Patent Publication Number: US-6658590-B1

Title: Controller-based transaction logging system for data recovery in a storage area network

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to data consistency in data storage systems, and more specifically, to a system for providing controller-based transaction logging to provide data recovery after an error event in a remote data replication system using a Storage Area Network. 
     BACKGROUND OF THE INVENTION AND PROBLEM 
     It is desirable to provide the ability for rapid recovery of user data from a disaster or significant error event at a data processing facility. This type of capability is often termed ‘disaster tolerance’ . In a data storage environment, disaster tolerance requirements include providing for replicated data and redundant storage to support recovery after the event. In order to provide a safe physical distance between the original data and the data to backed up, the data must be migrated from one storage subsystem or physical site to another subsystem or site. It is also desirable for user applications to continue to run while data replication proceeds in the background. Data warehousing, ‘continuous computing’, and Enterprise Applications all require remote copy capabilities. 
     Storage controllers are commonly utilized in computer systems to off-load from the host computer certain lower level processing functions relating to I/O operations, and to serve as interface between the host computer and the physical storage media. Given the critical role played by the storage controller with respect to computer system I/O performance, it is desirable to minimize the potential for interrupted I/O service due to storage controller malfunction. Thus, prior workers in the art have developed various system design approaches in an attempt to achieve some degree of fault tolerance in the storage control function. One such prior approach requires that all system functions be ‘mirrored’. While this type of approach is most effective in reducing interruption of I/O operations and lends itself to value-added fault isolation techniques, it has previously been costly to implement and heretofore has placed a heavy processing burden on the host computer. 
     One prior method of providing storage system fault tolerance accomplishes failover through the use of two controllers coupled in an active/passive configuration. During failover, the passive controller takes over for the active (failing) controller. A drawback to this type of dual configuration is that it cannot support load balancing, as only one controller is active and thus utilized at any given time, to increase overall system performance. Furthermore, the passive controller presents an inefficient use of system resources. 
     Another approach to storage controller fault tolerance is based on a process called ‘failover’. Failover is known in the art as a process by which a first storage controller, coupled to a second controller, assumes the responsibilities of the second controller when the second controller fails. ‘Failback’ is the reverse operation, wherein the second controller, having been either repaired or replaced, recovers control over its originally-attached storage devices. Since each controller is capable of accessing the storage devices attached to the other controller as a result of the failover, there is no need to store and maintain a duplicate copy of the data, i.e., one set stored on the first controller&#39;s attached devices and a second (redundant) copy on the second controller&#39;s devices. 
     U.S. Pat. No. 5,274,645 (Dec. 28, 1993), to Idleman et al. discloses a dual-active configuration of storage controllers capable of performing failover without the direct involvement of the host. However, the direction taken by Idleman requires a multi-level storage controller implementation. Each controller in the dual-redundant pair includes a two-level hierarchy of controllers. When the first level or host-interface controller of the first controller detects the failure of the second level or device interface controller of the second controller, it re-configures the data path such that the data is directed to the functioning second level controller of the second controller. In conjunction, a switching circuit re-configures the controller-device interconnections, thereby permitting the host to access the storage devices originally connected to the failed second level controller through the operating second level controller of the second controller. Thus, the presence of the first level controllers serves to isolate the host computer from the failover operation, but this isolation is obtained at added controller cost and complexity. 
     Other known failover techniques are based on proprietary buses. These techniques utilize existing host interconnect “hand-shaking” protocols, whereby the host and controller act in cooperative effort to effect a failover operation. Unfortunately, the “hooks” for this and other types of host-assisted failover mechanisms are not compatible with more recently developed, industry-standard interconnection protocols, such as SCSI, which were not developed with failover capability in mind. Consequently, support for dual-active failover in these proprietary bus techniques must be built into the host firmware via the host device drivers. Because SCSI, for example, is a popular industry standard interconnect, and there is a commercial need to support platforms not using proprietary buses, compatibility with industry standards such as SCSI is essential. Therefore, a vendor-unique device driver in the host is not a desirable option. 
     U.S. patent application, Ser. No. 08/071,710 to Sicola et al., describes a dual-active, redundant storage controller configuration in which each storage controller communicates directly with the host and its own attached devices, the access of which is shared with the other controller. Thus, a failover operation may be executed by one of the storage controller without the assistance of an intermediary controller and without the physical reconfiguration of the data path at the device interface. 
     U.S. Pat. No. 5,790,775 (Aug. 4, 1998) to Marks et al., discloses a system comprising a host CPU, a pair of storage controllers in a dual-active, redundant configuration. The pair of storage controllers reside on a common host side SCSI bus, which serves to couple each controller to the host CPU. Each controller is configured by a system user to service zero or more, preferred host side SCSI IDs, each host side ID associating the controller with one or more units located thereon and used by the host CPU to identify the controller when accessing one of the associated units. If one of the storage controllers in the dual-active, redundant configuration fails, the surviving one of the storage controllers automatically assumes control of all of the host side SCSI IDs and subsequently responds to any host requests directed to the preferred, host side SCSI IDS and associated units of the failed controller. When the surviving controller senses the return of the other controller, it releases to the returning other controller control of the preferred, SCSI IDS of the failed controller. In another aspect of the Marks invention, the failover is made to appear to the host CPU as simply a re-initialization of the failed controller. Consequently, all transfers outstanding are retried by the host CPU after time outs have occurred. Marks discloses ‘transparent failover’, which is an automatic technique that allows for continued operation by a partner controller on the same storage bus as the failed controller. This technique is useful in situations where the host operating system trying to access storage does not have the capability to adequately handle multiple paths to the same storage volumes. Transparent failover makes the failover event look like a ‘power-on reset’ of the storage device. However, transparent failover has a significant flaw: it is not fault tolerant to the storage bus. If the storage bus fails, all access to the storage device is lost. 
