Patent Publication Number: US-6212531-B1

Title: Method for implementing point-in-time copy using a snapshot function

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to co-pending and commonly-assigned patent application Ser. No. 09/006,638, filed on same date herewith, by Mark A. Haye, Ronald M. Kern, and David M. Shackelford, entitled “POINT-IN-TIME BACKUP UTILIZING MULTIPLE COPY TECHNOLOGIES,” which application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to improvements in the field of computer systems having backup/restore or archive/retrieve subsystems, and more particularly, to a method for implementing point-in-time copy operations using snapshot functions. 
     2. Description of Related Art 
     In a data processing system, a backup/restore subsystem is typically used to save a recent copy or version of one or more data sets or portion thereof on some form of backup data storage device, such as magnetic or optical disk drives, tape drives, or other memory. The backup/restore subsystem is used to protect against loss of data. For example, if an on-line version of one or more data sets is destroyed, corrupted, deleted, or changed because of power failure, hardware, or software error, user error or some other type of problem, the latest version of those data sets which are stored in a backup/restore subsystem can be restored and therefore the risk of loss of data is minimized. It is readily apparent that backup/restore subsystems are and will remain an important part of the field of data processing. 
     Successful recovery of data to a known consistent state requires a backup of all components of the data at the same consistent point in time. Generally, a point in time backup is a copy of the data which is logically consistent to a given point in time, with the: restriction that the amount of time to obtain logical consistency is significantly less than the amount of time to actually copy the data. 
     Concurrent copy, also known as time-zero copy, provides the ability to create a point-in-time backup. Concurrent copy is a point in time backup which uses a combination software and microcode architecture to obtain a copy of the original data at the time the backup was initiated. However, concurrent copy is usually only supported for data residing on storage subsystems which implement the concurrent copy functions. 
     One storage subsystem that does not support concurrent copy functions is the log-structured array subsystem (LSA). An LSA subsystem implements “virtual volumes”, wherein each virtual volume is created using a “virtual track table” having pointers to “virtual tracks” (i.e., records) in a sequential byte stream, wherein updated tracks are written to a new location at the logical end of the byte stream and their associated pointers, are reset to the new locations. Thereafter, the tracks at the old location in the sequential byte stream are no longer needed and can be released as free space for reclamation and reuse. 
     In certain products, emulation functions performed by the LSA subsystem permit computer programs executed by a host processor coupled to the LSA subsystem to “view” the LSA subsystem as a standard direct access storage device (DASD) with sequentially numbered tracks. However, such emulation does not necessarily extend to all functions generally capable of being supported with standard DASD. 
     Currently available LSA subsystems do support a very fast copy function called a snapshot function, which operates by copying pointers between virtual track tables representing different virtual data volumes without actually moving any data. On the other hand, the snapshot function suffers from a restriction that the source and target locations must be within the subsystem, and the same device types. As a result, a straightforward backup copy using current snapshot functions does not achieve point-in-time backup on LSA subsystems. 
     Thus, there is a need in the art for methods for providing point in time backups on LSA subsystems, wherein the point-in-time backups use snapshot functions. 
     SUMMARY OF THE INVENTION 
     To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method, apparatus, and article of manufacture for performing a point-in-time backup using a snapshot function. Work space on a work virtual volume is obtained to hold the snapshot copy of the source data being copied from a source virtual volume. Updates to the source virtual volume are suspended during the snapshot function, so as to provide a logically consistent view of the source virtual volume at a common point in time. Pointers are copied from a virtual track table of the source virtual volume to a virtual track table of the work virtual volume without actually moving any data on data storage devices referenced by those pointers. As a result, the snapshot function provides a method for copying the source virtual volume to the work virtual volume very quickly. Upon completion of the snapshot function, updates to the source virtual volume may be resumed. Subsequently, a backup is performed in the usual manner, except that the backup retrieves the source data from the work virtual volume rather than the source virtual volume. 
     One object of the present invention is that it the point-in-time backup using the snapshot function may be performed on multiple subsystems or target data storage devices that are different from the source data storage device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
     FIG. 1 is an exemplary hardware environment used to implement the preferred embodiment of the invention; 
     FIGS. 2A,  2 B, and  2 C illustrate a logical view of virtual volumes provided by the log-structured array subsystem before and after a snapshot function is executed; and 
     FIG. 3 is a flowchart illustrating the logic performed by a point in time backup using the snapshot function according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     HARDWARE ENVIRONMENT 
     FIG. 1 is an exemplary hardware environment used to implement the preferred embodiment of the invention. A host computer  10  is coupled via a bus  12  to a storage controller  14 , which itself is coupled via an I/O channel  16  to one or more data storage devices  18  and  20 . In the preferred embodiment, the data storage devices  18  and  20  each comprise RAID (redundant arrays of inexpensive disks) storage subsystems, such as the RAMAC Virtual Array Storage System sold by IBM Corporation, the assignee of the present invention. Of course, those skilled in the art will recognize that any data storage device may be used with the present invention. 
