Data and parity prefetching for redundant arrays of disk drives

DETAILED DESCRIPTION The present invention provides a system which improves the I/O performance of a computer system including a central processor unit or "host", an array of disk drives and a memory buffer cache.
The disk drives store data in stripes, each stripe includes a plurality of data blocks and a parity block for storing parity data generated from the data blocks.
The cache stores data read from, and data to be written to the disk drives.
The cache is organized into blocks compatible with the block structure of the disk drives.
The blocks in cache are managed in a least recently used (LRU) manner.
A low and a high threshold signal are provided to indicate the relative fullness of the cache.
The system for prefetching data blocks and parity blocks uses the low and high threshold signals to determine whether or not to prefetch the data and the parity blocks for blocks to be written.
If the low threshold signal is detected, a prefetch procedure is started to read the blocks of a stripe necessary to generate parity data.
That is the data blocks stored in cache are traversed in a LRU order, and for each data block in cache to be written to the disk drive, the block to be overwritten and the corresponding parity block of the stripe are read into cache.
Each block which has been prefetched is marked as such.
Prefetching continues, until the high threshold signal is detected.
The high threshold signal causes the blocks in cache to be written to the disk drives in LRU order, until sufficient blocks have been written to disk to disable the low threshold signal.