Why specialized disks for composite operations may be unnecessary

Abstract

Disk arrays with erasure coding such as RAID5 and RAID6 incur four and six disk accesses respectively for updating data and check blocks. The small write penalty can be reduced by the Read-Modify-Write (RMW) composite operations to update data and associated check blocks. The Disk Architecture with Composite Operation (DACO) is a proposal to eliminate the disk rotation associated with RMWs, by using a complex read/write head, which allows the writing of a block immediately after reading and modifying it without needing an extra disk rotation. We argue that the extra cost associated with DACO may not be justifiable, because it is not expected to have a significant impact on RAID performance. Furthermore an XOR capability is still required at the disk array controller for reconstructing missing data blocks. A duplexed Nonvolatile Storage (NVS) cache at the disk array controller provides the same reliability as magnetic disks and allows fast writes, i.e., writing to disk is considered completed as soon as data is written onto NVS. Deferring the destaging of data blocks from NVS allows these blocks to be be overwritten, obviating unnecessary disk writes. This also allows neighboring dirty blocks to be destaged in batches, so that a higher disk access efficiency is attained. Disks with multiple arms can also be used to make the processing of RMW requests more efficient, while disks with multiple R/W heads on one arm have little effect on RMW requests. In addition there are alternative methods to update check blocks, such as floating parities, parity logging, the reconstruct write method, log structured arrays, and variable scope parity protection.