     The host computer  10  executes one or more computer programs  22 ,  24 ,  26 , and  28  that control the operation of the host computer  10  and its interaction with the storage controller  14 . In the preferred embodiment, the computer programs  22 ,  24 ,  26 , and  28  comprise a backup program  22 , a snapshot program  24 , an extent determination program  26 , and a work space locator program  28 , respectively, although other computer programs may be used as well. 
     Similarly, the storage controller  14  includes one or more computer programs  30  or other logic that controls the operation of the storage controller  14  and its interaction with the host computer  10  and data storage devices  18  and  20 . In the preferred embodiment, the computer program  30  comprises a log-structured array (LSA) subsystem  30  that provides the access logic for the data storage devices  18  and  20 , although other functions may be provided as well. 
     The LSA subsystem  30  constructs “virtual volumes”  32  and  34  (also labeled as A and B) in the memory of the storage controller  14  for access by the host computer  10 . The virtual volumes  32  and  34  emulate standard DASD for data actually stored by the LSA subsystem  30  on the data storage devices  18  and  20 . In this emulation, the host computer  10  (or any computer program executed thereby) “views” the virtual volumes  32  and  34  of the LSA subsystem  30  as normal data volumes, i.e., standard DASD, with sequentially numbered tracks. To accomplish this emulation, the LSA subsystem  30  maintains a virtual track table in the memory of the storage controller  12  for each virtual volume  32  or  34 , wherein the virtual track table comprises one or more pointers representing one or more sequentially numbered virtual tracks and the pointers reference data stored on the data storage devices  18  and  20 . 
     The present invention provides a point-in-time backup on the LSA subsystem  30  that is termed “Concurrent Copy Compatible Snapshot.” The Concurrent Copy Compatible Snapshot function can be coordinated across multiple LSA subsystems  30  and also can be used to copy data to a target data storage device that is not an LSA subsystem  30 . 
     In the Concurrent Copy Compatible Snapshot function, work space on a work virtual volume is obtained to hold the snapshot copy of the data from the source virtual volume. During the operation of the Concurrent Copy Compatible Snapshot function, updates to the source virtual volume are suspended (although for a very short time) so as to provide a logically consistent view of the source data on the source virtual volume at a common point in time. The Concurrent Copy Compatible Snapshot function operates by copying the pointers for the virtual tracks containing the source data from the virtual track table of the source virtual volume to the virtual track table of the work virtual volume without actually moving any of the source data on the data storage devices  18  or  20  referenced by those pointers. As a result, the snapshot function provides a method for copying the source data from the source virtual volume to the work virtual volume very quickly. Upon completion of the operation of the Concurrent Copy Compatible Snapshot function, updates to the source data on the source virtual volume may be resumed. Subsequently, a backup is performed in the usual manner, except that the backup retrieves the source data from the work virtual volume rather than the source virtual volume. 
     Those skilled in the art will recognize that the exemplary environment illustrated in FIG. 1 is not intended to limit the present invention. Indeed, those skilled in the art will recognize that other alternative hardware environments may be used without departing from the scope of the present invention. 
     Those skilled in the art will also recognize that the present invention may be implemented as a method, apparatus, or article of manufacture using standard programming and/or engineering techniques to produce software, firmware, hardware, or any combination thereof. The term “article of manufacture” (or alternatively, “computer program carrier”) as used herein is intended to encompass any device, carrier, or media that provides access to instructions and/or data useful in performing the same or similar functionality. Of course, those skilled in the art will recognize many modifications may be made to this configuration without departing from the scope of the present invention. 
     VIRTUAL VOLUMES 
     FIGS. 2A,  2 B, and  2 C illustrate the logical view of virtual volumes A and B ( 32  and  34 ) provided by the LSA subsystem  30  before and after a snapshot function is executed. The virtual volumes  32  and  34  and their contents are viewed by the host computer  10  and its programs as standard DASD having sequential tracks of data, and the logical and physical structure of the data storage devices  18  and  20  providing the actual storage space for these virtual volumes  32  and  34  are hidden from view by the LSA subsystem  30 . Computer programs executed by the host computer  10  address data in the LSA subsystem  30  by virtual tracks of the virtual volumes  32  and  34 , wherein each virtual volume  32  and  34  is comprised of a virtual track table having one or more pointers representing one or more sequential virtual tracks, and each of the pointers reference a corresponding location (or multiple locations) on the associated data storage device  18  and  20 . 
     In FIG. 2B, a snapshot function has been executed by the LSA subsystem  30  to copy the virtual tracks containing the source data on virtual volume A ( 32 ) to virtual volume B ( 34 ). The result of the snapshot function is to set the pointers in the virtual track table of virtual volume B ( 34 ) to reference the same data locations on the data storage device  18  as the pointers in the virtual track table of virtual volume A ( 32 ). The data on the data storage device  20  previously pointed to by the pointers in the virtual track table of virtual volume B ( 34 ) is now inaccessible and the space it occupies on the data storage device  18  is available for reclamation and reuse. FIG. 2C illustrates a logical view of virtual volumes A and B provided by the LSA subsystem  30  during a point in time backup using the snapshot function. The LSA subsystem  30  is shown in the state after a snapshot function from the source virtual volume A ( 32 ) to a work virtual volume B ( 34 ) is executed, as described in FIG.  2 A. The backup program  22 , snapshot program  24 , extent determining program  26 , and work space locator  28  are all computer programs which are executed by the host processor  10  to perform these functions, wherein the backup program  22  is in control. These programs work together to back up the source data from the work virtual volume B ( 34 ) to a backup data storage subsystem, such as controller  40  and DASD  42 . The steps performed in conjunction with this Figure are described in more detail below. 
