# Patent

## Title
Replication using a virtual distributed volume

## Abstract

In one aspect, a method includes mirroring a logical unit between a first site and a second site, tracking the changes to the logical unit on a backlog mirrored between the first site and the second site, replicating a virtual machine from the first site to a third site, storing virtual machine disks on a file system on the mirrored logical unit and resuming replication of the virtual machine to the third site from the second site using the mirrored logical unit after failure of the first site.

## Background
BACKGROUND
Computer data is vital to today's organizations and a significant part of protection against disasters is focused on data protection. As solid-state memory has advanced to the point where cost of memory has become a relatively insignificant factor, organizations can afford to operate with systems that store and process terabytes of data.
Conventional data protection systems include tape backup drives, for storing organizational production site data on a periodic basis. 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 mirroring a logical unit between a first site and a second site, tracking the changes to the logical unit on a backlog mirrored between the first site and the second site, replicating a virtual machine from the first site to a third site, storing virtual machine disks on a file system on the mirrored logical unit and resuming replication of the virtual machine to the third site from the second site using the mirrored logical unit after failure of the first site.
In another aspect, an apparatus includes electronic hardware circuitry configured to mirror a logical unit between a first site and a second site, track the changes to the logical unit on a backlog mirrored between the first site and the second site, replicate a virtual machine from the first site to a third site, store virtual machine disks on a file system on the mirrored logical unit and resume replication of the virtual machine to the third site from the second site using the mirrored logical unit after failure of the first site.
In a further aspect, an article includes a non-transitory computer-readable medium that stores computer-executable instructions. The instructions cause a machine to mirror a logical unit between a first site and a second site, track the changes to the logical unit on a backlog mirrored between the first site and the second site, replicate a virtual machine from the first site to a third site, store virtual machine disks on a file system on the mirrored logical unit and resume replication of the virtual machine to the third site from the second site using the mirrored logical unit after failure of the first site.



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 another example of the data protection system in a virtual environment, according to one embodiment of the disclosure.
FIG. 4 is a block diagram of the data protection system in FIG. 3 after failure at a site, according to one embodiment of the disclosure
FIG. 5 is a flowchart of an example of a process to manage a backlog, according to one embodiment of the disclosure.
FIG. 6 is a flowchart of an example of a process to recover from a failure of a site, according to one embodiment of the disclosure.
FIG. 7 is a computer on which any of the processes of FIGS. 5 and 6 may be implemented, according to one embodiment of the disclosure.



