Abstract:
A method and system for providing logically consistent backup copies of a logical unit provided by a data storage device, and for managing and maintaining a pool of increasingly stale backup copies within the data storage device. Backup logical units within the pool are continuously recycled, so that the least recently current backup logical unit is next synchronized and activated to receive mirror I/O requests. A trigger I/O request is provided to allow an application program running on a host computer to signal points within a stream of I/O requests that represent logically consistent states.

Description:
CROSS REFERENCE  
       [0001]    This application is a continuation of patent application Ser. No. 09/895,474, filed Jun. 28, 2001. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to the use of logical unit mirroring within a data storage device and, in particular, to a method and system for providing a pool of logically consistent backup logical units that lag, in data consistency, a local and/or remote primary LUN by varying periods of time.  
         BACKGROUND OF THE INVENTION  
         [0003]    The present invention relates to mirror split operations conducted by the controllers of various types of data storage devices. The described embodiment relates to disk-array data-storage devices and disk-array controllers. Therefore, a concise background of disk and disk-array technologies is provided below.  
           [0004]    [0004]FIG. 1 is a block diagram of a standard disk drive. The disk drive  101  receives I/O requests from remote computers via a communications medium  102  such as a computer bus, fibre channel, or other such electronic communications medium. For many types of storage devices, including the disk drive  101  illustrated in FIG. 1, the vast majority of I/O requests are either READ or WRITE requests. A READ request requests that the storage device return to the requesting remote computer some requested amount of electronic data stored within the storage device. A WRITE request requests that the storage device store electronic data furnished by the remote computer within the storage device. Thus, as a result of a READ request carried out by the storage device, data is returned via communications medium  102  to a remote computer, and as a result of a write request, data is received from a remote computer by the storage device via communications medium  102  and stored within the storage device.  
           [0005]    The disk drive storage device illustrated in FIG. 1 includes controller hardware and logic  103  including electronic memory, one or more processors or processing circuits, and controller firmware, and also includes a number of disk platters  104  coated with a magnetic medium for storing electronic data. The disk drive contains many other components not shown in FIG. 1, including read/write heads, a high-speed electronic motor, a drive shaft, and other electronic, mechanical, and electromechanical components. The memory within the disk drive includes a request/reply buffer  105 , which stores I/O requests received from remote computers, and an I/O queue  106  that stores internal I/O commands corresponding to the I/O requests stored within the request/reply buffer  105 . Communication between remote computers and the disk drive, translation of I/O requests into internal I/O commands, and management of the I/O queue, among other things, are carried out by the disk drive I/O controller as specified by disk drive I/O controller firmware  107 . Translation of internal I/O commands into electromechanical disk operations, in which data is stored onto, or retrieved from, the disk platters  104 , is carried out by the disk drive I/O controller as specified by disk media read/write management firmware  108 . Thus, the disk drive I/O control firmware  107  and the disk media read/write management firmware  108 , along with the processors and memory that enable execution of the firmware, compose the disk drive controller.  
           [0006]    Individual disk drives, such as the disk drive illustrated in FIG. 1, are normally connected to, and used by, a single remote computer, although it has been common to provide dual-ported disk drives for use by two remote computers and multi-port disk drives that can be accessed by numerous remote computers via a communications medium such as a fibre channel. However, the amount of electronic data that can be stored in a single disk drive is limited. In order to provide much larger-capacity electronic data-storage devices that can be efficiently accessed by numerous remote computers, disk manufacturers commonly combine many different individual disk drives, such as the disk drive illustrated in FIG. 1, into a disk array device, increasing both the storage capacity as well as increasing the capacity for parallel I/O request servicing by concurrent operation of the multiple disk drives contained within the disk array.  
           [0007]    [0007]FIG. 2 is a simple block diagram of a disk array. The disk array  202  includes a number of disk drive devices  203 ,  204 , and  205 . In FIG. 2, for simplicity of illustration, only three individual disk drives are shown within the disk array, but disk arrays may contain many tens or hundreds of individual disk drives. A disk array contains a disk array controller  206  and cache memory  207 . Generally, data retrieved from disk drives in response to READ requests may be stored within the cache memory  207  so that subsequent requests for the same data can be more quickly satisfied by reading the data from the quickly accessible cache memory rather than from the much slower electromechanical disk drives. Various elaborate mechanisms are employed to maintain, within the cache memory  207 , data that has the greatest chance of being subsequently re-requested within a reasonable amount of time. The data contained in WRITE requests may also be stored first in cache memory  207 , in the event that the data may be subsequently requested via READ requests or in order to defer slower writing of the data to physical storage medium.  
