Electrical computers and digital processing systems: memory – Storage accessing and control – Specific memory composition
Reexamination Certificate
1998-10-19
2001-12-18
Bragdon, Reginald G. (Department: 2751)
Electrical computers and digital processing systems: memory
Storage accessing and control
Specific memory composition
C711S004000, C711S203000
Reexamination Certificate
active
06332177
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to data storage subsystems and, more particularly, to block mapping methods and associated structures for mirroring N images of a block of data across M drives to more effectively combine mirroring and non-mirroring LUNs within a group of drives in an array storage subsystem.
2. Discussion of Related Art
Typical computing systems store data within devices such as hard disk drives, floppy drives, tape, compact disk, etc. These devices are otherwise known as storage devices. The storage capacity of these storage devices has rapidly increased as computing applications' demand for storage have increased. Simultaneous with the increase in capacity, computer applications and users have demanded increased performance. Computing applications have become highly dependent on high performance, high capacity storage devices. However, such increased dependency on storage devices underscores the need for increased reliability of such storage subsystems. Failure of such high capacity, high performance storage devices and subsystems can cripple vital computing applications.
Disk array storage systems provide both improved capacity and performance as compared to single disk devices. In a disk array storage system, a plurality of disk drives are used in a cooperative manner such that multiple disk drives are performing, in parallel, the tasks normally performed by a single disk drive. Striping techniques are often used to spread large amounts of information over a plurality of disk drives in a disk array storage system. So spreading the data over multiple disk drives improves perceived performance of the storage system in that a large I/O operation is processed by multiple disk drives in parallel rather than being queued awaiting processing by a single disk drive.
However, adding multiple disk drives to a storage system reduces the reliability of the overall storage system. In particular, spreading data over multiple disk drives in a disk array increases the potential for system failure. Failure of any of the multiple disk drives translates to failure of the storage system because the data stored thereon cannot be correctly retrieved.
RAID techniques are commonly used to improve reliability in disk array storage systems. RAID techniques generally configure multiple disk drives in a disk array in geometries that permit redundancy of stored data to assure data integrity in case of various failures. In many such redundant subsystems, recovery from many common failures can be automated within the storage subsystem itself due to the use of data redundancy, error codes, and so-called “hot spares” (extra disk drives which may be activated to replace a failed, previously active disk drive). The 1987 publication by David A. Patterson, et al., from University of California at Berkeley entitled
A Case for Redundant Arrays of Inexpensive Disks
(
RAID
), reviews the fundamental concepts of RAID technology.
RAID level zero, also commonly referred to as disk striping, distributes data stored on a storage subsystem across a plurality of disk drives to permit parallel operation of a plurality of disk drives thereby improving the performance of I/O write requests to the storage subsystem. Though RAID level zero functionality improves I/O write operation performance, reliability of the disk array subsystem is decreased as compared to that of a single large disk drive. To improve reliability of disk arrays, other RAID geometries for data storage include generation and storage of redundancy information to permit continued operation of the disk array through certain common failure modes of the disk drives in the disk array.
There are six other “levels” of standard RAID geometries which include redundancy information as defined in the Patterson publication. Other RAID geometries have been more recently adopted and utilize similar concepts. For example, RAID level six provides additional redundancy to enable continued operation even in the case of failure of two disk drives in a disk array.
The simplest array, a RAID level 1 system, comprises one or more disks for storing data and an equal number of additional “mirror” disks for storing copies of the information written to the data disks. The remaining RAID levels, identified as RAID levels 2, 3, 4 and 5 systems by Patterson, segment the data into portions for storage across several data disks. One or more additional disks are utilized to store error check or parity information. RAID level 6 further enhances reliability by adding additional redundancy information to permit continued operation through multiple disk failures. The methods of the present invention may be useful in conjunction with any of the standard RAID levels including level 0.
RAID storage subsystems typically utilize a control module (controller) that shields the user or host system from the details of managing the redundant array. The controller makes the subsystem appear to the host computer as one (or more), highly reliable, high capacity disk drive. In fact, the RAID controller may distribute the host computer system supplied data across a plurality of the small independent drives with redundancy and error checking information so as to improve subsystem reliability. The mapping of a logical location of the host supplied data to a physical location on the array of disk drives is performed by the controller in a manner that is transparent to the host system. RAID level 0 striping for example is transparent to the host system. The data is simply distributed by the controller over a plurality of disks in the disk array to improve overall system performance.
To further improve performance. RAID subsystems frequently provide large cache memory structures. The cache memory is associated with the control module such that the storage blocks on the disk array are mapped to blocks in the cache. This mapping is also transparent to the host system. The host system simply requests blocks of data to be read or written and the RAID controller manipulates the disk array and cache memory as required.
A RAID level 1 disk array storage system, as presently known in the art, comprises one or more disks for storing data and an equal number of additional duplicate or mirror disks for storing copies of the information written to the data disks.
RAID level 1 techniques as presently practiced generally use a pair of disk drives for mirroring of data. A first disk drive is used to store the original data and the second is used to mirror the data. Striping of data (also known as RAID level 0), as noted above, is often used to improve performance. It is therefore known in prior techniques to combine striping (RAID level 0) with mirroring (RAID level 1) to provide improved performance and improved reliability through redundancy.
Whether striped or non-striped, RAID level 1 as presently practiced utilizes a single duplicate copy of the data—a single mirror. Some present striped RAID level 1 implementations allow for an odd number of disk drives to be used in the RAID level 1 mirroring. However, such storage device still maintain only a single mirror copy of the stored data. Each striped block has a single mirrored block associated therewith. The striping may distribute the original and mirrored block over any number of disk drives so long as the original block and mirrored block are resident on different disk drives to assure the needed reliability.
RAID level 1 improves reliability as compared to RAID level 0 by adding the capability to mirror or copy data on duplicate disk drives. RAID level 0, also known as disk striping, distributes data across the disks in the array but does not provide any data redundancy.
RAID levels 2-6 utilize RAID level 0 striping techniques to distribute the data over the plurality of disk drives in the array. RAID levels 2-6 use redundancy information which requires less storage capacity as compared to RAID level 1 mirroring. Specifically, redundancy information such as bitwise parity exclusive-OR (XOR) computation
Bragdon Reginald G.
LSI Logic Corporation
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