DETAILED DESCRIPTION
A site A performs replication to a site C; however site A may crash. Described herein are techniques to continue replication if site A crashes using a site B so that the replication will continue immediately from site B to site C with no full resynchronization needed. In particular, site B continues replication of a virtual machine disk.
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)
SPLITTER ACKNOWLEDGEMENT—an acknowledgement from a DPA to the protection agent (splitter) that data has been received at the DPA; this may be achieved by an SCSI status command.
SAN—a storage area network of nodes that send and receive an I/O and other requests, each node in the network being an initiator or a target, or both an initiator and a target;
SOURCE SIDE—a transmitter of data within a data replication workflow, during normal operation a production site is the source side; and during data recovery a backup site is the source side, sometimes called a primary side;
STORAGE SYSTEM—a SAN entity that provides multiple logical units for access by multiple SAN initiators
TARGET—a node in a SAN that replies to I/O requests;
TARGET SIDE—a receiver of data within a data replication workflow; during normal operation a back site is the target side, and during data recovery a production site is the target side, sometimes called a secondary side;
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 generates files in the file system and exposes the files as logical units to the virtual machines (each virtual machine disk 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 write activity performed in the multiple I/O requests, and sends the snapshot to DPA 124, for journaling and for incorporation in target storage system 120. In snapshot mode DPA 112 also sends an acknowledgement to protection agent 144 upon receipt of each I/O request, before receiving an acknowledgement back from DPA 124.
For the sake of clarity, the ensuing discussion assumes that information is transmitted at write-by-write granularity.
While in production mode, DPA 124 receives replicated data of LU A from DPA 112, and performs journaling and writing to storage system 120. When applying write operations to storage system 120, DPA 124 acts as an initiator, and sends SCSI commands to LU B.
During a recovery mode, DPA 124 undoes the write transactions in the journal, so as to restore storage system 120 to the state it was at, at an earlier time.
As described hereinabove, LU B is used as a backup of LU A. As such, during normal production mode, while data written to LU A by host computer 104 is replicated from LU A to LU B, host computer 116 should not be sending I/O requests to LU B. To prevent such I/O requests from being sent, protection agent 164 acts as a target site protection agent for host Device B and fails I/O requests sent from host computer 116 to LU B through host Device B.
Target storage system 120 exposes a logical unit 176, referred to as a “journal LU”, for maintaining a history of write transactions made to LU B, referred to as a “journal”. Alternatively, journal LU 176 may be striped over several logical units, or may reside within all of or a portion of another logical unit. DPA 124 includes a journal processor 180 for managing the journal.
Journal processor 180 functions generally to manage the journal entries of LU B. Specifically, journal processor 180 enters write transactions received by DPA 124 from DPA 112 into the journal, by writing them into the journal LU, reads the undo information for the transaction from LU B. updates the journal entries in the journal LU with undo information, applies the journal transactions to LU B, and removes already-applied transactions from the journal.
Referring to FIG. 2, which is an illustration of a write transaction 200 for a journal. The journal may be used to provide an adaptor for access to storage 120 at the state it was in at any specified point in time. Since the journal contains the “undo” information necessary to roll back storage system 120, data that was stored in specific memory locations at the specified point in time may be obtained by undoing write transactions that occurred subsequent to such point in time.
Write transaction 200 generally includes the following fields: one or more identifiers; a time stamp, which is the date & 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 & time, a write size, a beginning address in LU B for writing the new data in, and a pointer to the offset in the DO stream where the corresponding data is located. Similarly, a third stream, referred to as an UNDO stream, includes old data that was overwritten in LU B; and a fourth stream, referred to as an UNDO METADATA, include an identifier, a date & time, a write size, a beginning address in LU B where data was to be overwritten, and a pointer to the offset in the UNDO stream where the corresponding old data is located.
In practice each of the four streams holds a plurality of write transaction data. As write transactions are received dynamically by target DPA 124, they are recorded at the end of the DO stream and the end of the DO METADATA stream, prior to committing the transaction. During transaction application, when the various write transactions are applied to LU B, prior to writing the new DO data into addresses within the storage system, the older data currently located in such addresses is recorded into the UNDO stream. 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 400. The system 400 is spread over three sites: site A, site B and site C.
Site A includes a virtual host 402a and a service layer 460a. The virtual host 402a includes a virtual data protection appliance (vDPA) 410a, a virtual machine (VM) 412a, a splitter 422a and a delta marker 490a. The host also includes a virtual machine 492. The service layer 460a includes and exposes a logical unit 486a, on which a file system 430a is formatted. The file system 430a includes a virtual machine disk 432a, which is accessed by the VM 412a and a virtual machine disk 494a which is accessed by the VM 492. The service layer 460a also includes a backlog 462a. 