           [0008]    Electronic data is stored within a disk array at specific addressable locations. Because a disk array may contain many different individual disk drives, the address space represented by a disk array is immense, generally many thousands of gigabytes to tens or hundreds of terabytes. The overall address space is normally partitioned among a number of abstract data storage resources called logical units (“LUNs”). A LUN includes a defined amount of electronic data storage space, mapped to the data storage space of one or more disk drives within the disk array, and may be associated with various logical parameters including access privileges, backup frequencies, and mirror coordination with one or more LUNs. LUNs may also be based on random access memory (“RAM”), mass storage devices other than hard disks, or combinations of memory, hard disks, and/or other types of mass storage devices. Remote computers generally access data within a disk array through one of the many abstract LUNs  208 - 215  provided by the disk array via internal disk drives  203 - 205  and the disk array controller  206 . Thus, a remote computer may specify a particular unit quantity of data, such as a byte, word, or block, using a bus communications media address corresponding to a disk array, a LUN specifier, normally a 64-bit integer, and a 32-bit, 64-bit, or 128-bit data address that specifies a LUN, and a data address within the logical data address partition allocated to the LUN. The disk array controller translates such a data specification into an indication of a particular disk drive within the disk array and a logical data address within the disk drive. A disk drive controller within the disk drive finally translates the logical address to a physical medium address. Normally, electronic data is read and written as one or more blocks of contiguous 32-bit or 64-bit computer words, the exact details of the granularity of access depending on the hardware and firmware capabilities within the disk array and individual disk drives as well as the operating system of the remote computers generating I/O requests and characteristics of the communication medium interconnecting the disk array with the remote computers.  
           [0009]    In many computer applications and systems that need to reliably store and retrieve data from a mass storage device, such as a disk array, a primary data object, such as a file or database, is normally backed up to backup copies of the primary data object on physically discrete mass storage devices or media so that if, during operation of the application or system, the primary data object becomes corrupted, inaccessible, or is overwritten or deleted, the primary data object can be restored by copying a backup copy of the primary data object from the mass storage device. Many different techniques and methodologies for maintaining backup copies have been developed. In one well-known technique, a primary data object is mirrored. FIG. 3 illustrates object-level mirroring. In FIG. 3, a primary data object “O 3 ”  301  is stored on LUN A  302 . The mirror object, or backup copy, “O 3 ”  303  is stored on LUN B  304 . The arrows in FIG. 3, such as arrow  305 , indicate I/O write requests directed to various objects stored on a LUN. I/O WRITE requests directed to object “O 3 ” are represented by arrow  306 . When object-level mirroring is enabled, the disk array controller providing LUNs A and B automatically generates a second I/O write request from each I/O write request  306  directed to LUN A, and directs the second generated I/O write request via path  307 , switch “S 1 ”  308 , and path  309  to the mirror object “O 3 ”  303  stored on LUN B  304 . In FIG. 3, enablement of mirroring is logically represented by switch “S 1 ”  308  being on. Thus, when object-level mirroring is enabled, any I/O write request, or any other type of I/O request that changes the representation of object “O 3 ”  301  on LUN A, is automatically mirrored by the disk array controller to identically change the mirror object “O 3 ”  303 . Mirroring can be disabled, represented in FIG. 3 by switch “S 1 ”  308  being in an off position. In that case, changes to the primary data object “O 3 ”  301  are no longer automatically reflected in the mirror object “O 3 ”  303 . Thus, at the point that mirroring is disabled, the stored representation, or state, of the primary data object “O 3 ”  301  may diverge from the stored representation, or state, of the mirror object “O 3 ”  303 . Once the primary and mirror copies of an object have diverged, the two copies can be brought back to identical representations, or states, by a resync operation represented in FIG. 3 by switch “S 2 ”  310  being in an on position. In the normal mirroring operation, switch “S 2 ”  310  is in the off position. During the resync operation, any I/O operations that occurred after mirroring was disabled are logically issued by the disk array controller to the mirror copy of the object via path  311 , switch “S 2, ” and pass  309 . During resync, switch “S 1 ” is in the off position. Once the resync operation is complete, logical switch “S 2 ” is disabled and logical switch “S 1 ”  308  can be turned on in order to reenable mirroring so that subsequent I/O write requests or other I/O operations that change the storage state of primary data object “O 3 ,” are automatically reflected to the mirror object “O 3 ”  303 .  
           [0010]    In many data storage devices, including disk arrays, mirroring is conducted by a disk array controller on a per-LUN basis. A LUN may be mirrored for various reasons, including for preparation of a backup copy of a primary LUN that can be used for database backup and for archival purposes. For these purposes, a LUN may be mirrored for some interval of time, and the mirroring may then be disabled, or, in other words, the mirrored LUN pair may then be split, so that the backup LUN of a the primary-LUN/backup-LUN mirror pair can be used as a consistent snapshot of the data state of the primary LUN at the point in time that mirroring is disabled. Thus, the mirroring capability built into disk array controllers and controllers of other types of data storage devices can be exploited as an efficient data backup mechanism by database management systems and other application programs.  
           [0011]    Unfortunately, there are a number of significant deficiencies in using data-storage-device-based mirroring for generating backup copies of primary LUNs by application programs and system routines running on a host computer. First, data storage devices cannot determine when a primary LUN or a backup LUN of a mirrored LUN pair is in a logically consistent state with respect to host-computer transaction processing, database management, or other such activities involving data maintenance. Logically consistent states may occur quite infrequently, and, therefore, a backup LUN generated by a mirror split operation is generally logically inconsistent. In some cases, the backup LUN can be repaired by backing out incomplete transactions, for example, to produce a logically consistent backup, but, in other cases, the backup LUN cannot be repaired. Another deficiency in using data-storage-device-based mirroring for generating backup copies is the overhead involved, from the perspective of an application program, in regularly directing splitting of a mirrored LUN pair, including overhead involved in managing backup LUNs and the overhead of sometimes lengthy unavailability of a primary LUN during the mirror split operation. For these reasons, designers and manufacturers of data storage devices, and developers of application programs and database management systems have recognized the need for a more convenient and efficient method for employing data-storage-device mirroring to generate backup LUNs for database backup, archival purposes, and for other purposes.  
