>>> On Tue, 25 Dec 2007 19:08:15 +0000,
>>> pg_lxra@xxxxxxxxxxxxxxxxxxx (Peter Grandi) said:
[ ... ]
>> It's the raid10,f2 *read* performance in degraded mode that is
>> strange - I get almost exactly 50% of the non-degraded mode
>> read performance. Why is that?
> [ ... ] the mirror blocks have to be read from the inner
> cylinders of the next disk, which are usually a lot slower
> than the outer ones. [ ... ]
Just to be complete there is of course the other issue that
affect sustained writes too, which is extra seeks. If disk B
fails the situation becomes:
DISK
A X C D
1 X 3 4
. . . .
. . . .
. . . .
-------
4 X 2 3
. . . .
. . . .
. . . .
Not only must block 2 be read from an inner cylinder, but to
read block 3 there must be a seek to an outer cylinder on the
same disk. Which is the same well known issue when doing
sustained writes with RAID10 'f2'.
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