Site B includes a virtual host 402b and a service layer 460b. The virtual host 402b includes a vDPA 410b, a splitter 422b and a delta marker 490b. The service layer 460b includes a logical unit 486b, which is a synchronous active/active mirror of the logical unit 486a (i.e., this is LU 486b is basically the same LU 486a which can be accessed simultaneously at both sites A and B). Since the LU is mirrored then it data is the same and thus LU 486b includes a mirror of file system 430a, file system 430b; a mirror of virtual machine disk 432a, virtual machine disk 432b; and a mirror of virtual machine disk 494a, virtual machine disk 494b. The service layer also includes a backlog 462b. 
Site C includes a virtual host 402c and a virtual machine disk 432c (on a file system 430c on LU 490), which is a remote copy of virtual machine disk 432a, which is the storage of VM 412a. VMFS 430c includes a full copy of VM 412a, as it also includes the metadata describing the VM. In one example, the virtual host 402a-402c are each hypervisors (e.g., running VMWARE® ESXi OS).
In one example, the file systems 430a-430c are each a VMWARE virtual machine file system (VMFS). In another example, the file systems 430a-430c are each a cluster shared volume file system (CSVFS).
The virtual machine disk 432a at site A is replicated to virtual machine disk 432c at site C, as part of replication of the VM412a, the metadata of VM 412a and its configuration files are also replicated and thus there is a full copy of the virtual machine on site C. That is, when an I/O is generated from the VM 412a, the splitter writes the I/O to the virtual machine disk 432a and also sends the I/O to the vDPA 410a to be sent to the vDPA 410c to be written to the virtual machine disk 432c. 
In the system 400, the service layers 460a, 460b are depicted as physical devices. One of ordinary skill in the art would recognize that the service layers 460a, 460b may be virtual service layers, for example, within virtual hosts 402a, 402b respectively. In one example, the service layers are an EMC® VPLEX®.
The service layers 460a, 460b mirror the data between site A and site B. A link 496 between the service layers 460a, 460b, ensures that the backlog 462a and the backlog 462b are synchronized. As will be described further herein, the backlogs 462a, 462b are important during a site failure.
VM 412a has copy of its data in site A, site B and site C. There is a major difference between the method of replication between site A to site B and to site C, while the replication from site A to site B is an active/active replication and is done at the physical LU level, the replication to site C is at the virtual machine level. The I/Os from the VM 412a are intercepted at the host level, by the splitter and not at the LU level. Another important difference is that when the replication is at the LU level the virtual machines which reside on the file system 430a are replicated together, while when replicating at the virtual machine level only, selected VMs can be replicated. For example, the virtual machine disk 494a of VM 492 is mirrored to site B but not replicated to site C.
One of the challenges of the system 400 is that if site A crashes and the replication between site A and site C is asynchronous, there is a need to understand which data has not yet been transferred from site A to site C. The challenge is for the VM 412a that is running only at site A, because site B splitter 422b has no knowledge of any I/Os arriving to the VM 412a running at Site A. However, to account for the I/Os a backlog is used for each virtual machine disk to understand which locations needs resynchronization after a crash. For example, the backlog 462a tracks I/Os to the LU 486a. The backlog 462a has no knowledge of the virtual machine disk 432a because it is only aware of the address changes in the LU 486a containing the virtual machine disk 432a. After site A crashes and the VM 412a starts running on site B, the backlog 462b (a copy of backlog 462a) is used to determining which locations in the virtual machine disk need resynchronization. The backlog 462b saves LUN 486a addresses not virtual machine disk addresses so a translation will be performed as described in conjunction with process 600 (FIG. 6).
Referring to FIG. 4, the system 400 is depicted after site A fails. The VM 412a is initiated on the virtual host 402b at site B, for example, by VMWare HA mechanism or by user or any other mechanism. Since the LU 486b is a mirror of LU 486a, LU 486b contains the latest version of file system 430a and thus of virtual machine disk 432a (i.e., virtual machine disk 432b). The system 400 then resumes replication from the virtual machine disk 432b at site B to the replica, the virtual machine disk 432c at site C. The replication may be resumed using data in the backlogs (e.g., backlog 462b) to understand which locations needs to be resynchronized between site B and site C.
Referring to FIG. 5, process 500 is an example of a process to manage the backlog (e.g., backlog 462a, 462b). As will be described further herein the backlog 462a contains changes that were not sent to the LU 486a. In one example, the process 500 is a protocol performed between the backlog and the vDPA 410a. 
Process 500 maintains a list of the virtual machines for each file system (i.e. the list of virtual machines which are replicated for each mirrored LU) (502). For example, for LU 486a a list of the virtual machine disks it has which belong to replicated VMs is maintained (e.g., a list of the virtual machine disks which belong to some replicated virtual machine which is replicated to site C, which reside on the file system 430a which resides on LU 486a). The data which already reached site C from the backlog 462a is erased, so that the backlog 462a is as small as possible so that a resynchronization from the backlog 462a will be as short of a time as possible.
In order to release data from the backlog 462a, there needs to be validation that the data erased is now at site C. That is, the data for the VMs replicated which are stored at LU 386a has reached site C. To ensure this, the vDPA 410a maintains a list of the VMs which are replicated and stored on LU 486a. 
Process 500 notifies the backlog that a timestamp is being taken (506). For example, vDPA 410a notifies the backlog 462a that a timestamp is being taken.
Process 500 marks a timestamp for the file systems (e.g., file system 430a) (508) and for each VM waits for the data up until the timestamp selected has reached site C and flushed to the replica file storage (510). In one example, vDPA 410a performs processing blocks 508 and 510.