         SUMMARY OF THE INVENTION  
         [0012]    In one embodiment of the present invention, the controller of a data storage device recognizes a special I/O request received from a host computer as a TRIGGER event, and inserts a corresponding TRIGGER message into a sequenced stream of I/O requests for a particular primary LUN of a primary LUN/backup LUN mirror pair. The TRIGGER message indicates to the data storage device controller that, when all I/O requests preceding the TRIGGER message are successfully executed on the primary LUN, the primary LUN will be in a logically consistent state. The TRIGGER message can be, in turn, inserted into the I/O request stream directed to the backup LUN, so that the backup LUN can also detect a logically consistent state. In the described embodiment, a mirror split occurs at a point in time that both the primary LUN and backup LUN of a mirrored LUN pair are logically consistent, and the data states of the primary LUN and backup LUN are consistent. In the described embodiment, a pool of circularly linked backup LUNs is managed by a data storage device controller. Generally, one backup LUN of the pool of circularly linked backup LUNs is active, and actively receives mirror WRITE requests, and the other backup LUNs of the pool of circularly linked backup LUNs are inactive. The backup LUNs are rotated through the active state at regular intervals, so that the backup LUNs represent successively less recent, logically consistent snapshots of the primary LUN. An application program or system routine running on a host computer can thus easily use the mirroring functionality provided by the data storage device to generate backup copies of a primary LUN. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a block diagram of a standard disk drive.  
         [0014]    [0014]FIG. 2 is a simple block diagram of a disk array.  
         [0015]    [0015]FIG. 3 illustrates object-level mirroring.  
         [0016]    FIGS.  4 A-D illustrate problems that prevent logically consistent and quick mirror split operations.  
         [0017]    FIGS.  5 A-D illustrate operation of a trigger message in the context of the example illustrated in FIGS.  4 A-I.  
         [0018]    In FIG. 6 illustrates a pool of backup LUNs employed for generating backups at regular intervals.  
         [0019]    [0019]FIG. 7 illustrates an enhanced backup-LUN-pool technique that decreases the synchronization overhead for newly activated backup LUNs.  
         [0020]    [0020]FIG. 8 illustrates a primary LUN mirrored to a local circularly-linked pool of backup LUNs.  
         [0021]    [0021]FIG. 9 illustrates a primary LUN within a first data storage device mirrored to a remote primary backup LUN in a remote data storage device  908 .  
         [0022]    [0022]FIG. 10 shows an N-by-N data-storage-device configuration employing pools of circularly-linked backup LUNs. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    Embodiments of the present invention incorporate a TRIGGER event technique that allows an application program or system routine running on a host computer to identify, to the controller of a data storage device, points in a stream of I/O requests directed to a primary LUN that represent logically consistent data states when all preceding I/O requests in the stream of I/O requests have been successfully executed. These embodiments also employ pools of backup LUNs managed by the data-storage-device controller. In order to describe the TRIGGER technique and the backup-LUN-pool technique, the problems of backup-LUN synchronization and logical consistency need further description.  
         [0024]    FIGS.  4 A-D illustrate problems that prevent logically consistent and quick mirror split operations. In FIGS.  4 A-D, a simplified, abstract view of mirror-related I/O request handling is provided. The view is simplified because only a single mirrored LUN pair is discussed, although a disk array controller needs to concurrently handle processing of I/O requests directed to from tens to thousands of mirrored LUN pairs. Furthermore, many details of disk arrays unnecessary for explaining backup-LUN synchronization and logical consistency problems are omitted. Nevertheless, the problems illustrated in FIGS.  4 A-D are representative of the many concurrently overlapping problems experienced by a disk array controller.  
         [0025]    FIGS.  4 A-D employ similar illustrative conventions. These conventions are described with reference to FIG. 4A, and many of the numerical labels introduced in FIG. 4A will be used in FIGS.  4 A-D, as well as in FIGS.  5 A-D, discussed below. In Figure A, an application program running on a host computer  402  generates I/O requests and adds them to an output queue  404  for transmission, via a communications medium, to a disk array  418 . There are a number of WRITE requests already queued to the output queue  404 , including WRITE requests  406 ,  408 , and  412  associated with a first transaction, darkly colored in FIG. 4A to indicate the association with the first transaction, WRITE requests  407  and  411  associated with a second transaction, cross-hatched in FIG. 4A to indicate the association with the second transaction, and additional WRITE requests  409 ,  410 ,  413 , and  414  not explicitly associated with a transaction described in the current example. The WRITE requests  406 - 414  are directed to a primary LUN  420  in the disk array  418 . The primary LUN is mirrored to a backup LUN  428  in a second disk array  424 . A WRITE request from output queue  404  thus is transmitted first to the first disk array  418 , queued to the input queue  416  of the first disk array  418 . The controller of the first disk array  418  dequeues WRITE requests from the input queue  416 , executes the WRITE requests on the primary LUN  420  to write data to the primary LUN, and queues mirror WRITE requests to output queue  422  for transmission to the input queue  426  of the second disk array  424  for writing to the backup LUN  428 . Note that, in the current example, only WRITE requests are discussed, because, in general, WRITE requests represent the vast majority of I/O requests that alter the data state of a LUN.  
