Electrical computers and digital processing systems: memory – Storage accessing and control – Specific memory composition
Reexamination Certificate
1998-05-22
2001-01-09
Robertson, David L. (Department: 2759)
Electrical computers and digital processing systems: memory
Storage accessing and control
Specific memory composition
Reexamination Certificate
active
06173361
ABSTRACT:
This invention relates generally to a disk control device for disk array systems and more particularly, to a disk control device adapted to be used with RAID-4 and RAID-5 disk systems for increasing the data access speed by reducing the number of accesses by the disk control device to the disk devices.
BACKGROUND OF THE INVENTION
Disk array systems devices which allow great volumes of data to be transferred in and out and from many disks at high transfer speed and with relatively good reliability have been in use for many years. These disk array systems were classified as Redundant Arrays of Inexpensive Disks (RAID) 1 to 5 by Dr. David A. Patterson et al. of the University of California (see ACM SIGMOD Conference, Chicago, Ill., Jun. 1-3, 1988, P.109-166). The present invention relates primarily to RAID-4 and RAID-5, which will now be discussed.
It is known in the art that the reliability of the RAID-4 and the RAID-5 systems is provided by a parity or redundant data which is created from the combination of data stored in the same memory block location of each of the disk devices. The data which create the parity data are formed into a group called a parity group. The parity data of each parity group is stored in each of the disk devices in the array and are included in the parity group.
One of the characteristics of the RAID-4 and the RAID-5 systems is that each time data of a particular parity group is altered, the parity data of that parity group is recalculated and updated so that the redundancy of the parity data is maintained. This requires the disk control device to not only access the memory block of a disk containing the desired data, but also the memory block of the disk containing the corresponding parity data, each time the data is accessed from the disk array. Consequently, this causes a delay in operation, since the disk control device for the RAID-4 and the RAID-5 systems must always access an additional memory block each time data is accessed. In other words, the inherent characteristics of the RAID-4 and the RAID-5 systems result in reduced access speed during the reading and writing processes. Thus, there is a need for a disk control device for the RAID-4 and the RAID-5 systems which is able to increase the access speed.
Accordingly, it is one object of this invention to provide a new and improved disk control device for the RAID-4 and the RAID-5 systems.
Another object is to provide a new and improved disk control device having increased access speed for use with the RAID-4 and the RAID-5 systems.
Yet another object of this invention is to provide a new and improved disk control device for the RAID-4 and the RAID-5 systems which does not access the parity data each time the nonparity data is accessed.
Still another object of this invention is to provide a new and improved disk control device for the RAID-4 and the RAID-5 systems which holds the parity data in a cache until all the nonparity data of the corresponding parity group are transferred out of the cache.
BRIEF SUMMARY OF THE INVENTION
In keeping with one aspect of this invention, a disk control device is operationally connected to a high-order device and operates a plurality of disk devices. A cache is included in the disk control device for storing data including a plurality of parity data from a corresponding plurality of parity groups in the plurality of disk devices. The disk control device also includes a cache management table for indicating the positions of the data stored in the cache. Cache management means is provided for transferring a selected data request by the host control from the plurality of disk devices to the cache when the cache management table indicates that said selected data is not stored in the cache. Control means transfers the selected data from the cache to the high-order device when the cache management means informs the control means that the selected data is stored in the cache. Moreover, the cache management means holds selected parity data in the cache when data belonging to the same one of the plurality of parity groups as the selected parity data are stored in the cache, and expels the selected parity data from the cache when no data belonging to the same one of the plurality of parity groups as the selected parity data is stored in the cache.
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Mishra et al., Performance Study of RAID-5 Disk Arrays with Data and Parity Cache, 1996 Internation Conference on Parallel Processing, 1996, pp. 1-222 through 1-229.*
Fujitsu Limited
Greer Burns & Crain Ltd.
Robertson David L.
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