The Architecture Of A Fault-tolerant Cached RAID Controller

RAID-5 arrays need 4 disk accesses to update a data block—2 to read old data and parity, and 2 to write new data and parity. Schemes previously proposed to improve the update performance of such arrays are the Log-Structured File System [10] and the Floating Parity Approach [6]. Here, we consider a third approach, called Fast Write, which eliminates disk time from the host response time to a write, by using a Non-Volatile Cache in the disk array controller. We examine three alternatives for handling Fast Writes and describe a hierarchy of destage algorithms with increasing robustness to failures. These destage algorithms are compared against those that would be used by a disk controller employing mirroring. We show that array controllers require considerably more (2 to 3 times more) bus bandwidth and memory bandwidth than do disk controllers that employ mirroring. So, array controllers that use parity are likely to be more expensive than controllers that do mirroring, though mirroring is more expensive when both controllers and disks are considered.