On Jan 4, 2007, at 3:25 AM, [EMAIL PROTECTED] wrote:
Is there some reason why a small read on a raidz2 is not
statistically very
likely to require I/O on only one device? Assuming a non-degraded
pool of
course.
ZFS stores its checksums for RAIDZ/RAIDZ2 in such a way that all
disks must be read to compute and
verify the checksum.
But why do ZFS reads require the computation of the RAIDZ checksum?
If the block checksum is fine, then you need not care about the
parity.
It's the block checksum that requires reading all of the disks. If
ZFS stored sub-block checksums
for the RAID-Z case then short reads could often be satisfied without
reading the whole block (and
all disks).
So actually I mis-spoke slightly; rather than "all disks", I should
have said "all data disks."
In practice this has the same effect: No more than one read may be
processed at a time.
Anton
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