     However, none of the above references disclose a system having a remote backup site connected to a host site via a dual fabric link, where the system provides in-order operations while a link is down as well as when the link returns to operation while the transaction log is ‘merged’ back with the remote site. 
     Therefore, there is a clearly felt need in the art for a disaster tolerant data storage system capable of rapid recovery from disruptions such as short-term link failure and remote site failover, without the direct involvement of the host computer, wherein both original and backup copies of user data are returned to the same state without incurring the overhead of a full copy operation. 
     SOLUTION TO THE PROBLEM 
     Accordingly, the above problems are solved, and an advance in the field is accomplished by the system of the present invention which provides a completely redundant configuration including dual Fibre Channel fabric links interconnecting each of the components of two data storage sites, wherein each site comprises a host computer and associated data storage array, with redundant array controllers and adapters. The present system is unique in that each array controller is capable of performing all of the data replication functions, and each host ‘sees’ remote data as if it were local. The array controllers also perform a command and data logging function which stores all host write commands and data ‘missed’ by the backup storage array during a situation wherein the links between the sites are down, the remote site is down, or where a site failover to the remote site has occurred. Log ‘units’ are used to store, in order, all commands and data for every transaction which was ‘missed’ by the backup storage array when one of the above system error conditions has occurred. The present system provides rapid and accurate recovery of backup data at the remote site by sending all logged commands and data from the logging site over the link to the backup site in order, while avoiding the overhead of a full copy operation. 
     The ‘mirroring’ of data for backup purposes is the basis for RAID (‘Redundant Array of Independent [or Inexpensive] Disks’) Level 1 systems, wherein all data is replicated on N separate disks, with N usually having a value of 2. Although the concept of storing copies of data at a long distance from each other (i.e., long distance mirroring) is known, the use of a switched, dual-fabric, Fibre Channel configuration as described herein is a novel approach to disaster tolerant storage systems. Mirroring requires that the data be consistent across all volumes. In prior art systems which use host-based mirroring (where each host computer sees multiple units), the host maintains consistency across the units. For those systems which employ controller-based mirroring (where the host computer sees only a single unit), the host is not signaled completion of a command until the controller has updated all pertinent volumes. The present invention is, in one aspect, distinguished over the previous two types of systems in that the host computer sees multiple volumes, but the data replication function is performed by the controller. Therefore, a mechanism is required to communicate the association between volumes to the controller. To maintain this consistency between volumes, the system of the present invention provides a mechanism of associating a set of volumes to synchronize the logging to the set of volumes so that when the log is consistent when it is “played back” to the remote site. 
     Each array controller in the present system has a dedicated link via a fabric to a partner on the remote side of the long-distance link between fabric elements. Each dedicated link does not appear to any host as an available link to them for data access, however, it is visible to the partner array controllers involved in data replication operations. These links are managed by each partner array controller as if being ‘clustered’ with a reliable data link between them. 
     The fabrics comprise two components, a local element and a remote element. An important aspect of the present invention is the fact that the fabrics are ‘extended’ by standard e-ports (extension ports). The use of e-ports allow for standard Fibre Channel cable to be run between the fabric elements or the use of a conversion box to covert the data to a form such as telco ATM or IP. The extended fabric allows the entire system to be viewable by both the hosts and storage. 
     The dual fabrics, as well as the dual array controllers, dual adapters in hosts, and dual links between fabrics, provide high-availability and present no single point of failure. A distinction here over the prior art is that previous systems typically use other kinds of links to provide the data replication, resulting in the storage not being readily exposed to hosts on both sides of a link. The present configuration allows for extended clustering where local and remote site hosts are actually sharing data across the link from one or more storage subsystems with dual array controllers within each subsystem. 
     The present system is further distinguished over the prior art by other additional features, including independent discovery of initiator to target system and automatic rediscovery after link failure. In addition, device failures are detected by ‘heartbeat’ monitoring by each array controller. Furthermore, no special host software is required to implement the above features because all replication functionality is totally self contained within each array controller and automatically done without user intervention. 
     An additional aspect of the present system is the ability to function over two links with data replication traffic. If failure of a link occurs, as detected by the ‘initiator’ array controller, that array controller will automatically ‘failover’, or move the base of data replication operations to its partner controller. At this time, all transfers in flight are discarded, and therefore discarded to the host. The host simply sees a controller failover at the host OS (operating system) level, causing the OS to retry the operations to the partner controller. The array controller partner continues all ‘initiator’ operations from that point forward. The array controller whose link failed will continuously watch that status of its link to the same controller on the other ‘far’ side of the link. That status changes to a ‘good’ link when the array controllers have established reliable communications between each other. When this occurs, the array controller ‘initiator’ partner will ‘failback’ the link, moving operations back to newly reliable link. This procedure re-establishes load balance for data replication operations automatically, without requiring additional features in the array controller or host beyond what is minimally required to allow controller failover. 
     Because the present system provides a logging mechanism for storing all commands and data for every transaction that occurs in the failure situations described above, the system is thus is capable of rapid recovery from disruptions such as short-term link failure and remote site failover, without the direct involvement of the host computer, and without incurring the overhead of a full copy operation. Furthermore, the present system&#39;s method of logging upon site failover provides for rapid site failback and resynchronization in situations wherein there is only a temporary downtime at the primary site. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects, features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a diagram showing long distance mirroring; 
     FIG. 2 illustrates a switched dual fabric, disaster-tolerant storage system; 
     FIG. 3 is a block diagram of the system shown in FIG. 2; 
     FIG. 4 is a high-level diagram of a remote copy set operation; 
     FIG. 5 is a block diagram showing exemplary controller software architecture; 
     FIG. 6A is a flow diagram showing inter-site controller heartbeat timer operation; 
     FIG. 6B is a flow diagram showing intra-site controller heartbeat timer operation; 
     FIG. 7 is a flowchart showing synchronous system operation; 
     FIG. 8A is a flowchart showing asynchronous system operation; 
     FIG. 8B is a flowchart showing a ‘micro-merge’ operation; 
     FIG. 9 is a diagram showing an example of a link failover operation; 
     FIG. 10 is a flow diagram showing a log operation when both links are down, or when the remote site is down; 
     FIG. 11 is a flowchart showing log unit writing and merging operations; 
     FIG. 12 is a flow diagram showing a log operation in response to a site failover; and 
     FIG. 13 is a diagram showing an exemplary format of data and extent information stored on a log unit. 