         [0026]    [0026]FIG. 4A thus represents an initial set of conditions for a two-transaction example that is provided with reference to FIGS.  4 B- 4 I. Initially, it is assumed that primary LUN  420  and backup LUN  428  contain the same data, and are thus in a data-consistent state. They are also assumed to be logically consistent, or, in other words, do not contain partial data associated with one or more host-application transactions.  
         [0027]    In FIG. 4B, WRITE requests  406 - 408  have been transmitted from output queue  404  through a communications medium to input queue  416  of the first disk array  418 . In FIG. 4C, WRITE request  409  has joined WRITE requests  408  and  407  on the input queue  416  of the first disk array  418 , and WRITE request  406  has been executed on the primary LUN  420 , resulting in data  430  written to primary LUN  420 . In addition, WRITE request  406  has been placed onto the output queue  422  of the first disk array  418  for transmission to the second disk array  424 . Thus, WRITE request  406  is queued as a mirror WRITE request directed to backup LUN  428 .  
         [0028]    In FIG. 4D, WRITE request  406  has been transmitted via the communications medium to the input queue  426  of the second disk array  424 , but, due to bottlenecks in communications between the host computer  402  and the first disk array  418 , or because of other higher-priority intervening tasks requiring the host computer&#39;s processing cycles, no further WRITE requests have been transmitted from the host computer  402  to the input queue  416  of the first disk array  418 . In FIG. 4E, WRITE request  406  has been executed to the backup LUN  428  resulting in data  432  written to the backup LUN  428 . Processing of WRITE requests from the input queue  416  by the controller of the first disk array  418  is temporarily stalled, although an additional WRITE request  411  has been transferred from the host computer to input queue  416 . At the point in time represented by FIG. 4E, the data state of the primary LUN  420  is consistent with the data state of the backup LUN  428 . All WRITE requests executed on the primary LUN  420  have also been executed on the backup LUN  428 . This data-state consistency can be determined by the disk array controllers of disk arrays  418  and  424 , and such data-consistent states are normally selected by data storage device controllers for mirror splitting.  
         [0029]    With respect to the first and second transactions carried out by the host computer  402  involving WRITE requests  406 ,  408 , and  412  and WRITE requests  407  and  411 , respectively, the data state of both the primary LUN  420  and backup LUN  428  are not logically consistent, in FIG. 4E. For logical, transaction-based consistency, all WRITE requests associated with multiple-WRITE-request transactions need to be either fully executed on a LUN or not executed on the LUN. However, at the point in time shown in FIG. 4E, WRITE request  406  has been executed to primary LUN  420  and backup LUN  428 , but related WRITE requests  408  and  412  have not been executed to either the primary LUN or backup LUN. Should disk array  418  fail, at this point in time, primary LUN  420  will be left in a logically inconsistent state, containing only partial data for the first transaction, and backup LUN  428  will also be in a logically inconsistent state, and will not be useable for backup purposes until all partially completed transactions, such as data  432  written to the backup LUN  428 , are recognized and removed. In certain cases, partially executed transactions can be identified and removed in order to produce a logically consistent backup. In other cases, the task is impossible.  
         [0030]    As illustrated in FIGS.  4 A-E, the problem of logical inconsistency of primary and backup LUNs results from the inability of a data storage device controller to recognize transaction boundaries within a stream of WRITE requests. Only the application program, system routine, or other host-computer-resident process issuing WRITE requests has sufficient information to recognize points in a stream of WRITE requests that represent clean transaction boundaries with respect to all ongoing transactions that correspond to points of logical consistency. Note that a LUN may be logically inconsistent with respect to one transaction, or with respect to multiple concurrent host-application transactions. As the number of transactions with respect to which a LUN is logically inconsistent grows, the task of repairing the logical inconsistency in case of a failure may increase greatly in complexity.  
         [0031]    FIGS.  4 A-I illustrate another problem associated with using data-storage-device mirroring to generate backup copies of a primary LUN. Although few WRITE requests are shown queued to output queue  422  and input queue  426  of the first and second disk arrays, respectively, there may be megabytes of WRITE request data backed up on queues. When the disk-array controller of the first disk array  418  receives a mirror split request, the disk-array controller and the disk-array controller of the second disk array  424  need to cooperatively flush all such queued WRITE requests and execute them on the backup LUN  428  in order to bring the backup LUN  428  to a data consistent point. Moreover, when a new backup LUN is paired with the primary LUN to reconstitute a mirrored LUN pair, the data state of the new backup LUN must be brought to consistency with that of the primary LUN in a synchronization process. Thus, a mirror split operation may take considerable time, and the primary LUN may be unavailable for additional WRITE request execution until the mirror split operation completes.  
