Comment on "DACO: A High Performance Disk Architecture”

DACO is a proposal to provide disk head separation so that a Read-Modify-Write to update check blocks in RAID arrays with erasure encoding can be processed with a minimal rotational delay. Aside from the difficulty associated with implementation of such R/W heads, there are various reasons why such a disk architecture which runs contrary to the write verify SCSI (read after write) command is not highly desirable. DACO is not utilized by the Disk Array Controller in XORing data from multiple disks and processing updates to prevent write holes. We discuss alternate methods to cope with the small write penalty and quantify the effect of DACO on performance via a simple queueing analysis.