System and method for synchronizing data mirrored by storage subsystems

[0002] The present invention generally relates to the field of mirrored data storage systems, and particularly to a system and method for synchronizing data mirrored by storage systems.

[0003] The reliable storage of computer data is one of the most important aspects of modem life. From applications and programs, to credit information, web sites, and the like, some of the most important aspects of an individual's life are stored in an electronic form. One of the problems facing electronic storage is device failure and the subsequent loss of data. One of the methods utilized to lessen the chance of data device failure, and the resulting loss of information, is the use of mirrored data storage devices.

[0004] Mirrored data storage devices are devices in which data is written to at least two devices simultaneously. In this way, if one of the devices fail, the system may switch to the other device without a loss of data or service. Mirroring is useful as a way to keep data accessible in a reliable manner. However, although the data is typically available when one of the devices is suspended, such as when an interruption in operation occurs, it is still desirable to enable the data on the working subsystem to be mirrored on an additional device. In other words, to return the system to full operational status in case the remaining working subsystem suffers an operational failure, such as by returning the suspended device to operational status, using a replacement device, and the like. To accomplish this, mirrored devices previously copied all the data from the operational device to the suspended storage device. However, this may take a significant amount of time to perform, thereby causing a decrease in system resources. One method utilized to address this problem involved the transfer of only the data changed since the mirrored storage device became suspended. This was accomplished with a type of log or map that was maintained to track the data not transferred to the mirror. However, as storage devices became larger, the storage and processing required for maintaining this map has become increasingly larger.

[0005] Therefore, it would be useful to provide an improved system and method for synchronizing data between mirrored subsystems.

[0006] The present invention is directed to a system and method for synchronizing data between mirrored subsystems. In a first aspect of the present invention, a method for storing data includes receiving data suitable for storage to a first storage device and a second storage device, wherein the first storage device and the second storage device are mirrored. A map is created including at least one map entry having an identifier suitable for describing a range of addressable data blocks, wherein the map entry corresponds to a data block modified after operation of the first storage device is suspended. A map is stored including the map entry on the second storage device, wherein the map is suitable for being utilized to restore data stored on the second storage device to at least one of the first storage device and a third storage device.

[0007] In a second aspect of the present invention, a system for storing data includes a first storage device and a second storage device. The first storage device and the second storage device are suitable for receiving and storing data in a mirrored fashion. When operation of the first storage device is suspended, at least one map entry of a map including an identifier suitable for describing a range of addressable data blocks, modified after operation of the first storage device is suspended, is incremented. The map is suitable for being utilized to restore data stored on the second storage device to at least one of the first storage device and a third storage device.

[0008] In a third aspect of the present invention, a method for restoring mirrored data includes creating a map including at least one map entry having an identifier suitable for describing a range of addressable data blocks. In which, the map entry corresponds to a data block modified after operation of a first storage device is suspended. The map is stored, the map including the at least one map entry on a second storage device. At least one of the first storage device and a third storage device are restored. In which, the map stored on the second storage device is suitable for being utilized to restore data stored on the second storage device to at least one of the first storage device and the third storage device. Thus, data on the second storage device is mirrored by at least one of the first storage device and the third storage device.

[0009] It is to be understood that both the forgoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles of the invention.

[0010] The numerous advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which:

[0011] FIG. 1 is a flow chart depicting an exemplary embodiment of the present invention wherein data is restored to a mirrored device utilizing a map including a map entry having an identifier which describes a range of addressable data blocks;

[0012] FIG. 2 is a flow chart illustrating an exemplary embodiment of the present invention wherein modification of data blocks is monitored and a map entry incremented if the data block is not described in a previously stored map entry;

[0013] FIGS. 3A and 3B are block diagrams illustrating the adjustment of a range included in a map entry to modify the granularity of a map; and

[0014] FIG. 4 is an flow chart depicting an exemplary embodiment of the present invention wherein a mirrored storage device is restored based upon a map including a map entry having an identifier which describes a range of addressable data blocks at least one of which was modified after the suspension of a mirrored storage device.

[0015] Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.

[0016] Referring generally now to FIGS. 1 through 4, exemplary embodiments of the present invention are shown. Mirrored storage systems, such as mirrored disk drives and the like, generally include two mirrored sets of data to permit access to the data in the event one of the subsystems fails. However, in the event of failure of one of the subsystems, it is still desirable to restore the system to full operational status to further protect the data from further system failures in the most efficient manner possible.

[0017] Referring now to FIG. 1, an exemplary embodiment 100 of the present invention is shown wherein data is restored to a mirrored device utilizing a map including a map entry having an identifier, which describes a range of addressable data blocks. A mirrored storage device receives data, the mirrored storage device including a first storage device and a second storage device 102. However, operation of the first storage device may be suspended 104, such as due to operational failure, data transfer failure, and the like, thereby preventing the first storage device from receiving and storing data.

[0018] Therefore, a map is created, the map including a map entry having an identifier that describes a range of addressable data blocks 106. The map and the map entries include addressable data blocks that were modified after the suspension of the first storage device. Preferably, the map entry must have enough resolution to include a number from zero to n, wherein n is the number of data or cache blocks in the range. Thus, once operation of the first storage device is re-established 108, the range of addressable data blocks described by the map and the included map entries are copied to the first storage device 110. In this way, the storage system is returned to full operational status, with both the first storage device and the second storage device having the same, updated, mirrored data. Moreover, each map entry of the map may describe a greater amount of modified data in an efficient and space-saving manner.