    
    
     DETAILED DESCRIPTION 
     The system of the present invention comprises a data backup and remote copy system which provides disaster tolerance. In particular, the present system provides a peer-to-peer remote copy (backup) function which is implemented as a controller-based replication of one or more LUNs (logical units) between two remotely separated pairs of array controllers connected by redundant links. The present system further provides a data logging mechanism (a write history ‘log unit’) for storing commands and data for every transaction that occurs in the situation where the remote backup storage device is unavailable because both links have failed, a remote site is down, or because of a site failover. The system performs an in-order merging of the log unit data with the data on the previously unavailable backup device to quickly return both local and remote sites to the same data state after link restoration or remote site restoration. 
     FIG. 1 is a diagram showing long distance mirroring, which is an underlying concept of the present invention. The present system  100  employs a switched, dual-fabric, Fibre Channel configuration to provide a disaster tolerant storage system. Fibre Channel is the general name of an integrated set of standards developed by the American National Standards Institute (ANSI) which defines protocols for information transfer. Fibre Channel supports multiple physical interface types, multiple protocols over a common physical interface, and a means for interconnecting various interface types. A ‘Fibre Channel’ may include transmission media such as copper coax or twisted pair copper wires in addition to (or in lieu of) optical fiber. 
     As shown in FIG. 1, when host computer  101  writes data to its local storage array, an initiating node, or ‘initiator’  111  sends a backup copy of the data to remote ‘target’ node  112  via a Fibre Channel switched fabric  103 . A ‘fabric’ is a topology (explained in more detail below) which supports dynamic interconnections between nodes through ports connected to the fabric. In FIG. 1, nodes  111  and  112  are connected to respective links  105  and  106  via ports  109 . A node is simply a device which has at least one port to provide access external to the device. In the context of the present system  100 , a node typically includes an array controller pair and associated storage array. Each port in a node is generically termed an N (or NL) port. Ports  109  (array controller ports) are thus N ports. Each port in a fabric is generically termed an F (or FL) port. In FIG. 1, links  105  and  106  are connected to switched fabric  103  via F ports  107 . More specifically, these F ports may be E ports (extension ports) or E port/FC-BBport pairs, as explained below. 
     In general, it is possible for any node connected to a fabric to communicate with any other node connected to other F ports of the fabric, using services provided by the fabric. In a fabric topology, all routing of data frames is performed by the fabric, rather than by the ports. This any-to-any connection service (‘peer-to-peer’ service) provided by a fabric is integral to a Fibre Channel system. It should be noted that in the context of the present system, although a second host computer  102  is shown (at the target site) in FIG. 1, this computer is not necessary for operation of the system  100  as described herein. 
     An underlying operational concept employed by the present system  100  is the pairing of volumes (or LUNs) on a local array with those on a remote array. The combination of volumes is called a ‘remote copy set’. A remote copy set thus consists of two volumes, one on the local array, and one on the remote array. For example, as shown in FIG. 1, a remote copy set might consist of LUN  1  ( 110 ) on a storage array at site  101  and LUN  1 ′( 110  ′) on a storage array at site  102 . The array designated as the ‘local’ array is called the initiator, while the remote array is called the target. Various methods for synchronizing the data between the local and remote array are possible in the context of the present system. These synchronization methods range from full synchronous to fully asynchronous data transmission, as explained below. The system user&#39;s ability to choose these methods provides the user with the capability to vary system reliability with respect to potential disasters and the recovery after such a disaster. The present system allows choices to be made by the user based on factors which include likelihood of disasters and the critical nature of the user&#39;s data. 
     System Architecture 
     FIG. 2 illustrates an exemplary configuration of the present invention, which comprises a switched dual fabric, disaster-tolerant storage system  100 . The basic topology of the present system  100  is that of a switched-based Storage Area Network (SAN). As shown in FIG. 2, data storage sites  218  and  219  each respectively comprise two hosts  101 / 101 A and  102 / 102 A, and two storage array controllers  201 / 202  and  211 / 212  connected to storage arrays  203  and  213 , respectively. Alternatively, only a single host  101 / 102 , or more than two hosts may be connected to system  100  at each site  218 / 219 . Storage arrays  203  and  213  typically comprise a plurality of magnetic disk storage devices, but could also include or consist of other types of mass storage devices such as semiconductor memory. 
     In the configuration of FIG. 2, each host at a particular site is connected to both fabric elements (i.e., switches) located at that particular site. More specifically, at site  218 , host  101  is connected to switches  204  and  214  via respective paths  231 A and  231 B; host  101 A is connected to the switches via paths  241 A and  241 B. Also located at site  218  are array controllers A 1  (ref. no.  201  and A 2  (ref. no.  202 ). Array controller A 1  is connected to switch  204  via paths  221 H and  221 D; array controller A 2  is connected to switch  214  via paths  222 H and  222 D. The path suffixes ‘H’ and ‘D’ refer to ‘Host’ and ‘Disaster-tolerant’ paths, respectively, as explained below. Site  219  has counterpart array controllers B 1  (ref. no  211 ) and B 2  (ref. no.  212 ), each of which is connected to switches  205  and  215 . Note that array controllers B 1  and B 2  are connected to switches  205  and  215  via paths  251 D and  252 D, which are, in effect, continuations of paths  221 D and  222 D, respectively. 
