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Is RAID just one Big Disk?

Intuitively, the reader may wish to equate the destage pointer in WOW to a disk head. It is as if WOW or CSCAN are simulating the position of the disk head, and destaging accordingly. In practice, this intuition is not strictly correct since (i) concurrent read misses may be happening which can take the disk heads to arbitrary locations on disks; and (ii) the position of the heads cannot be strictly controlled, for example, due to read-modify-write in RAID-5; and (iii) at a lower level, either the RAID controller or the individual disks may re-order concurrent write requests. In view of these limitations, the purpose of WOW or CSCAN is to spatially localize the disk heads to a relatively narrow region on the disks with the idea that the resulting disk seeks will be less expensive than random disk seeks which may move the head across a larger number of cylinders on the disks. In practice, we have seen that these observations indeed hold true.

Binny Gill 2005-10-17