Abstract:
A secondary volume of a mirrored volume pair is replicated by suspending the mirroring operations, associating a selected volume identifier with the secondary volume, replicating the secondary volume to a backup volume, and associating the original secondary volume identifier with the backup volume. In some embodiments the original secondary volume identifier is written to a hidden field on the secondary volume and the hidden field is copied to the backup volume identifier field after the replication. In some embodiments the actions of suspending the mirror operations, managing the volume identifiers, replicating the secondary volume to a backup volume, synchronizing the secondary volume with the primary volume, and reestablishing the mirror pair are performed as an automated sequence. The resultant replication method is less costly and error prone because it may be created by an automated process rather than manual commands issued by a system administrator.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to data replication means and methods. More particularly, the invention relates to an apparatus, system and method for replicating a secondary volume of a mirrored volume pair to a backup volume. 
   2. Description of the Related Art 
   It is well known that during operation a CPU may update one or more data storage volumes in an attached storage subsystem. It is further known that replication of data storage volumes is a frequently used strategy for maintaining continuously available information systems in the presence of system level faults or failures. Among several replication techniques, mirroring is often favored over point-in-time copying in that a data mirror is continuously updated and may be quickly substituted for an unavailable primary volume. 
   Data mirroring involves maintaining identical copies of data on a primary volume and a secondary volume. Volume-to-volume mirroring from a primary volume to a secondary volume may be accomplished either synchronously (in real time) or asynchronously (at selected occasions or intervals). In either case, the primary volume is typically available for use by a host processor and the secondary volume is offline. 
   Referring to  FIG. 1 , a prior art peer-to-peer remote copy (PPRC) system  100  is illustrated. The PPRC system  100  is one example of a synchronously mirrored system and includes a primary storage system  110  and a secondary storage system  120 . A host  130  is connected to the primary storage system  110 . The host  130  stores data by sending write requests to the primary storage system  110 . 
   Data written to primary storage system  110  is copied to the secondary storage system  120 , creating a mirror image of the data residing on the primary storage system  110  on the secondary storage system  120 . In the PPRC system  100 , a write made by the host  130  is considered complete only after the data written to the primary storage system  110  is also written to the secondary storage system  120 . The primary host  130  may take various forms, such as a server on a network, a Web server on the Internet, or a mainframe computer. In the depicted examples, the primary storage system  110  and secondary storage system  120  are disk systems. 
   A communication path  140  connects the host  130  to the primary storage system  110 . A communication path  150  connects the primary storage system  110  with the secondary storage system  120 . The communication paths  140 / 150  may comprise various links, such as fiber optic lines, packet switched communication links, enterprise systems connection (ESCON) fibers, small computer system interface (SCSI) cable, and wireless communication links. 
   The primary storage system  110  includes at least one storage volume  160  typically referred to as a primary volume and other well-known components such as a controller, cache, and non-volatile storage. The secondary storage system  120  includes at least one storage volume  170 , typically referred to as a secondary volume. The primary volume  160  and secondary volume  170  are set up in PPRC pairs. PPRC pairs are synchronous mirror sets in which a storage volume in the primary storage system  110  has a corresponding storage volume in the secondary storage system  120  with data that is identical. This pair is referred to as an established PPRC pair or synchronous mirror set. 
   In operation, each time a write request is sent to the primary volume  160  by the host  130 , the primary storage system  110  stores the data on the primary volume  160  and also sends the data over the communication path  150  to the secondary storage system  120 . The secondary storage system  120  then copies the data to the secondary volume  170  to form a mirror of the primary volume  160 . 
     FIG. 2  depicts a prior art asynchronously mirrored data system  200  including a host  210 , one or more application programs  220 , and a data mover  230 . A primary storage system  240  is connected to the host  210  by one or more channels, for example, fiber optic channels. At least one primary volume  250  is contained within or connected to the primary storage system  240 . 
   A secondary storage system  260  is connected to the host  210  by one or more channels or alternatively by a communication link. Contained within or connected to the secondary storage system  260  is at least one secondary volume  270 . In some systems, a direct communication link may be established between the primary storage system  240  and the secondary storage system  260 . In such systems, the data mover  230  may reside within the primary storage system  240 . 
   The asynchronously mirrored data system  200  collects data from the primary storage systems  240  so that all write requests from the host  210  to the primary volume  250  are preserved and applied to the secondary volume  270  without significantly impacting access rates for the host  210 . The data and control information transmitted to the secondary storage system  260  is sufficient such that the presence of the primary storage system  240  is no longer required to preserve data integrity. 