     FLOWCHART 
     FIG. 3 is a flowchart that further illustrates the steps performed in a point in time backup using the snapshot function according to the present invention. The backup program  22  is in control of the execution of the following steps. 
     Block  44  represents the extent determining program  26  locating the virtual tracks on the source virtual volume A ( 32 ) that contain the source data which is to be backed up, and then returning information concerning the virtual tracks to the backup program  22 . 
     Block  46  represents the host computer  10  suspending any further updates of the source data on the source virtual volume A ( 32 ). Generally, this step is invoked by an operator, by the backup program  22 , or by some other program. 
     Block  48  represents the work space locator program  28  locating sufficient work space, i.e., unused virtual tracks, on the work virtual volume B ( 34 ) to contain the source data which is to be backed up from the source virtual volume A ( 32 ), and then returning information concerning the virtual tracks to the backup program  22 . The LSA subsystem  20  considers the located work space as being reserved for the backup program  22 , so that any subsequent request, whether internal or external, for this work space will be blocked. 
     Block  50  represents the snapshot program  24  updating the pointers from the virtual track table of the work virtual volume B ( 34 ) representing the work space so that they have the same values as the pointers from the virtual track table of the source virtual volume B ( 34 ) representing the source data. This is done by copying the pointers in the virtual track table for source virtual volume A ( 32 ) to the pointers in the virtual track table for the work virtual volume B ( 34 ). The backup program  22  records a correspondence between the virtual track location of the source data on the source virtual volume A ( 32 ) and the virtual track location of the work space on the work virtual volume B ( 34 ). 
     Block  52  represents the host computer  10  resuming further updates to the source data on the source virtual volume A ( 32 ). As with step  44  above, this step is invoked by an operator, by the backup program  22 , or by some other program. Note, however, that these further updates affect only the virtual tracks of the source virtual volume A ( 32 ) representing the source data and do not affect the virtual tracks of the work virtual volume B ( 34 ) representing the work space. Since the LSA subsystem  20  implements log-structured storage, wherein updated virtual tracks of the source virtual volume A ( 32 ) are written to a new location at the end of the log-structured storage and their associated pointers in the virtual track table are reset to the new locations. However, the virtual tracks at the old location in the log-structured storage are not released as free space, even though they are no longer needed by the source virtual volume A ( 32 ), because they are still needed and pointed to by the work virtual volume B ( 34 ). 
     Block  54  represents the backup program  22  performing the steps normally used to backup the source data and then writing the backed up source data to the backup destination device, for example, controller  40  and DASD  42 . However, in the present invention, the source data is retrieved from the virtual tracks on the work virtual volume B ( 34 ) representing the work space and containing the source data. These steps are performed by the backup program  22  translating requests for the source data to the work space using the recorded correspondence between the virtual track location of the source data on the source virtual volume A ( 32 ) and the virtual track location of the work space on the work virtual volume B ( 34 ). 
     Finally, Block  56  represents the LSA subsystem  20  releasing or freeing the work space obtained on the work virtual volume B ( 34 ) when the backup program  22  completes. 
     CONCLUSION 
     This concludes the description of the preferred embodiment of the invention. The following paragraphs describe some alternative methods of accomplishing the same objects. 
     In alternative embodiments of the present invention, other types and configurations of computers could be used. For example, the invention need not be restricted to the hardware and software configuration illustrated herein. For example, other mainframes, minicomputers, personal computers, or networks of computers could be used with the present invention. In another example, peripherals other than those illustrated herein could benefit from the present invention. 
     In alternative embodiments of the present invention, data storage devices other than those described herein could be used. For example, the log-structured storage could be implemented on different data storage devices than the log-structured array subsystem as illustrated herein. 
     In alternative embodiments of the present invention, other logic than that described herein could be performed without departing from the scope of the present invention. For example, the invention need not be restricted to the exact steps or elements illustrated herein. 
     In summary, the present invention discloses a method, apparatus, and article of manufacture for performing a point-in-time backup using a snapshot function. Work space on a work virtual volume is obtained to hold the snapshot copy of the source data being copied from a source virtual volume. Updates to the source virtual volume are suspended during the snapshot function, so as to provide a logically consistent view of the source virtual volume at a common point in time. Pointers are copied from a virtual track table of the source virtual volume to a virtual track table of the work virtual volume without actually moving any data on data storage devices referenced by those pointers. As a result, the snapshot function provides a method for copying the source virtual volume to the work virtual volume very quickly. Upon completion of the snapshot function, updates to the source virtual volume may be resumed. Subsequently, a backup is performed in the usual manner, except that the backup retrieves the source data from the work virtual volume rather than the source virtual volume. 
     The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.