Process 500 notifies the backlog that data up to the timestamp was flushed (512). For example, vDPA 410a notifies the backlog 462a that data until timestamp was flushed (is now on site C) and that the backlog 462 can now erase data it received before the timestamp generated in processing block 506.
Referring to FIG. 6, process 600 is an example of a process to recover from a failure. For example, a first site (site A) has failed and a second site (site B) takes over. Process 600 detects failure (602) and initiates running of the virtual machine previously running at the first site, at the second site. For example, the VM 412a is now running at site B. In some examples, the detection may be done by a witness machine running at a third site. In other examples, the failure may be detected by a hypervisor High Availability (HA) or a service layer.
Process 600 mounts file system to vDPA (606). In one example, the file system 430b is mounted by the vDPA 410b in a read only mode. In one example, the file system 430b is mounted after exposing the LU 486b containing the file system 430b to vDPA 410b. 
Process 600 retrieves changes in backlog (614). The vDPA 410b accesses the changes recorded in the backlog 462b. As previously described the backlog 462b is a mirror of the backlog 462a and the changes are record in the backlog 462b using LU addresses, but the delta markers 490a, 490b record changes using virtual machine disk addresses. The following processing blocks describe a translation from LU addresses to virtual machine disk addresses.
Process 600 parses metadata by each replicated virtual machine disk (618). For example, vDPA 410b pares the metadata of the file system 430b by virtual machine disk.
Process 600 maps blocks by replicated virtual machine disk (622). For example, the blocks changes in the LU are correlated to blocks in the replicated virtual machine disks (e.g., virtual machine disk 432b), and thus determines which offsets were changed in each replicated virtual machine disk. In one example, for each replicated virtual machine disk, the vDPA 410b maps the changed blocks to a virtual machine disk,
Process 600 sends changed blocks by virtual machine disk to a delta marker (626). For example, the vDPA 410b sends the changed blocks to the delta marker 490b. 
Process 600 starts replication (636). For example, vDPA 410b starts replicating virtual machine disk 432b to virtual machine disk 432a. 
Referring to FIG. 7, in one example, a computer 700 includes a processor 702, a volatile memory 704, a non-volatile memory 706 (e.g., hard disk) and the user interface (UI) 708 (e.g., a graphical user interface, a mouse, a keyboard, a display, touch screen and so forth). The non-volatile memory 706 stores computer instructions 712, an operating system 716 and data 718. In one example, the computer instructions 712 are executed by the processor 702 out of volatile memory 704 to perform all or part of the processes described herein (e.g., processes 500 and 600).
The processes described herein (e.g., processes 500 and 600) are not limited to use with the hardware and software of FIG. 7; they may find applicability in any computing or processing environment and with any type of machine or set of machines that is capable of running a computer program. The processes described herein may be implemented in hardware, software, or a combination of the two. The processes described herein may be implemented in computer programs executed on programmable computers/machines that each includes a processor, a non-transitory machine-readable medium or other article of manufacture that is readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and one or more output devices. Program code may be applied to data entered using an input device to perform any of the processes described herein and to generate output information.
The system may be implemented, at least in part, via a computer program product, (e.g., in a non-transitory machine-readable storage medium such as, for example, a non-transitory computer-readable medium), for execution by, or to control the operation of, data processing apparatus (e.g., a programmable processor, a computer, or multiple computers)). Each such program may be implemented in a high level procedural or object-oriented programming language to communicate with a computer system. However, the programs may be implemented in assembly or machine language. The language may be a compiled or an interpreted language and it may be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program may be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. A computer program may be stored on a non-transitory machine-readable medium that is readable by a general or special purpose programmable computer for configuring and operating the computer when the non-transitory machine-readable medium is read by the computer to perform the processes described herein. For example, the processes described herein may also be implemented as a non-transitory machine-readable storage medium, configured with a computer program, where upon execution, instructions in the computer program cause the computer to operate in accordance with the processes. A non-transitory machine-readable medium may include but is not limited to a hard drive, compact disc, flash memory, non-volatile memory, volatile memory, magnetic diskette and so forth but does not include a transitory signal per se.
The processes described herein are not limited to the specific examples described. For example, the processes 500 and 600 are not limited to the specific processing order of FIGS. 5 and 6, respectively. Rather, any of the processing blocks of FIGS. 5 and 6 may be re-ordered, combined or removed, performed in parallel or in serial, as necessary, to achieve the results set forth above.
The processing blocks (for example, in the processes 500 and 600) associated with implementing the system may be performed by one or more programmable processors executing one or more computer programs to perform the functions of the system. All or part of the system may be implemented as, special purpose logic circuitry (e.g., an FPGA (field-programmable gate array) and/or an ASIC (application-specific integrated circuit)). All or part of the system may be implemented using electronic hardware circuitry that include electronic devices such as, for example, at least one of a processor, a memory, a programmable logic device or a logic gate.
Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Other embodiments not specifically described herein are also within the scope of the following claims.