         [0032]    In FIG. 4F, processing of WRITE requests by the controller of the first disk array  418  has resumed, and WRITE requests  407  and  408  have been executed on primary LUN  420 , resulting in data  434  and  436  written to primary LUN  420 . WRITE requests  407  and  408  have been placed on the output queue  422  of the first disk array  418  for forwarding, as mirror WRITE requests, to the second disk array  424 . WRITE requests  411  and  412  have been successfully transmitted to the first disk array  418  through a communications medium. At the point in time shown in FIG. 4F, the data state of the primary LUN  420  is no longer consistent with the data state of the backup LUN  428 , and the data state of the primary LUN  420  is now logically inconsistent with respect to both the first transaction and the second transaction, whereas the data state of the backup LUN  428  is logically inconsistent only with respect to the first transaction.  
         [0033]    In FIG. 4G, all WRITE requests related to the first and second transactions have been executed on the primary LUN  420 , which is now in a logically consistent state. WRITE request  412  has been placed on output queue  422  of the first disk array  418  for transmission as a mirror WRITE request to the second disk array  424 . WRITE request  411  resides on the input queue  426  of the second disk array  424 . The backup LUN  428  contains data associated with WRITE requests  406 ,  408 , and  407 , and is therefore logically inconsistent with respect to both the first and second transactions. Note that the data state of the primary LUN  420  is inconsistent with the data of the backup LUN  428 . Thus, were the first disk array  418  to fail, the backup LUN  428  would need to be cleansed of data associated with WRITE requests  406 - 408  in order to be placed in a logically consistent state prior to being used as the primary LUN following fail over.  
         [0034]    In FIG. 4H, WRITE request  411  has been executed on the backup LUN  428 , which is now logically consistent with respect to the second transaction, but logically inconsistent with respect to the first transaction. WRITE request  412  has yet to be mirrored to the backup LUN  428 . As before, the primary LUN  420  is logically consistent with respect to both the first and second transactions. The situation in FIG. 4H demonstrates that a LUN may be logically consistent with respect to one transaction, but logically inconsistent with respect to others. Finally, in FIG. 41, all WRITE requests associated with the first and second transactions are fully executed on both the primary LUN  420  and the backup LUN  428 . At the point in time shown in FIG. 41, the data states of the primary LUN and backup LUN are consistent, and both are logically consistent, at least with respect to transactions emanating from host computer  402 . Thus, either the point in time shown in FIG. 4A or the point in time shown in FIG. 4I would be appropriate points in time for disabling mirroring in order that backup LUN  428  become an inactive, archival backup LUN. The intervening points in time illustrated in FIGS.  4 C- 4 H would not be appropriate points in time for generating a backup copy of the primary LUN. However, as discussed above, the controllers of the first disk array  418  and second disk array  424  have no way of determining those points in time at which the primary LUN and backup LUN are in a logically consistent state. They can only determine points in time when the data states of the primary LUN and backup LUN are consistent with one another, such as the points in time shown in FIGS. 4A, 4B,  4 E, and  4 I.  
         [0035]    The present invention addresses the logical consistency problem illustrated in FIGS.  4 A-I by introducing a special trigger message. The trigger message, from the standpoint of the host computer, is an I/O request directed to either a special LUN or volume of the first disk array, or an I/O request directed to the primary LUN of a mirrored LUN pair by an alternate path designated as the path for trigger messages, or some other kind of out-of-band signal sent from the host computer to the disk array.  
         [0036]    FIGS.  5 A-D illustrate operation of a trigger message in the context of the example illustrated in FIGS.  4 A-I. FIG. 5A corresponds to FIG. 4A, with the exception that a trigger I/O request  502  is inserted following WRITE request  412  in the input queue  404  of the host computer  402 . The host computer inserts the trigger I/O request at a point in the I/O request stream at which, if all previous queued I/O requests are successfully executed, and no subsequently queued I/O requests are executed, the primary LUN will be in a logically consistent state with respect to transactions generated by host computer  402 . FIG. 5B corresponds to FIG. 4F, and shows the trigger I/O request  502  having been successfully transmitted to the first disk array  418  and placed in the input queue  416  as a special TRIGGER message in correct sequential order with respect to the WRITE requests queued to the input queue. Thus, the controller of the disk array  418  interprets the I/O request directed to the special LUN or volume, or to the primary LUN via a special path as a trigger request and places a special TRIGGER request into the input queue.  
         [0037]    Note that the trigger-request mechanism depends on faithful sequencing of I/O requests within the input queues of the first disk array  418  and the second disk array  428 . As discussed in a previously filed application, U.S. application Ser. No. ______, faithful sequencing of I/O requests within local and remote disk arrays can be accomplished using universal sequence number generation components within the disk arrays. Details for sequencing of I/O requests within disk arrays can be found in that application.  
