On Mar 5, 2004, at 5:57 AM, Jan Grant wrote:
How did you come to this conclusion? For a RAID 5 with a single parity
drive, the reason you zero the disks out completely on initialisation is
to set up the integrity of the parity check. Then any update to any
RAID5 with single parity requires a read of two drives (the target
sector and the corresponding parity drive), an in-memory exclusive or
against the new data, and two writes. Reads and writes can be in
parallel.

You're right, which means I came to my conclusion wrongly, I guess. :-) Part of this was because I was also thinking about how the array behaves after a failure, as you mention next:


The "work" for parity updates only scales linearly with number of disks
if you use a naive parity algorithm. Or, obviously, if a drive fails.


Even using a non-naive :-) algorithm, RAID-5 writes still take somewhat more work than RAID-1 writes do in terms of I/O ops.


--
-Chuck

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