD  316   c  and the journal  320   b  are part of a file system  310   b . In one example, the file system  310   a  is a CSVFS or a VMFS. In one example, the journals  320   a ,  320   b  are each VHDs. 
     In one example, an I/O from a VM  308   a  that is directed to a virtual disk  316   a  is split by the splitter  314   a  to the DPA  312   b  (via the DPA  312   a  and the WAN  304 ) recorded by the journal  320   b  to be stored on the virtual disk  316   c.    
       FIG. 4  is a simplified block diagram to depict when virtual machines are added or relocated to the replication site in a data replication system  400 . The data replication system  400  is similar to the data replication system  100  and  300  but includes a process (e.g., a process  500  ( FIG. 5 )) 
     The data replication system  400  includes a first VM  408   a , and a second VM  408   a  at a production site that is replicated at the replication site as first VM replica  408   a ′ and second VM replica  408   b ′, respectively using the first VM journal  420   a  and the second VM journal  420   b , respectively. In one example, the journals  420   a ,  420   b  are each configured to include write transactions similar to the write transaction  200  ( FIG. 2 ). 
     In the example depicted in  FIG. 4 , VMs  408   a ,  408   b ,  408   c  are part of a single application and a user is attempting to relocate the application from the production site to the replication site. In one particular example, the virtual machines  408   a ,  408   b ,  408   c  was or will be relocated from the production site to the replication site using VMware® vSphere@ vMotion®. 
     The third VM machine  408   c  is replicated at the productions site to a third VM replica  408   c ′ using a third VMjournal  420   c . In one example, the journal  420   c  is configured to include write transactions similar to the write transaction  200  ( FIG. 2 ). 
     When trying to move an application which includes multiple VMs it may not be possible due to bandwidth and other performance/scale limitation to do add the VMs remotely simultaneously. In such cases, the data replication system may move a single VM at a time, and during the entire movement process a failure of one of the sites or the connectivity between the sites will result in application unavailability. Since disasters and especially WAN disasters can happen, it is important to maintain a consistent point-in-time, which will be available at both sites, so that if one site is lost the application may be recoverable from the other site. 
     Referring to  FIGS. 4 and 5 , a process  500  is an example of a process to maintain a consistent point-in-time at a production site in an asynchronous replication after relocation (or addition) of a virtual machine at the replication site. One of ordinary skill in the art would recognize that the process  500  may be repeated for the replication site as well so that there is a recent (e.g., few minutes old) consistent image of the virtual machines at both the replication and production sites. Thus, a failure will not cause major data loss. 
     Process  500  pauses write I/Os for VMs running at the replication site ( 502 ). For example, the third VM  408   c  is quiesced or paused from acknowledging write I/Os. 
     Process  500  generates consistent snapshots at the production site for the VMs hard drives ( 508 ). In some examples, a snapshot may be a hypervisor-based virtual machine snapshot for a file system or an array-based snapshot for a virtual hard drive that is a VVOL. For example, a first VM snapshot  410   a  of the first VM  408   a  is generated and second VM snapshot  410   b  of the second VM  408   b  is generated. 
     Process  500  generates a bookmark for each VM running at the replication site ( 512 ) and sends the bookmark(s) to the production site ( 514 ). For example, a bookmark is generated for the third VM  408   c  at the replication site and sent to the production site. In other examples, if there are multiple VMs, the bookmark generated for each VM, marks the same point-in-time since the VMs are quiesced at the same time. 
     Process  500  resumes I/Os for VMs at the replication site ( 516 ). For example, the third VM  408   c  resumes acknowledging write I/Os. 
     Process  500  waits for the bookmark(s) to arrive at journal at the production ( 520 ). For example, process  500  waits for the bookmark to arrive at the third VM journal  420   c . Process  500  rolls the journal to the bookmark point-in-time ( 524 ). For example, the third VM journal  420   c  is rolled to the bookmark point-in-time using the processes described herein including with respect to  FIGS. 1 and 2 , for example. 