   The application programs  220  generate write requests, which update data on the primary volume  250 . The locations of the data updates are tracked by the primary storage system  240 . Often, updates to the primary volume  250  are tracked on a track-by-track basis. A two dimensional array of bits (a bit map), often referred to as an active track array or changed track array, is typically used to keep a real-time record of tracks on the primary volume that have been changed since the last synchronization. The changed track array is maintained in the primary storage system  240 . The primary storage system  240  may group the updates and conduct a synchronization session to provide the updates to the data mover  230 . The updates are transmitted from the data mover  230  to the secondary storage system  260 , which writes the updates to the secondary volume  270 . 
   Asynchronous mirroring has minimal impact on the access rate between the primary host  210  and the primary storage system  240  because a subsequent I/O operation may start directly after receiving acknowledgement that data has been written to the primary volume  250 . While write requests may occur as demanded by the application programs  220 , synchronization of the secondary volume  270  is an independent, asynchronous event. For example, synchronization sessions may be scheduled periodically throughout the day as directed by settings managed by a system administrator, typically several times per hour. Thus, the asynchronous secondary volume  270  may be only rarely identical to the primary volume  250 , since additional writes requests to the primary volume  250  may occur during the copy operation necessary to synchronize the secondary volume. 
   In some systems, both synchronous and asynchronous data mirror pairs are maintained. This configuration permits rapid promotion of a synchronous mirror system to become a replacement primary storage system in the event that the original primary storage system becomes unavailable. The configuration also provides for the maintenance of a nearly real-time remote copy of the primary storage system data for use if the primary site becomes unavailable. In this configuration, the storage volumes on the primary storage system may act as the primary volumes for both the synchronously mirrored volumes and asynchronously mirrored volumes. 
   In disk mirroring environments, system administrators may desire to create a point-in-time archive or backup copy. In order to minimize the effect on system performance, it is desirable to use the secondary volume as the data source for the copy while allowing the host to access the primary volume in a normal fashion. However, since the secondary volume is an exact copy of the primary volume, the volume identifier is the same on both the primary volume and the secondary volume. The secondary volume cannot be brought online to perform the copy since doing so would introduce duplicate volume identifiers on the system. 
   In order to backup a mirrored volume pair, the user may bring the secondary volume online to a different system and perform the backup operation on that system. This method eliminates the problem of duplicate volume identifiers. Nevertheless, since multiple systems are required to perform the backup, the solution typically necessitates the purchase of another system. 
   Alternately, the user may change the volume identifier of the secondary volume, then bring the secondary volume online to the same system as the primary volume and use the renamed secondary volume as the data source for the copy. A disadvantage of this solution is that the backup or archive volume does not have the original secondary volume identifier. During a restore operation, the user is required to remember the original volume identifier of the secondary volume and manually rename the restored volume with the original volume identifier after the restore operation. This procedure is error-prone and often results in system downtime. 
   Given the aforementioned alternatives, a need exists for an apparatus, method, and system to replicate a secondary volume of a mirrored volume pair including the volume identifier on a backup storage volume. Beneficially, such an apparatus, method, and system would simplify the creation of a point-in-time backup on a mirrored system and decrease the probability of error in restoring the backup. 
   SUMMARY OF THE INVENTION 
   The present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available mirror volume replicaters. Accordingly, the present invention has been developed to provide a method, apparatus, and system for replicating a secondary volume of a mirrored pair that overcomes many or all of the above-discussed shortcomings in the art. 
   The apparatus for replicating a secondary volume of a mirrored pair is provided with logic containing a plurality of modules configured to functionally execute the necessary steps of replicating the mirror pair secondary volume. These modules in the described embodiments include a mirror module, a volume identification module, and a data replication module. 
   The apparatus, in one embodiment, includes a mirror module that suspends mirroring operations between a primary volume and a secondary volume and, in some embodiments, also resynchronizes the secondary volume to the primary volume and reestablishes the mirror pair. A data replication module copies the data on the secondary volume to a backup volume. 
   A volume identification module associates a secondary volume with a selected volume such that the secondary volume may be brought online without introducing duplicate volume identifiers. The volume identification module also associates the suspend-time secondary volume identifier to the backup volume. In some embodiments, the volume identification module copies the suspend-time secondary volume identifier to a hidden field on the secondary volume and associates the contents of the hidden field to the backup volume subsequent to the volume replication. 