     In the present system shown in FIG. 2, all storage subsystems ( 201 / 202 / 203  and  211 / 212 / 213 ) and all hosts ( 101 ,  101 A,  102 , and  102 A) are visible to each other over the SAN  103 A/ 103 B. This configuration provides for high availability with a dual fabric, dual host, and dual storage topology, where a single fabric, host, or storage can fail and the system can still continue to access other system components via the SAN. As shown in FIG. 2, each fabric  103 A/ 103 B employed by the present system  100  includes two switches interconnected by a high-speed link. More specifically, fabric  103 A comprises switches  204  and  205  connected by link  223 A, while fabric  103 B comprises switches  214  and  215  connected by link  223 B. 
     Basic Fibre Channel technology allows the length of links  223 A/ 223 B (i.e., the distance between data storage sites) to be as great as 10 KM as per the FC-PH3 specification (see Fibre Channel Standard: Fibre Channel Physical and Signaling Interface, ANSII X3T11). However, distances of 20 KM and greater are possible given improved technology and FC-PH margins with basic Fibre Channel. FC-BB (Fibre Channel Backbone) technology provides the opportunity to extend Fibre Channel over leased Telco lines (also called WAN tunneling). In the case wherein FC-BB is used for links  223 A and  223 B, FC-BB ports are attached to the E ports to terminate the ends of links  223 A and  223 B. 
     It is also possible to interconnect each switch pair  204 / 205  and  214 / 215  via an Internet link ( 223 A/ 223 B). If the redundant links  223 A and  223 B between the data storage sites  218 / 219  are connected to different ISPs (Internet Service Providers) at the same site, for example, there is a high probability of having at least one link operational at any given time. This is particularly true because of the many redundant paths which are available over the Internet between ISPs. For example, switches  204  and  214  could be connected to separate ISPs, and switches  205  and  215  could also be connected to separate ISPS. 
     FIG. 3 is an exemplary block diagram illustrating additional details of the system shown in FIG.  2 . The configuration of the present system  100 , as shown in FIG. 3, depicts only one host per site for the sake of simplicity. Each host  101 / 102  has two adapters  308  which support the dual fabric topology. The hosts typically run multi-pathing software that dynamically allows failover between storage paths as well as static load balancing of storage volumes (LUNs) between the paths to the controller-based storage arrays  201 / 202  and  211 / 212 . The configuration of system  100  allows for applications using either of the storage arrays  203 / 213  to continue running given any failure of either fabric  103 A/ 103 B or either of the storage arrays. 
     The array controllers  201 / 202  and  211 / 212  employed by the present system  100  have two host ports  109  per array controller, for a total of four connections (ports) per pair in the dual redundant configuration of FIG.  3 . Each host port  109  preferably has an optical attachment to the switched fabric, for example, a Gigabit Link Module (‘GLM’) interface at the controller, which connects to a Gigabit Converter (‘GBIC’) module comprising the switch interface port  107 . Switch interconnection ports  306  also preferably comprise GBIC modules. Each pair of array controllers  201 / 202  and  211 / 212  (and associated storage array) is also called a storage node (e.g.,  301  and  302 ), and has a unique Fibre Channel Node Identifier. As shown in FIG. 3, array controller pair A 1 /A 2  comprise storage node  301 , and array controller pair B 1 /B 2  comprise storage node  302 . Furthermore, each storage node and each port on the array controller has a unique Fibre Channel Port Identifier, such as a World-Wide ID (WWID). 
     The array controllers&#39; ports  109  are connected somewhat differently than typical dual controller/adapter/channel configurations. Normally, the controller ports&#39; connections to dual transmission channels are cross-coupled, i.e., each controller is connected to both channels. However, in the present system configuration  100 , both ports on array controller A 1 , for example, attach directly to a single fabric via switch  204 . Likewise, both ports on array controller A 2  attach directly to the alternate fabric, via switch  214 . The exact same relative connections exist between array controllers B 1 /B 2  and their respective switches  205 / 215  and associated fabrics. One port of each controller is the ‘host’ port that will serve LUN(s) to the local host  101 / 102 . The other port of each controller is the ‘remote copy’ port, used for disaster tolerant backup. 
     Remote Copy Sets 
     FIG. 4 is a high-level diagram of a ‘remote copy set’ operation. The present system  100  views volumes (or LUNs) on a local array as being paired with counterpart volumes on a remote array. A remote copy set comprises a pair of same-sized volumes, one on the local array, and one on the remote array. When a local host computer  101 , for example, requests a storage array I/O operation, the local array controller, or ‘initiator’  301 , presents a local volume that is part of the remote copy set to the local host. The host  101  performs writes to the local volume on the local array  203  , which copies the incoming write data to the remote volume on the target array  213 . 
     As shown in FIG. 4, two LUNs (logical units), LUN X ( 410 ) and LUN X′( 410 ′), attached to controllers B 1 /B 2  ( 302 ) and A 1 /A 2  ( 301 ), respectively, are bound together as a remote copy set  401 . A remote copy set (RCS), when added on array  203 , points to array  213 , and will cause the contents of the local RCS member on array  203  to be immediately copied to the remote RCS member on array  213 . When the copy is complete, LUN X′( 410 ′) on array  213  is ready to be used as a backup device. In order to preserve the integrity of the backup copy, local host  101  access to LUN  410 ′ is not allowed during normal operations. 