## Claims

1. A method comprising:
mirroring a logical unit between a first site and a second site;
tracking the changes to the logical unit on a backlog mirrored between the first site and the second site;
replicating a virtual machine from the first site to a third site;
storing virtual machine disks on a file system on the mirrored logical unit; and
resuming replication of the virtual machine to the third site from the second site using the mirrored logical unit after failure of the first site.

2. The method of claim 1, wherein resuming replication of the virtual machine comprises resuming replication of the virtual machine by obtaining the changes to the virtual machine that were not sent to the third site.

3. The method of claim 2, wherein obtaining the changes comprises:
mounting the file system;
parsing the files system to obtain the virtual machine disk locations on the logical unit containing the file system; and
comparing the virtual machine disk location with the changes tracked in the backlog.

4. The method of claim 3, further comprising:
mapping blocks on the logical unit to the blocks of the virtual machine disks;
determining which of the blocks changed on the logical unit are blocks inside replicated virtual machine disks;
adding the changed blocks to a list of dirty block; and
resynchronizing the blocks which are marked as changed in synchronized virtual machine disks.

5. The method of claim 1, further comprising erasing changes in the backlog after the virtual machines disks on the logical unit have their data reach the third site.

6. The method of claim 5, wherein erasing changes in the backlog after the virtual machines disks on the logical unit have their data reach the third site comprises:
marking a timestamp in the file systems;
for each virtual machine, waiting for the data up until the timestamp selected has reached the third site and is flushed to the replica file storage;
notifying the backlog that data up to the timestamp was flushed; and
erasing data received before the timestamp.

7. An apparatus, comprising:
electronic hardware circuitry configured to:
mirror a logical unit between a first site and a second site;
track the changes to the logical unit on a backlog mirrored between the first site and the second site;
replicate a virtual machine from the first site to a third site;
store virtual machine disks on a file system on the mirrored logical unit; and
resume replication of the virtual machine to the third site from the second site using the mirrored logical unit after failure of the first site.

8. The apparatus of claim 7, wherein the circuitry comprises at least one of a processor, a memory, a programmable logic device or a logic gate.

9. The apparatus of claim 7, wherein the circuitry configured to resume replication of the virtual machine comprises circuitry configured to resume replication of the virtual machine by obtaining the changes to the virtual machine that were not sent to the third site.

10. The apparatus of claim 9, wherein the circuitry configured to obtain the changes comprises circuitry configured to:
mount the file system;
parse the files system to obtain the virtual machine disk locations on the logical unit containing the file system; and
compare the virtual machine disk location with the changes tracked in the backlog.

11. The apparatus of claim 10, further comprising circuitry configured to:
map blocks on the logical unit to the blocks of the virtual machine disks;
determine which of the blocks changed on the logical unit are blocks inside replicated virtual machine disks;
add the changed blocks to a list of dirty block; and
resynchronize the blocks which are marked as changed in synchronized virtual machine disks.

12. The apparatus of claim 7, further comprising circuitry configured to erase changes in the backlog after the virtual machines disks on the logical unit have their data reach the third site.

13. The apparatus of claim 12, wherein the circuitry configured to erase changes in the backlog after the virtual machines disks on the logical unit have their data reach the third site comprises circuitry configured to:
mark a timestamp in the file systems;
for each virtual machine, wait for the data up until the timestamp selected has reached the third site and is flushed to the replica file storage;
notify the backlog that data up to the timestamp was flushed; and
erase data received before the timestamp.

14. An article comprising:
a non-transitory computer-readable medium that stores computer-executable instructions, the instructions causing a machine to:
mirror a logical unit between a first site and a second site;
track the changes to the logical unit on a backlog mirrored between the first site and the second site;
replicate a virtual machine from the first site to a third site;
store virtual machine disks on a file system on the mirrored logical unit; and
resume replication of the virtual machine to the third site from the second site using the mirrored logical unit after failure of the first site.

15. The article of claim 14, wherein the instructions causing the machine to resume replication of the virtual machine comprises instructions causing the machine to resume replication of the virtual machine by obtaining the changes to the virtual machine that were not sent to the third site.

16. The article of claim 15, wherein the instructions causing the machine to obtain the changes comprises instructions causing the machine to:
mount the file system;
parse the files system to obtain the virtual machine disk locations on the logical unit containing the file system; and
compare the virtual machine disk location with the changes tracked in the backlog.

17. The article of claim 16, further comprising instructions causing the machine to:
map blocks on the logical unit to the blocks of the virtual machine disks;
determine which of the blocks changed on the logical unit are blocks inside replicated virtual machine disks;
add the changed blocks to a list of dirty block; and
resynchronize the blocks which are marked as changed in synchronized virtual machine disks.

18. The article of claim 14, further comprising instructions causing the machine to erase changes in the backlog after the virtual machines disks on the logical unit have their data reach the third site.

19. The article of claim 18, wherein the instructions causing the machine to erase changes in the backlog after the virtual machines disks on the logical unit have their data reach the third site comprises instructions causing the machine to:
mark a timestamp in the file systems;
for each virtual machine, wait for the data up until the timestamp selected has reached the third site and is flushed to the replica file storage;
notify the backlog that data up to the timestamp was flushed; and
erase data received before the timestamp.