   A system of the present invention is also presented for replicating a secondary volume of a mirrored pair. The system may be embodied with a host, a primary storage system, a secondary storage system functioning to provide a synchronous data mirror, and a backup system. The mirroring operations may be suspended and the secondary volume associated with a selected identifier such that the secondary volume may be brought online without introducing duplicate volume identifiers. The secondary volume may be replicated to a backup volume, and the backup volume associated with the suspend-time secondary volume identifier. 
   In some embodiments, the suspend-time secondary volume identifier is written to a hidden field on the secondary volume and the contents of the hidden field are associated with the backup volume after the replication to the backup volume is complete. In some embodiments, the operations of suspending mirroring operations, managing the volume identifiers, replicating the secondary volume to a backup volume, and reestablishing mirroring operations between the primary volume and the secondary volume are performed as an automated sequence responsive to a single command from a system administrator. 
   A method of the present invention is also presented for replicating a secondary volume of a mirrored pair. The method in the disclosed embodiments substantially includes the steps necessary to carry out the functions presented above with respect to the operation of the described apparatus and system. In one embodiment, the method includes suspending mirror operations between a primary volume and a secondary volume, associating the secondary volume with a selected volume identifier, replicating the secondary volume to a backup volume, and associating the suspend-time secondary volume identifier to a backup volume. 
   In one embodiment, the method also includes writing the suspend-time secondary volume identifier to a hidden field on the secondary volume and associating the contents of the hidden field with the backup after the replication of the secondary volume to the backup volume. In some embodiments, the method further includes resynchronizing the secondary volume to the primary volume and reestablishing mirroring operations between the primary volume and the secondary volume. 
   Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
   Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
   These features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which: 
       FIG. 1  is a schematic block diagram illustrating a prior art peer-to-peer remote copy (PPRC) system; 
       FIG. 2  is a schematic block diagram illustrating a prior art asynchronously mirrored data system; 
       FIG. 3  is a schematic block diagram illustrating one embodiment of a mirrored volume replication system of the present invention; 
       FIG. 4  is a schematic block diagram illustrating one embodiment of a mirrored volume replication apparatus of the present invention; and 
       FIG. 5  is a schematic flow chart diagram illustrating one embodiment of a method for replicating a mirrored volume of the present invention; 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Many of the functional units described in this specification have been labeled as modules, in order to more particularly emphasize their implementation independence. For example, a module may be implemented as a hardware circuit comprising custom VLSI circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. A module may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices or the like. 
   Modules may also be implemented in software for execution by various types of processors. An identified module of executable code may, for instance, comprise one or more physical or logical blocks of computer instructions that may, for instance, be organized as an object, procedure, or function. Nevertheless, the executables of an identified module need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the module and achieve the stated purpose for the module. 
   Indeed, a module of executable code could be a single instruction, or many instructions, and may even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data may be identified and illustrated herein within modules, and may be embodied in any suitable form and organized within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices, and may exist, at least partially, merely as electronic signals on a system or network. 
   Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
   Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of programming, software modules, user selections, network transactions, database queries, database structures, hardware modules, hardware circuits, hardware chips, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
   The present invention sets forth an apparatus, system and method to replicate a secondary volume of a mirrored volume pair. The invention may be embodied in a system with one or more mirror pairs, each mirror pair including a primary storage volume and a secondary storage volume. The mirroring operations may be synchronous or asynchronous. The resultant replicated copy of the secondary volume contains the suspend-time secondary volume identifier. 
     FIG. 3  is a schematic block diagram illustrating one embodiment of a mirrored volume replication system of the present invention. The system  300  includes a host  310  operably connected to a primary storage system  320 , a secondary storage system  330 , and a backup system  340 . In the depicted embodiment, a secondary volume replication module  350  resides on the host  310 . In some embodiments, the secondary volume replication module  350  may reside on an external storage system. In certain embodiments, the secondary storage system  330  may be directly connected to the primary storage system  320  in order to facilitate remote synchronous mirroring operations. 
   The primary storage system  320  includes at least one primary volume  355  configured as a mirror pair primary volume, and the secondary storage system  330  includes at least one secondary volume  360  configured as a mirror pair secondary volume. During mirroring operations, the primary volume identifier is identical to the secondary volume identifier. 