     Software Architecture 
     FIG. 5 is a block diagram showing exemplary array controller software architecture employed by the present system  100 . As shown in FIG. 5, peer-to-peer remote copy software (‘PPRC manager’)  515  is layered in between host port initiator module  510  and VA (‘Value Added’, such as RAID and caching) software module  520  within each controller (A 1 /A 2 /B 1 /B 2 ). VA layer  520  is not aware of any PPRC manager  515  context (state change or transfer path). Host port target code  505  allows only host initiators to connect to the controller port which is a dedicated data replication port. 
     The PPRC manager module  515  uses containers and services that the VA layer  520  exports. PPRC manager  515  uses interfaces between host port initiator module  510  and VA module  520  for signaling, transfer initiation, and transfer completions. PPRC manager  515  is responsible for managing functions including initiating the connection and heartbeat with the remote controller and initiating the remote copy for incoming host writes (via host port initiator  510 ); initiating I/O operations for performing full copy, log, and merge; handling error recovery (link failover) and peer communication; and maintaining state information. Device Services layer  525  handles the physical I/O to external devices including the local data storage array and switch. 
     Inter-Site Controller Heartbeat Timer Operation 
     FIG. 6A is an exemplary flow diagram showing the operation of two of the array controller ‘heartbeat’ timers. The operation described in FIG. 6A is best understood in conjunction with reference to the system architecture shown in FIGS. 2 and 3. In the embodiment described in FIG. 6A, during the course of normal system operation, host computer  101  sends requests to write data to array  203  via controller A 1  ( 201 ). At step  600 , in response to a write request, array controller A 1  sends a write command and the host write data to target array controller B 1  via fabric  103 A (referred to as ‘link  1 ” in FIG.  6 ), so that the data is backed up on array  213 . At step  605 , controller A 1  starts a command (‘heartbeat’) timer which keeps track of the time between issuance of the write command and a response from the target controller B 1 . If link  1  and controller B 1  are operational, then controller B 1  writes the data to array  213  and, at step  610 , sends an acknowledgement (‘ACK’) back to controller A 1  via link  1 , indicating successful completion of the command. 
     Asynchronously with respect to the command timer described above, at step  601 , controller A 1  may also periodically send a Fibre Channel ‘echo’ extended link service command to controller B 1  via link  1 . In one embodiment of the present system, the link echo is sent every 10 seconds; however, the exact frequency of the echoes is not critical, nor is it necessary to have the echoes synchronized with any specific source. At step  603 , controller A 1  sets a second ‘heartbeat’ timer or counter, which can simply be a counter which counts-down using a clock to keep track of the time elapsed since the sending of the link echo. At step  610 , in the normal course of operation, controller A 1  receives an ‘ACK’ from controller B 1 , indicating that link  1  is operational. The command and link timers are preferably set to time out at intervals which are best suited for the cross-link response time between controllers A 1  and B 1 . It is to be noted that a single inter-site link/command timer may be employed in lieu of the two timers described above. A periodic ‘echo’ and associated timer may entirely supplant the command timer, or, alternatively, the echo timer may be replaced by the use of a single timer to ensure that each command sent over each inter-site link is responded to within a predetermined time. 
     At step  615 , due to a failure of link  1  or controller B 1 , at least one of two situations has occurred—(1) controller A 1 &#39;s command timer has timed out, or (2) controller A 1 &#39;s link timer has timed out. In either event, a link failover operation is initiated. At step  620 , controller A 1  transfers control to controller A 2 , causing A 2  to assume control of backup activities. Next, at step  625 , controller A 2  proceeds to back up data on storage array  213  by communicating with controller B 2  via link  2  (fabric  103 B). Since controller B 2  shares storage array  213  with controller B 1 , at step  630 , B 2  now has access to the volume (e.g., LUN X′) which was previously created by controller B 1  with data sent from controller A 1 . The failover process is further described below with respect to FIG.  6 B. 
     Intra-Site Controller Heartbeat Timer Operation 
     FIG. 6B is a flow diagram showing the operation of controller-based ‘heartbeat’ timers, wherein a controller failover operation is effected by a ‘surviving’ controller. In the example illustrated in FIG. 6B, controllers A 1  ( 201 ) and A 2  ( 202 ) are interchangeably represented by the letters ‘C’ and ‘C!’, where “C!” represents C&#39;s ‘companion’ controller, i.e., where controller C can be either controller A 1  or A 2 , and controller C! is the companion controller A 2  or A 1 , respectively. This terminology is chosen to illustrate the symmetrical relationship between the two controllers. In the present example, the data from host computer  101  is sent over C&#39;s link (e.g., link  1 ) to a backup volume (e.g., LUN X) via its counterpart controller (e.g., controller B 1 ) at the remote target site. 
     Initially, at step  635 , controllers C and C! set a ‘controller heartbeat’ timer or counter to keep track of the time elapsed between receiving consecutive heartbeat signals (hereinafter referred to as ‘pings’) from the other controller. The controller heartbeat timer is set to time out at a predetermined interval, which allows for a worst-case elapsed time between receiving two consecutive pings from the other controller. Next, during normal operation, at step  640 , controllers C and C! periodically send pings to each other via DUARTs (Dual Asynchronous Receiver/Transmitters) located at both ends of bus  330 . Assuming that neither controller C nor controller C!&#39;s heartbeat timer has timed out, at step  643 , both controllers C and C! receive a ping from their companion controller. Both controllers then reset their heartbeat timers at step  645 , and each controller awaits another ping from its companion controller. 
     In the situation where, for example, controller C fails (step  647 ), allowing controller C!&#39;s heartbeat timer to time out (at step  650 ), then, at step  655 , controller C! initiates a controller failover operation to move the target LUN on remote storage array to the other controller (e.g., from controller B 1  to controller B 2 ). At step  660 , controller C! proceeds by sending backup data to alternate controller (e.g., controller B 2 ) via the alternate link (e.g., link  2 ). At this point, controller C! has access to the backup volume (e.g., LUN X′) on array  213 . 