         [0038]    The presence of the trigger message  502  in input queue  416  indicates to the disk array controller that, when the trigger message is dequeued from the input queue, the primary LUN is in a logically consistent state, at least with respect to I/O requests generated by host computer  402 . FIG. 5C corresponds to FIG. 4G. The trigger message  502  has been processed by the first disk array  418  and is queued for transmission to the second disk array  424 . At the point in time shown in FIG. 5C, the controller of the first disk array  418  may undertake steps to prepare for splitting of the mirror pair comprising primary LUN  420  and backup LUN  428 . For example, the controller of the first disk array  418  may temporarily discontinue processing WRITE requests directed to the primary LUN until the mirror split has occurred. FIG. 5D corresponds to FIG. 4I in which the primary LUN and backup LUN are both logically consistent, and the data states of the primary LUN and backup LUN are consistent, as well. The trigger message  502  will be next dequeued from input queue  426  of the second disk array  424 . Upon dequeuing the trigger message, the controller of the second disk array  424  will know that the backup LUN  428  is in a logically consistent state, and can be disabled or deactivated in order to generate an archival backup copy. Should execution of a WRITE request fail, or other error conditions occur, during the time interval illustrated in FIGS.  4 C- 4 G, then failed WRITES will need to be repeated and completed prior to considering the backup LUN  428  to be in a logically consistent or even data consistent state.  
         [0039]    The trigger-message mechanism allows the controller of the data storage device to recognize logically consistent points within an I/O request stream. However, for effective use of data-storage-device mirroring for backup copy generation, a second, automated technique for generating backup copies at regular intervals is needed. FIGS.  6 - 10  illustrate this second technique. In FIG. 6, a primary LUN  602  is paired with a currently active backup LUN  604  to form a mirrored LUN pair. The locations of the primary LUN and backup LUN are not important for this example, and are thus not specified in FIG. 6. Backup LUN  604  is part of a pool of backup LUNs that also includes inactive backup LUNs  606 ,  608 , and  610 . The pool of backup LUNs is organized as a circularly linked list of backup LUNs. This circularly linked list can be thought of as rotating, over time, in a forward direction. In FIG. 6, the forward direction is indicated by arrows, such as arrow  612 . The inactive backup LUNs  606 ,  608 , and  610  represent logically consistent backup copies taken at times increasingly distant in the past from the current time. For example, in FIG. 6, there are three inactive backup LUNs that may represent backups generated one hour previous to the time point shown in FIG. 6, in the case of inactive backup LUN  606 , two hours previous to the current time, in the case of inactive backup LUN  608 , and three hours previous to the current time, in the case of inactive backup LUN  610 . At the next mirror split time, currently active backup LUN  604  will be disabled in a logically consistent state as determined by the presence of a trigger message, and will become the most recent backup copy, and inactive backup LUN  610  will be activated to become the currently active backup LUN via synchronizing by incorporating data added or updated to the primary LUN while backup LUN  610  was inactive.  
         [0040]    Although the circularly-linked pool of backup LUNs, illustrated in FIG. 6, automatically managed by the controller of the data storage device, provides for regular backup copy generation, the activation of an inactive backup LUN following a mirror split can involve a rather lengthy period of time during which the newly activated backup LUN must be synchronized with the primary LUN. Synchronization involves detecting differences in the data state of the primary LUN with respect to that of the newly activated backup LUN, and executing WRITE requests on the newly activated backup LUN in order to bring the backup LUN to a data state consistent with that of the primary LUN. In order to avoid a lengthy synchronization period, mirror WRITE requests can be executed on the currently active backup LUN as well as on the backup LUN that will next become the currently active backup LUN. FIG. 7 illustrates this enhanced backup-LUN-pool technique that decreases synchronization overheads for newly activated backup LUNs. As shown in FIG. 7, after an initial synchronization operating, new mirror WRITES corresponding to WRITES executed on the primary LUN  602  are executed both on the currently active backup LUN  604  and on backup LUN  610 , which will next become the currently active backup LUN. Thus, during activation of currently inactive backup LUN  610 , only those WRITE requests executed at the time that inactive backup LUNs  606  and  608  were previously active need be repeated in order to synchronize backup LUN  610  with primary LUN  602 .  
         [0041]    The pool-of-circularly-linked-backup-LUNs technique may be employed for local mirroring as well as for remote mirroring. In FIG. 8, a primary LUN  802  is mirrored to a local, circularly linked pool of backup LUNs  804 - 807 . In addition, the primary LUN  802  is mirrored to a circularly linked pool of backup LUNs  808 - 811  within a remote data storage device  812 . Alternatively, a primary LUN may be mirrored to a primary backup LUN, and both the primary LUN and primary backup LUN may be locally mirrored to a circularly-linked pool of backup LUNs. FIG. 9 shows a primary LUN  902  within a first data storage device  904  mirrored to a remote primary backup LUN  906  in a remote data storage device  908 . Both the primary LUN  902  and the primary backup LUN  906  are additionally mirrored to pools of circularly-linked backup LUNs  910  and  912 , respectively. In certain cases, a data protection group, or multiple-LUN entity, may be distributed across several local data storage devices and mirrored to one or more remote data storage devices. Such N×M configurations are described in U.S. application Ser. No. ______ in detail. FIG. 10 shows an N-by-N configuration employing pools of circularly-linked backup LUNs. In FIG. 10, a distributed primary LUN  1002  and  1004  resides in local data storage devices  1006  and  1008 , respectively. The distributed primary LUN is mirrored to a distributed backup LUN  1010  and  1012  residing in remote data storage devices  1014  and  1016 , respectively. Each of the primary and primary backup LUNs  1002 ,  1004 ,  1010 , and  1012  are additionally mirrored to pools of circularly-linked backup LUNs  1018 - 1021 .  