   The secondary volume replication module  350  suspends the mirroring operation between the primary volume  355  and the secondary volume  360 , and associates the secondary volume with a unique identifier such that the secondary volume may be brought online without introducing a duplicate volume identifier. In one embodiment, the secondary volume is associated with a unique identifier by overwriting the secondary volume identifier field  365  with the unique identifier. The secondary volume replication module  350  copies the data from the secondary volume  360  to a backup volume  370  and writes the suspend-time secondary volume identifier to a backup volume identifier field  375 . 
   In some embodiments, the secondary volume replication module  350  may write the suspend-time secondary volume identifier to a hidden field  380  on the secondary volume  360  and, after the replication of the secondary volume  360  is complete, copy the contents of the hidden field  380  to the backup volume identifier field  375 . In some embodiments, the secondary volume replication module  350  resynchronizes the secondary volume  360  to the primary volume  355  and reestablishes the mirroring operations between the primary volume  355  and the secondary volume  360 . 
     FIG. 4  is a schematic block diagram illustrating one embodiment of a mirrored volume replication apparatus  400  of the present invention. A host  310  is operably connected to a primary storage volume  355  and a secondary volume  360  configured as a mirror pair, and a backup volume. The depicted host  310  includes a secondary volume replication module  350 . The depicted secondary volume replication module  350  includes a mirror module  410 , a volume identification module  420 , and a data replication module  430 . In some embodiments the secondary volume replication module  350  may reside on an external storage system. 
   The secondary volume  360  contains a volume identifier field  365 , and the backup volume  370  contains a volume identifier field  375 . The volume identifier field  375  contains a volume identifier associated with the volume on which the field  375  resides. Because the primary volume  355  and the secondary volume  360  operate as a mirror pair, the secondary volume identifier is identical to the primary volume identifier. The value in the secondary volume identifier field  365  at the time the mirror operations are suspended is referred to as the suspend-time secondary volume identifier. 
   The mirror module  410  under certain circumstances suspends the mirror operations between the primary volume  355  and the secondary volume  360 . In some embodiments, the mirror module  410  also initiates resynchronization of the secondary volume  360  to the primary volume  355  and reestablishes the mirroring operations between the primary volume  355  and the secondary volume  360 . During a resynchronization operation, the secondary volume identifier field  365  may be overwritten by the primary volume identifier field  440 . 
   The volume identification module  420  associates the secondary volume with a unique volume identifier, such that the renamed secondary volume  360  may be brought online without introducing a duplicate volume identifier. The volume identification module  420  writes the suspend-time secondary volume identifier to the backup volume identifier field  375 . In some embodiments, the volume identification module  420  writes the suspend-time secondary volume identifier to a hidden field  380  on the secondary volume  360  and, subsequent to the replication, copies the contents of the hidden field  380  to the backup volume identifier field  375 . Consequently, if the replication operation is interrupted, the volume identification module  420  may recover the suspend-time secondary volume identifier from the hidden field  380  on the secondary volume  360  in order to write the backup volume identifier field  375 . 
   The data replication module  430  copies the data from the secondary volume  360  to the backup volume  370 . In some embodiments, the data replication module  430  may bring the secondary volume  360  online prior to the start of the replication operation and take the secondary volume  360  offline after the replication operation is complete. 
     FIG. 5  is a schematic flow chart diagram illustrating one embodiment of a method  500  for replicating a secondary volume of a mirrored volume pair of the present invention. The method  500  starts  510  when a user requests a point-in-time copy of a mirrored volume. The mirror module  410  suspends  520  the mirroring operations between a primary volume  355  and a secondary volume  360 . Then the volume identification module  420  copies  530  the secondary volume identifier to a hidden field  380  on the secondary volume  360 , and afterwards associates  540  the secondary volume  360  with a selected identifier such that the secondary volume identifier will not introduce a duplicate volume when the secondary volume  360  is brought online. 
   Subsequently, the data replication module  430  brings  550  the secondary volume  360  online and then replicates  560  the secondary volume  360  by copying all data resident on the secondary volume  360  to the backup volume  370 . The volume identification module  420  copies  570  the contents of the hidden field  380  to the backup volume identifier field  375 . Then the mirror module  410  resynchronizes  580  the secondary volume  360  to the primary volume  355  by copying the tracks of the primary volume  355  containing data that differs from the associated secondary volume track data to the secondary volume  360 . The mirror module  410  then reestablishes  590  the mirror relationship between the primary volume  355  and the secondary volume  360 , and the method  500  ends. 
   The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.