     Synchronous System Operation 
     FIG. 7 is a flowchart showing synchronous system operation. In synchronous operation mode, data is written simultaneously to local controller cache memory (or directly to local media if the write request is a write-through command), as well as to the remote subsystems, in real time, before the application I/O is completed, thus ensuring the highest possible data consistency. Synchronous replication is appropriate when this exact consistency is critical to an application such as a banking transaction. A drawback to synchronous operation is that long distances between sites mean longer response times, due to the transit time, which might reach unacceptable latency levels, although this situation is somewhat mitigated by write-back cache at the target. Asynchronous operation, described in the following section, may improve the response time for long-distance backup situations. 
     Steady state synchronous operation of system  100  proceeds with the following sequence. As shown in FIG. 7, at step  701 , host computer  101  issues a write command to local controller A 1  ( 201 ), which receives the command at host port  109  over path  221 h at step  705 . At step  710 , the controller passes the write command down to the VA level software  530  (FIG. 5) as a normal write. At step  715 , VA  530  writes the data into its write-back cache through the normal cache manager path (i.e., through the device services layer  525 ). On write completion, VA  530  retains the cache lock and calls the PPRC manager  515 . At step  720 , PPRC manager  515  sends the write data to remote target controller B 1  ( 211 ) via host port initiator module  510 . The data is sent through the remote copy dedicated host port  109  via path  221 D, and across fabric  103 A. Next, at step  725 , remote target controller B 1  writes data to its write-back cache (or directly to media if a write through operation). Then, at step  730 , controller B 1  sends the completion status back to initiator controller A 1 . Once PPRC manager  515  in controller A 1  has received a completion status from target controller, it notifies VA  530  of the completion, at step  735 . At step  740 , VA  530  completes the write in the normal path (media write if write through), releases the cache lock, and completes the operation at step  745  by sending a completion status to the host  101 . 
     Asynchronous System Operation 
     FIG. 8A is a flowchart showing asynchronous operation the present system  100 . Asynchronous operation provides command completion to the host after the data is safe on the initiating controller, and prior to completion of the target command. During system operation, incoming host write requests may exceed the rate at which remote copies to the target can be performed. Copies therefore can be temporarily out of synchronization, but over time that data will converge to the same at all sites. Asynchronous operation is useful when transferring large amounts of data, such as during data center migrations or consolidations. 
     Asynchronous operation of the present system  100  proceeds with the following sequence. As shown in FIG. 8A, at step  801 , host computer  101  issues a write command to local controller A 1  ( 201 ), which receives the command at host port  109  over path  221 h at step  805 . At step  810 , the controller passes the write command down to the VA level software  530  (FIG. 5) as a normal write. At step  815 , VA  530  writes the data into its write-back cache through the normal cache manager path (i.e., through the device services layer  525 ). On write completion, VA  530  retains the cache lock and calls the PPRC manager  515 . At step  820 , PPRC Manager “micro-logs” the write transfer LBN extent in the controller&#39;s non-volatile write-back cache ‘micro-log’. This is done in all situations (not just in error situations), in case the initiator controller (A 1 ) crashes after status is returned to the host, but before the remote copy completes. A small reserved area of cache is dedicated for the micro-log. 
     Micro-logging is done during steady state operation for each asynchronous transfer, not just during error situations. The micro-log information is only used when the controller crashes with outstanding remote copies (or with outstanding logging unit writes). The micro-log contains information to re-issue (‘micro-merge’) the remote copies by either the ‘other’ controller (in this example, controller A 2 ) upon controller failover, or when ‘this’ controller (A 1 ) reboots, in the situation wherein both controllers A 1  and A 2  are down. 
     At step  825 , PPRC manager  515  calls back VA  530  to complete the host write request, and the host is given the completion status. VA  530  retains the cache lock and Data Descriptor data structure. At step  830 , PPRC manager  515  (via host port initiator module  510 ) sends the write data to the remote target. Order preserving context is also passed to host port initiator module  510 . At step  835 , remote target controller B 1  ( 211 ) writes data to its write-back cache (or associated media if a write-through operation). A check is then made by controller A 1  at step  840  to determine whether the remote copy successfully completed. If so, then, at step  845 , target controller B 1  sends the completion status back to initiator controller A 1 . At step  850 , PPRC manager  515  marks the micro-log entry that the write has completed. The PPRC manager also unlocks the controller cache and de-allocates the Data Descriptor. 
     If, at step  840 , if it was determined that the remote copy operation did not complete successfully, then at step  855 , if the initiator controller (A 1 ) failed while the remote copy was in transit, then a ‘micro-merge’ operation (described below with respect to FIG. 8B) is performed. If the remote copy was unsuccessful for other reasons, then at step  860 , other error recovery procedures (not part of the present disclosure) are invoked. 
     FIG. 8B is a flowchart showing a ‘micro-merge’ operation. A micro-merge operation is applicable during asynchronous operation when the controller has failed in the window where the host write status has already been returned to the host, but where the remote copy operation (or write history log operation) has not completed. As indicated above, these ‘outstanding’ writes were logged to the initiator controller A 1 &#39;s write-back cache, which is also mirrored in partner controller A 2 &#39;s write-back cache, so that it is available if controller A 1  fails. If a controller failover has taken place (as explained in the next section, below), then the partner controller (A 2 ) re-issues these remote copies from the micro-log. Alternatively, if both controllers A 1  and A 2  are down, then controller A 1  itself re-issues these writes when it restarts. 