         [0042]    The following is an abstract, C++-like model for the techniques of the present invention. This pseudocode model is not intended to show even a small fraction of the details of a data storage device controller, but is instead intended to illustrate the trigger message and pool-of-circularly-linked-backup-LUNs techniques in an abstract model example.  
         [0043]    First, the pseudocode example includes an enumeration and several class declarations:  
                                                    1 enum requestType {TRIGGER, WRITE, READ};            2 class IORequest            3 {            4   public:            5     requestType getType( );            6 };            7 class diskOp            8 {            9   public:           10     requestType getType( );           11 };           12 class disk           13 {           14   public:           15     void executeOperation(diskOp* op);           16 };                      
 
         [0044]    The enumeration “requestType,” declared above on line 1, includes three types of I/O requests, including a TRIGGER request type, a WRITE request type, and a READ request type. The class “IORequest,” declared above on lines 2-6, represents a generalized I/O request, and includes a single member function “getType,” declared on line 5, that returns an indication of the type of I/O request, where the type is one of the values of the enumeration “requestType.” The class “diskOp,” declared above on lines 7-11, represents a disk operation that can be directed towards a particular data storage device, such as a disk drive. The class “disk,” declared above on lines 12-16, represents a data storage device interface, such as a disk interface, and includes the member function “executeOperation,” declared on line 15, which executes a disk operation furnished by reference in the calling argument “op.” 
         [0045]    The class “LUN,” provided below, represents control functionality associated with a particular LUN within a data storage device, and a generalized LUN interface:  
                                                    1 class LUN            2 {            3   private:            4     disk dk;            5     bool rotatingMirrorGroup;            6     LUN* remote;            7     LUN* mirrorHead;            8     LUN* mirrorTail;            9     LUN* next;           10     LUN* previous;           11     void synchronize(LUN* Iptr);           12           13   public:           14     void executeOperation(diskOp* op);           15     LUN* getNext( ) {return next;};           16     LUN* getPrevious( ) {return previous;};           17     void setNext(LUN* n) {next = n;};           18     void setPrevious(LUN* p) {previous = p;};           19 };                      
 
         [0046]    The class “LUN” includes the following data members: (1) “dk,” declared above on line 4, the disk drive to which the LUN is mapped within a data storage device; (2) “rotatingMirrorGroup,” declared above on line 5, a Boolean value indicating whether or not the LUN is associated with a pool of circularly-linked backup LUNs, as illustrated in FIG. 6; (3) “remote,” declared above on line 6, a pointer to a primary backup LUN, if the LUN is mirrored to a primary backup LUN; (4) “mirrorHead,” declared above on line 7, a reference to the currently active backup LUN in an associated pool of backup LUNs; (5) “mirrorTail,” declared above on line 8, a pointer to the least recently active backup LUN in an associated pool of circularly-linked backup LUNs; (6) “next,” declared above on line 9, a pointer to the next LUN in a circularly-linked list of LUNs; and (7) “previous,” declared above on line 10, a pointer to the previous LUN in a circularly-linked list of LUNs. The class “LUN” includes a single private member function “synchronize,” declared on line 11, which synchronizes the current LUN and a LUN pointed to by the reference argument “lptr.” An implementation for this member function is not provided, as it is outside the scope of the present discussion. Implementations are provided only as needed to illustrate the techniques of the present invention. The class “LUN” includes the following public member functions: (1) “executeOperation,” a member function, declared above on line 14, that carries out a disk operation directed to the LUN; (2) “getNext,” a member function, declared above on line 15, that provides the next LUN in a circularly-linked list of LUNs including the current LUN; (3) “getPrevious,” declared above on line 16, a member function that returns a pointer to the previous LUN in a circularly-linked list of LUNs including the current LUN; and (4) member functions “setNext” and “setPrevious,” declared above on lines 17-18, that allow data members “next” and “previous” to be set to values provided as arguments.  
         [0047]    The class “LUNs,” provided below, is a class that manages the entire set of LUNs provided by a data storage device, and the class “inputQueue,” also provided below, corresponds to an input queue, such as input queue  416  in disk array  418  of FIGS.  4 A-I:  
                                                    1 class LUNs            2 {            3   public:            4     diskOp* translateRequest(LUN** Iptr, IORequest* r);            5            6 };            7 class inputQueue            8 {            9   public:           10     void inQueue(IORequest* r);           11     IORequest* outQueue( );           12     int numQueued( );           13     activateCallback(void (*a) (inputQueue* qu));           14     inputQueue( );           15 };                      
 
         [0048]    The class “LUNs” is shown only with a single member function “translateRequest” that translates an I/O request supplied as argument “r” into a returned disk operation that can be directed towards a LUN referenced by argument “lptr.” The member functions of the class “inputQueue” allow for queuing and dequeuing I/O requests to the input queue, for determining the number of I/O requests queued to the input queue, and for activating a callback function associated with the input queue so that, when an I/O request is queued to the input queue, the callback function is called. It is assumed that only a single thread or process corresponding to the callback function can be activated at any given point in time, so that if the callback function is active when a next I/O request is queued to the queue, a second process or thread will not be spawned.  
