Patent Application
20060206542
The present invention relates generally to transferring data over a data link and, in particular, to applying a differencing algorithm to reduce data link bandwidth usage during data transfers.
Remote copying of data is an integral part of disaster recovery for protecting critical data from loss and providing continuous data availability. In a disaster recovery support system, data write updates to a primary or central data store are reproduced at a secondary, remote site. The remote site is typically located at a distance from the primary data store if protection from natural disasters is a concern, but may be adjacent to the primary site if equipment failure is the main concern. In the event of a failure at the primary data store, the remote site can take over all operations, including data write updates, with confidence that no data has been lost. Later, after repair, the primary data store can be restored to the condition of the remote site and can resume all operations, including data write operations.
During remote copying, typically same-sized blocks of data are sent from the primary data store to the remote data site. In this way, data write updates at the primary data store are reproduced at the remote site so as to permit reconstruction of the data, including reconstruction of the exact sequence of data write updates that took place at the primary data store. This reproducibility can be especially important, for example, in a banking system or other transaction log system. Thus, data write updates at the primary data store are collected and are periodically sent to the remote site in a remote copy operation.
The various types of remote copy can require enormous amounts of bandwidth over the data lines between the primary data store and the remote site controller. For example, if a primary data store controller can support 20,000 input/output (I/O) operations per second, and if 50% of these operations are write operations, then the controller can handle 10,000 write operations per second. If each write update involves 4 K bytes, then bandwidth of 40 MB per second is required between the primary controller and the remote site controller. This is a significant amount of bandwidth to provide, given currently available pricing for data lines. Even though asynchronous remote copy can speed up write updates, it does not decrease the amount of bandwidth required.
One proposed system which addresses the issue of bandwidth usage is presented in U.S. Pat. No. 6,327,671 entitled DELTA COMPRESSED ASYNCHRONOUS REMOTE COPY and assigned to the assignee of the present application (“the '671 patent”), which patent is incorporated herein by reference in its entirety. As illustrated in
The present invention provides methods and apparatus for copying data from a primary storage facility to a secondary storage facility which reduce the workload on the storage controller in the primary facility and minimize bandwidth usage. The primary storage facility includes a primary data replication appliance which transfers data to a secondary replication appliance. Updated data from a host is both stored through a storage controller in the primary facility and also received by the primary replication appliance. Logic in the primary replication appliance determines whether the immediately previous version of the data is in a buffer from a prior storage operation. If so, the current (updated) version of the data is compared with the previous version and the difference, such as calculated through a bit-wise exclusive-OR operation, is compressed and transferred to the secondary replication appliance. The process is reversed in the secondary replication appliance and the recreated updated version of the data stored in the secondary facility.
While the primary appliance is performing these operations, the storage controller may be performing other tasks, including allowing access by the host. Therefore, the operations of the present invention, being performed in the primary replication appliance, do not adversely affect the workload of the storage controller or of the host. Additionally, because the difference between the previous version of the data and the current version, representing only bits which have changed, is typically highly compressible, when it is compressed and transmitted to the secondary replication appliance, bandwidth usage is reduced.
Other features and advantages of the present invention should be apparent from the following description of the preferred embodiments, which illustrates, by way of example, the principles of the invention.
The primary and secondary replication appliances 210, 260 both include: a processor 222, 262; a memory 224, 264 for storage of software instructions to be executed on the processor 222, 262; disk storage 226, 266; and a first-in-first-out (FIFO) buffer 228, 268 for temporary storage of data. The appliances 220, 260 also include disk storage 228, 268. Logic 230, 270 for performing the operations of the present invention may reside in the memory 224, 264, in firmware or in a separate processor.
Referring to the flowchart of
If the previous version of the data being updated is still present, the logic 230 directs that it be compared with the current (updated) version (step 320). Because of the easy availability and reversibility of an exclusive-OR (XOR) function, preferably a bit-wise XOR operation is performed on the two sets of data to calculate a “difference” (step 322). Other mathematical transformations, or sets of complementary transformations, may be substituted for the XOR operation. An appropriate other set of two transformations includes a first transformation T1 which is performed on the current version of the data and the immediately previous version of the data to generate a new version D. A second, complementary, transformation T2 is subsequently applied to the new version D and the immediately previous version to generate (recreate) the current version. A further property of the second transformation T2 is that, when it is applied to the new version D and the current version, the immediately previous version should be generated (recreated). When the bit-wise XOR operation is used, both transformations T1 and T2 are the same. As used herein, the term “difference” will refer to the new version D which is the result of any such transformation T1.
Once generated or calculated, the difference is compressed (step 324) to reduce its size. Subsequently, the compressed difference is transferred to the secondary replication appliance 260 (step 328). While the primary appliance 220 is performing these operations, the primary storage controller 212 may be performing other tasks, including allowing access by the host 204. Therefore, the operations of the present invention, being performed in the replication appliance 220, do not adversely affect the workload of the primary storage controller 212 or of the host 204. Additionally, because the typically highly compressible difference between the previous version of the data and the current version, representing only bits which have changed, is compressed and transmitted to the secondary replication appliance 260, bandwidth usage is reduced.
At the direction of the logic 230, the difference may be encrypted to improve security before being transferred (step 326). If the previous version of the data was not still in the buffer 226 and the unprocessed current version is to be transferred to the secondary appliance 260, it too may be compressed, encrypted or both before being transferred.
Referring to the flowchart of
If the previous version of the data is not still in the buffer 266 of the secondary replication appliance 260, it must be retrieved from the secondary storage 254 (step 414) before it can be compared with the difference (step 408) to recreate the current version (step 410) and be stored (step 412). If the secondary appliance employs a read_before_write function when storing data, the previous version of the data will first be read from the storage 254 and temporarily stored in a nonvolatile memory, which may include the buffer 266. Only after the previous version is hardened in this fashion will the recreated current version overwrite the previous version in the secondary storage 254. Thus, if the overwrite fails, the previous version is still available for recovery. Consequently, no extra step is required by the secondary appliance 260 to obtain the previous version of the data before recreating the current version.
In addition to the savings in bandwidth, the present invention beneficially permits the development and deployment of one logic to direct the process described herein, regardless of what type of storage controller the replication appliance is used with.
It is important to note that while the present invention has been described in the context of a fully functioning system, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms and that the present invention applies regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media such as a floppy disk, a hard disk drive, a RAM, and CD-ROMs and transmission-type media such as digital and analog communication links.
The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. Moreover, although described above with respect to methods and systems, the need in the art may also be met with a computer program product containing instructions for copying updated data from a primary storage facility to a secondary storage facility or a method for deploying computing infrastructure comprising integrating computer readable code into a computing system for performing the same.