     The following sequence takes place in the controller during micro-merging mode. At step  865 , access to the initiator unit by the host is inhibited until the micro-merge is complete. At step  870 , for each valid entry in the micro-log in the controller write-back cache, the initiator unit is read at the LBN described. If the read has an FE (forced error), then the FE will be copied to the target (which is highly unlikely, since the area was just written). If the read is unrecoverable, then the target member is removed, because it is impossible to make the target consistent. If the read is successful, the data is then written to the remote target member using the normal remote copy path. Alternatively, if write history logging is active, the data is written to the log unit described below in the ‘Write History Logging’ section. 
     In addition to command and LBN extent information, the micro-log contains the command sequence number and additional context to issue the commands in the same order received from the host. At step  875 , if the remote copy of the entry was successful, then at step  880 , the recorded entry in the micro-log is cleared, and the next entry is ‘re-played’, at step  870 . If the remote copy of the entry was not successful, then at step  895 , then error recovery procedures (not part of the present disclosure) are invoked. After completing all micro-merges (step  885 ), the initiator unit is made accessible to the host at step  890 . 
     Link Failover 
     ‘Link failover’ is recovery at the initiator site when one of the two links has failed. Examples of a link failover situation include a target controller rebooting, a switch failure, or an inter-site link failure. In a first situation, if the initiator controller has two consecutive failed heartbeats and its dual partner has two consecutive successful ‘heartbeats’, then a link failover is performed. It may also performed in a second situation wherein a remote write has failed due to a link error and its dual partner last had two successful heartbeats (a failed write is held for two successive heartbeats). 
     FIG. 9 is a diagram showing an example of a link failover operation. As shown in FIG. 9, link  901  is lost to initiator controller A 1 . In the present example, controller A 1  is in communication with partner controller A 2 , which indicates to A 1  that A 2 &#39;s link  902  to controller B 2  is operational. In this situation, initiator controller A 1  attempts link failover recovery procedures by attempting to communicate through its dual redundant partner controller A 2  and resume operations. In one embodiment of the present system, a link failover is accomplished by restarting (re-booting) controller A 1 , to force the initiator unit X on array  203  from controller A 1  to its partner controller A 2 . Once unit X is moved over from controller A 1  to controller A 2  on the initiator side, controller A 2  then ‘pulls’ target unit Y over to its dual redundant partner B 2  where controller A 2  (the ‘new’ initiator) can access it. Link failover is not performed upon receiving SCSI errors (unit failures) from the remote unit, because the other controller will likely encounter the same error. It is to be noted that the initiator controllers (A 1  and A 2 ) control the entire failover operation (the target controller, e.g., B 2  is the slave). 
     Operations resume between controllers A 2  and B 2  if the previous steps were successful. When link failover is successful, the host retries any writes, similar to a controller failover event. Incoming writes during this time are not queued, but rather rejected, so the host will retry them. If the link is restored, the host can move the unit back to the original side. The unit is not moved back automatically by the controller. In other words, there is no “link failback” performed by the controller. 
     Write History Logging 
     The present system  100  provides a unique storage set (typically, RAID level 1, level 0+1, or level 5 storage set) that is considered as a logical unit by the associated controller, and which is employed to create a write history (transaction) log comprising log commands and extents, as well as data, during situations where the remote member of a remote copy set (‘RCS’) is not available. This storage set, called a ‘log unit’, hereinafter, is subsequently ‘replayed’, in original transaction order, i.e., in the exact same order in which it was written, to the remote RCS member to merge the local and remote RCS members. The log unit is preferably located on the same storage array as the local remote copy set member, but in an alternative embodiment, the log unit could be located on a separate storage device coupled to the array controller associated with the local remote copy set member. 
     FIG. 10 is a high-level flow diagram showing a write history log operation performed by the present system  100  when both links are down, or when the remote site is down. The top section of FIG. 10 depicts normal operation of the present system  100 , where arrow  1005  shows write data from host computer  101  being stored on local (initiator) array  203 . Arrow  1010  indicates that the write data is normally backed up on remote (target) array  213 . The lower section of FIG. 10 shows system  100  operation when the links between the local and remote sites are down, or when the remote pair of array controllers  211 / 212  are inoperative, and thus array  213  is inaccessible to local site  218 , as indicated by the broken arrow  1015 . In this situation, as indicated by arrows  1020 , write operations from the local host (ref. no.  101 , shown in FIGS.  2  and  3 ), are directed by the initiator array controller (either  201  or  202  in FIGS. 2 and 3) to both array  203  and log unit  1000 . It is to be noted that a user of the present can control when logging and merging starts and stops, regardless of the link state (operational or not). This provides the ability, for example, to minimize bandwidth during times of high production volume. 
     Extents and data are both written to log unit  1000 , the format for which is described in detail below with respect to FIG.  13 . The logging is done via write through to media. The log unit  1000  is required to have write-back disabled. Enabling write-back would require a DMA copy of the data so that it could be written to media at a later time. The DMA copy process incurs extra overhead, consumes resources, and adds complexity, so write-back mode is not desirable for the present logging function. 
     A log unit is ‘replayed’ to the remote site ‘partner’ controller when the link is restored, the remote site has been restored, or when the local site has been restored (during a site failback, described below with respect to FIG.  12 ). Replaying the log means sending all commands and data over to the remote partner in the original transaction order to all remote copy sets associated with the log unit. A merging operation (hereinafter referred to as simply ‘merge’) is performed by system  100  to quickly return a remote copy set (both local and remote members) to the same data state (i.e., up to date) after link restoration or remote site restoration. A ‘mergeback’ operation is performed by system  100  to restore the local site back to the same state as the remote site during site failback. Log units  1000  and  1001  are used to replay the transactions for the merge and mergeback functions, respectively. 