     Process  500  generates consistent snapshots at the production site for VMs that already transferred to run on the replication site ( 528 ). In some examples, a snapshot may be a hypervisor-based snapshot for the virtual hard drives that are on a file system such as CSVFS or VMFS or an array-based snapshot if the virtual hard drive is a VVOL. In one example, a third VM snapshot  410   c  is generated from the third VM replica  408   c ′ for the point-in-time designated in the bookmark using the journal  420   c.    
     Process  500  resumes distribution of point-in-time to the third VM replica  408   c ′ ( 532 ). For example, the newest data is applied from journal volume  420   c  to the third VM replica  408   c ′ as described in  FIG. 2 , for example. Process  500  erases any older point-in-time snapshots at the production site if any ( 536 ). For example, previous snapshots generated from previous executions of the process  500  are deleted. 
     Process  500  waits a period of time ( 542 ) and repeats processing blocks  502 ,  508 ,  512 ,  514 ,  516 ,  520 ,  524 ,  528 ,  532 ,  536  and  542 . 
     Since the volumes at the replication site are paused while the consistent snapshot(s) is taken at production site, the image the bookmark represents is consistent with the image generated when the snapshot was generated at the production site. As virtual machines are moved from the production site to the replication site the set of machines which will be paused will change. 
     In other examples, the process is repeated also in the reverse order when the virtual disk snapshots are taken at the replication site and the machine are quiesced at the production site, so that both sites have consistent point in time. 
     Referring to  FIG. 6 , in one example, a computer  600  includes a processor  602 , a volatile memory  604 , a non-volatile memory  606  (e.g., hard disk) and the user interface (UI)  608  (e.g., a graphical user interface, a mouse, a keyboard, a display, touch screen and so forth). The non-volatile memory  606  stores computer instructions  612 , an operating system  616  and data  618 . In one example, the computer instructions  612  are executed by the processor  602  out of volatile memory  604  to perform all or part of the processes described herein (e.g., process  500 ). 
     The processes described herein (e.g., process  500 ) are not limited to use with the hardware and software of  FIG. 6 ; they may find applicability in any computing or processing environment and with any type of machine or set of machines that is capable of running a computer program. The processes described herein may be implemented in hardware, software, or a combination of the two. The processes described herein may be implemented in computer programs executed on programmable computers/machines that each includes a processor, a non-transitory machine-readable medium or other article of manufacture that is readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and one or more output devices. Program code may be applied to data entered using an input device to perform any of the processes described herein and to generate output information. 
     The system may be implemented, at least in part, via a computer program product, (e.g., in a non-transitory machine-readable storage medium such as, for example, a non-transitory computer-readable medium), for execution by, or to control the operation of, data processing apparatus (e.g., a programmable processor, a computer, or multiple computers). Each such program may be implemented in a high level procedural or object-oriented programming language to communicate with a computer system. However, the programs may be implemented in assembly or machine language. The language may be a compiled or an interpreted language and it may be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program may be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. A computer program may be stored on a non-transitory machine-readable medium that is readable by a general or special purpose programmable computer for configuring and operating the computer when the non-transitory machine-readable medium is read by the computer to perform the processes described herein. For example, the processes described herein may also be implemented as a non-transitory machine-readable storage medium, configured with a computer program, where upon execution, instructions in the computer program cause the computer to operate in accordance with the processes. A non-transitory machine-readable medium may include but is not limited to a hard drive, compact disc, flash memory, non-volatile memory, volatile memory, magnetic diskette and so forth but does not include a transitory signal per se. 
     The processes described herein are not limited to the specific examples described. For example, the process  500  is not limited to the specific processing order of  FIG. 5 . Rather, any of the processing blocks of  FIG. 5  may be re-ordered, combined or removed, performed in parallel or in serial, as necessary, to achieve the results set forth above. 
     The processing blocks (for example, in the process  500 ) associated with implementing the system may be performed by one or more programmable processors executing one or more computer programs to perform the functions of the system. All or part of the system may be implemented as, special purpose logic circuitry (e.g., an FPGA (field-programmable gate array) and/or an ASIC (application-specific integrated circuit)). All or part of the system may be implemented using electronic hardware circuitry that include electronic devices such as, for example, at least one of a processor, a memory, a programmable logic device or a logic gate. 
     Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Various elements, which are described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. Other embodiments not specifically described herein are also within the scope of the following claims.