[0019] Referring now to FIG. 2, an exemplary embodiment 200 of the present invention is shown wherein modification of data blocks is monitored and a map entry incremented if the data block is not described in a previously stored map entry. A mirrored storage system including a first storage device and a second storage device receives data for storage 202. Operation of the first storage device is suspended 204, such as due to hardware or software failure. A map is created including map entries having an identifier which describes a range of addressable data blocks 204, the data blocks modified after suspension of the first storage device. The map may be stored to nonvolatile storage so that the mirror state may be restored following a storage subsystem or other failure. An additional data block is then modified 208. If the additional data block is already in the range described by the map entry 210, the map entry is not incremented 212. However, if the data block is not in the range of data blocks described by the map entry 210, the map entry is incremented 214. Incrementing a map entry may include extending the range described by an existing map entry to include the modified data block 216, creation of a corresponding map entry having an identifier which describes a range of addressable data blocks including the modified data block 218, and the like as contemplated by a person of ordinary skill in the art. Thus, the size of the range may be modified, thereby adjusting the granularity of the map identifiers.

[0020] Referring now to FIGS. 3A and 3B, exemplary embodiments 300 and 350 of the present invention are shown wherein the adjustment of a range included in a map entry to modify the granularity of a map and map entries is described. Modified data blocks are depicted as including an“X”, wherein unmodified data blocks do not contain an“X”. As shown in FIG. 3A, a map may contain a map entry that describes a range of addressable address blocks 302 and 304. By limiting the range described by the identifier, a more precise description of the address blocks that are modified may be achieved and therefore less extraneous information needs to be copied to update the suspended device. However, such a range may result in a larger map, as more map entries are required to describe the modified address blocks. If a larger range is permitted to be described by an identifier included in a map entry 352, fewer map entries are needed, thereby permitting a smaller map to be created and stored. However, it may be more likely that unmodified data blocks are included in the map entry 352 description. Therefore, once the blocks are copied over to the previously suspended storage device, more blocks are transferred than are actually required, thereby requiring greater system resources.

[0021] The granularity of the map and map entries may be adjusted based upon the available resources. For example, a map containing map entries describing smaller ranges may be utilized if there is an abundance of storage space. However, upon approaching the limit of the storage space, the range described by the identifier may be expanded, thereby permitting a greater amount of addressable data block to be described by the map entry, thus, permitting a smaller map to be stored. It should be apparent that the granularity of the map entries of the present inventions may be modified for a variety of reasons and motivations as contemplated by a person of ordinary skill in the art without departing from the spirit and scope of the present invention.

[0022] Additionally, a map may be pre-allocated, such as to increase performance. For example, a map may be made of a fixed size, wherein each map entry describes a range. Therefore, each map entry may be incremented to describe a change to the corresponding range it describes. Further, the map entry may be decremented to described when that range was written successfully to the desired device. The size of the ranges described may be user-specified to permit a user-defined degree of granularity. In this way, a user may specify in which way to allocate a trade off between the range described and therefore the amount of information that must be written to the desired device and the size of the map itself.

[0023] Referring now to FIG. 4, an exemplary embodiment 400 of the present invention is shown wherein a mirrored storage device is restored based upon a map including a map entry having an identifier which describes a range of addressable data blocks at least one of which was modified after the suspension of a mirrored storage device. Data is received for storage to a mirrored storage device including a first storage device and a second storage device 402. Both the first storage device and the second storage device may include a map of the present invention. Operation of the first storage device, in this instance, the primary storage device, is suspended 404. Thus, to provide access to the data, the second storage device becomes the de facto primary device 406. A map is created including a map entry having an identifier, which describes a range of addressable data blocks 408. The map may be pre-allocated before device suspension to increase performance. The map is modified to describe changes to data blocks, such as creating and/or incrementing map entries 410. Thus, once the first storage device is reestablished 412, the range of addressable data blocks described by the map are copied to the first storage device 414. When the block is successfully written to the mirror, the map entry may be decremented. The first storage device may then become the primary storage device again, the map reassigned to the second storage device 416. Thus, the present invention allows fast and efficient mirror re-establish following a device or communications failure. A trade-off of storage size for larger established data blocks results in an efficient, cost effective use of storage device memory.

[0024] Although the invention has been described with a certain degree of particularity, it should be recognized that elements thereof may be altered by persons skilled in the art without departing from the spirit and scope of the invention. One of the embodiments of the invention can be implemented as sets of instructions resident in the memory of one or more information appliances or devices. Until required by the device, the set of instructions may be stored in another readable memory device, for example in a hard disk drive or in a removable memory such as an optical disk for utilization in a CD-ROM drive, a floppy disk for utilization in a floppy disk drive, a floppy/optical disk for utilization in a floppy/optical drive, or a personal computer memory card for utilization in a personal computer card slot. Further, the set of instructions can be stored in the memory of a device and transmitted over a local area network or a wide area network, such as the Internet, when desired by the user. Additionally, the instructions may be transmitted over a network in the form of an applet that is interpreted or compiled after transmission to the computer system rather than prior to transmission. One skilled in the art would appreciate that the physical storage of the sets of instructions or applets physically changes the medium upon which it is stored electrically, magnetically, chemically, physically, optically or holographically so that the medium carries computer readable information.

[0025] In exemplary embodiments, the methods disclosed may be implemented as sets of instructions or software readable by a device. Further, it is understood that the specific order or hierarchy of steps in the methods disclosed are examples of exemplary approaches. Based upon design preferences, it is understood that the specific order, or hierarchy of steps in the method can be rearranged while remaining within the scope of the present invention. The attached method claims present elements of the various steps in a sample order, and are not meant to be limited to the specific order or hierarchy presented.

[0026] It is believed that the system and method for synchronizing data mirrored by storage subsystems of the present invention and many of its attendant advantages will be understood by the forgoing description. It is also believed that it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.