     FIG. 11 is a flowchart showing an exemplary write history log operation followed by an exemplary merge performed by the present system  100 . As shown in FIG. 11, at step  1105 , access from site  218  to target array  213  is broken, as indicated by arrow  1015  in FIG.  10 . At step  1110 , the write history logging operation of the present system is initiated by array controller  201  in response to a link failover situation, as explained above with respect to FIG.  9 . Initiation of the logging function requires that assignment of a dedicated log unit  1000 / 1001  has been made by a system user. At step  1115 , write operations requested by host computer  101 / 102  are redirected by associated initiator array controller  201  (optionally, controller  202 ) from target controller  211  to log unit  1000 . The log unit descriptors reside at the beginning of the unit. The extent entries are logged before the data in a spiral fashion. FIG. 13, described below, shows the format of data and extent information stored on a log unit. 
     The present system allows different logging streams active at the same time to be intermixed. The log unit is not partitioned in any manner based on the presence of different log streams. If asynchronous operation is enabled, then asynchronous writes occur to the log unit, wherein completion status is returned prior to writing the log unit. 
     A step  1120 , access to target array  213  is re-established, and at step  1125 , the merging operation is initiated. At step  1130 , the data and extent information from host computer  101  is still written to log unit  1000 , but the host writes are delayed to allow the merge to catch up to the log writes. More specifically, the controller turns on a ‘command throttle’ to slow host I/O down, so the merge can make progress. Then at step  1135 , a data read stream is started at the appropriate LBN of the log unit. The data is written to the remote target member using the normal remote copy path. The command order is preserved with the context stored in the log unit. At step  1140 , writes to the log unit  1000  are paused, allowing merge read operations to completely catch up. At this point, there must be more merging I/O operations performed than host I/O operations to avoid reaching the end of the log unit. Therefore, when the merge stream catches up to the log stream, host writes are quiesced (temporarily queued) to make the transition. At step  1145 , the merge reads catch up with the log writes. Finally, at step  1150 , the log and merge operations are completed, and at step  1155 , normal backup operation of system  100  is resumed. 
     Note that during a merge operation, it is not sufficient to send the data over the link in an order compatible with the original write ordering—the data has to be written to “media” (either magnetic media or controller write-back cache) at the remote site in compatible order. This means the local and remote controllers have to control the number of outstanding write operations so that the execution of these commands cannot be reordered, even in the presence of Fibre Channel errors in the inter-controller link, to pervert this order. The present system merges the write commands in the proper order, including write commands which are allowed to overlap each other. For example, if during logging, the original host writes command A and it completes before it issues command C, then during merging, the “play back” must also finish command A before starting command C. 
     FIG. 12 is a flow diagram showing a log operation in response to a site failover. As shown in FIG. 12, during the course of normal operations, host writes to array  203  are backed up on the corresponding remote copy set LUN in array  213 , as indicated by arrow  1210 . If, for example, array  203  becomes inaccessible by local host  101 , as indicated by arrow  1215 , then site failover is performed, since host  101  cannot write the local array  203 , and the controllers at both sites cannot communicate with each other, so inter-site backup is not possible, as shown by arrow  1220 . When site failover takes place, the existing remote copy set is deleted, and a new remote copy set is created with the original target member as the new initiator (now at site  219 ), and the original initiator as the new target member. The remote set consists of two members, as before the site failover. The new initiator unit now presents the WWID of the original initiator (remote copy set&#39;s WWID) to the host at site  219 . In this situation, write operations from the host ( 102 ) at site  219  are directed by the initiator array controller (either  211  or  212 , FIGS. 2 and 3) to array  213 , as indicated by arrow  1225 , and to log unit  1001 , as shown by arrow  1230 . 
     Upon site failback, merge-back takes place. The merge-back operation is analogous to the merge operation described with respect to FIG. 11, except that the ‘initiator’ unit during the merge-back operation is the LUN resident on array  1001 , and the ‘target’ is the original initiator LUN. 
     FIG. 13 is a diagram showing an exemplary format  1300  of data and extent information stored on a log unit  1000 / 1001 . As shown in FIG. 13, the Log Container (log unit) Descriptor (‘LCD’)  1301  starts at logical block address (LBA)  0 , and is used to describe attributes of the log ‘container’ (i.e., the log unit). The LCD  1301  contains information comprising the log disk state (free, allocated, or in use), the current log position, and membership information, including (1) the initiator LUN ID, (2) the target LUN ID, and (3) the target name. The Log Stream Descriptor (‘LSD’)  1302  is used to describe the merging state and includes information such as the current log stream state (free, normal, logging, or merging), the current merge position, and target information which is bit-encoded to denote their specific LCD membership parameters. Following the LCD  1302  is a series of Extent Descriptor List/data segment pairs  1303 */ 1305 *, starting at LBA  100 . The Extent Descriptor List (‘EDL’)  1303 * (where ‘*’ denotes the rank in the series) is used to describe the host write data, and includes an Extent Descriptor header and Extent Descriptor array [*] member information. The Extent Descriptor array contains N number of extent descriptor elements which follow the extent descriptor header. 
     Each extent descriptor describes a contiguous chunk of data that is stored on the log media. The Extent Descriptor header contains pointers  1306  to the next EDL and the previous EDL, more specifically, the next/previous EDL logical block address (LBA). Each member (array element) in the Extent Descriptor includes (1) the LBA of the data at the target destination; (2) an offset  1307  from the EDL to the associated data segment  1303  on the log unit; (3) bit-encoded LCD membership parameters for the target(s) the data belongs to, (4) a ‘look-ahead limit’ used to describe the host data write ordering, and (4), the block count for the associated data in the data segment  1303  following the particular EDL. The terminal EDL segment is indicated by a ‘next EDL’ LBA (flag) value of −1. 
     Although the above description refers to specific embodiments of the invention, the invention is not necessarily limited to the particular embodiments described herein. It is to be understood that various other adaptations and modifications may be made within the spirit and scope of the invention as set forth in the appended claims.