         [0049]    The class “controller,” provided below, represents I/O request processing by the controller of the data storage device:  
                                                    1 class controller            2 {            3   private:            4     inputQueue* queue( );            5     LUNs luns;            6            7            8   public:            9     void handle(inputQueue* qu);           10     controller( );           11 };                      
 
         [0050]    The class “controller” includes the following data members: (1) “queue,” declared above on line 4, an input queue such as input queue  416  of disk array  418  in FIG. 4A; and (2) “luns,” declared above on line 5, the LUN management functionality of the controller. The class controller includes the following public member functions: (1) “handle,” declared above on line 9, a member function that serves as the callback function for an input queue serviced by the controller and that handles all I/O requests directed to the controller; and (2) “controller,” a constructor for the class “controller.” An implementation of the constructor is not shown, but includes initialization of the input queue to activate member function “handle” as the callback function.  
         [0051]    An implementation of the member function “executeOperation” of the class “LUN” is provided below:  
                                    1 void LUN::executeOperation(diskOp* op)        2 {        3        4   switch (op-&gt;getType( ))        5   {        6     case TRIGGER:        7       if (rotatingMirrorGroup)        8       {        9         mirrorHead = mirrorTail;       10         mirrorTail = mirrorTail-&gt;getPrevious( );       11         synchronize(mirrorHead);       12       }       13       if (remote != NULL) remote-&gt;executeOperation(op);       14       break;       15     case WRITE:       16       dk.executeOperation(op);       17       if (rotatingMirrorGroup)       18       {       19         mirrorHead-&gt;executeOperation(op);       20         mirrorTail-&gt;executeOperation(op);       21       }       22       if (remote != NULL) remote-&gt;executeOperation(op);       23       break;       24     case READ:       25       dk.executeOperation(op);       26       break;       27   }       28 }                  
 
         [0052]    The member function “executeOperation” receives a disk operation referenced by the argument “op” on line 1. On line 4, executeOperation determines the type of the operation. In the case that the operation is a TRIGGER message, as described above with reference to FIGS.  5 A-D, executeOperation determines, on line 7, whether or not the LUN is associated with a rotating mirror group. If so, then executeOperation rotates the circularly linked list of backup LUNs by one, on lines 9 and 10, and synchronizes the newly activated backup LUN on line 11. If the LUN is associated with a remote primary backup LUN, as detected on line 13, then the TRIGGER message is forwarded to that remote primary backup LUN on line 13. In the case of a WRITE disk operation, executeOperation executes the WRITE on the local LUN on line 16, forwards the WRITE to an associated pool of circularly-linked backup LUNs on lines 17-21, and forwards the WRITE to a remote primary backup LUN on line 22, if the LUN is associated with a remote primary backup LUN. In the case of a READ operation, executeOperation simply executes the READ on the local data storage device on line 25.  
         [0053]    Finally, an implementation of the member function “handle” for the class “controller” is provided below:  
                                                    1 void controller::handle(inputQueue* qu)            2 {            3   IORequest* r;            4   diskOp* d;            5   LUN* ptr;            6            7   while (qu-&gt;numQueued( ) &gt; 0)            8   {            9     r = qu-&gt;outQueue( );           10     d = luns.translateRequest(&amp;ptr, r);           11     ptr-&gt;executeOperation(d);           12   }           13 }                      
 
         [0054]    The functionality for the controller related to the present invention in the current pseudocode model is straightforward. While there are I/O requests queued to the input queue, as detected on line 7, the controller dequeues the next I/O request on line 9, translates the dequeued I/O request into a disk operation on line 10, and directs the disk operation to the appropriate LUN on line 11.  
         [0055]    Although the present invention has been described in terms of a particular embodiment, it is not intended that the invention be limited to this embodiment. Modifications within the spirit of the invention will be apparent to those skilled in the art. For example, both management of a pool of circularly linked backup LUNs and processing of TRIGGER I/O requests and internal TRIGGER messages can be implemented via an almost unlimited number of different hardware, firmware, and software implementations, or via hybrid combinations of hardware, firmware, and software. Any number of backup LUNs may included within a pool of backup LUNs, and the number and identify of the LUNs may be configured by configuration files or through data-storage-device administration tools. The time interval during a backup LUN is active may also be configurable. As discussed above, any of a variety of different out-of-band messaging techniques can be employed for the TRIGGER I/O request, including sending by an application program of an I/O request to a special LUN or volume, sending an I/O request by a speacial path to the primary LUN of a mirror LUM pair, sending a special TRIGGER  1 /O request recognized by the disk array controller, or by sending a message or signal over a separate communications medium. Many different backup LUN pool configurations and topologies are possibly, apart from those disclosed above. More complex strategies involving tiered hierarchies of backup LUNs, or topologies other than circularly linked lists, may be employed. While the discussed embodiments related to disk arrays, the techniques of the present invention are related to a wide range of data storage devices that offer data mirroring. Although the described embodiments and examples showed a single mirrored LUN pair, the techniques of the present invention are intended to applied, in parallel, for tens to hundreds of mirrored LUN pairs within one or more data storage devices.  
         [0056]    The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. The foregoing descriptions of specific embodiments of the present invention are presented for purpose of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations are possible in view of the above teachings. The embodiments are shown and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents: