On Wed, 22 Jul 2009, Roch wrote:
HI Bob did you consider running the 2 runs with
echo zfs_prefetch_disable/W0t1 | mdb -kw
and see if performance is constant between the 2 runs (and low).
That would help clear the cause a bit. Sorry, I'd do it for
you but since you have the setup etc...
Revert with :
echo zfs_prefetch_disable/W0t0 | mdb -kw
-r
I see that if I update my test script so that prefetch is disabled
before the first cpio is executed, the read performance of the first
cpio reported by 'zpool iostat' is similar to what has been normal for
the second cpio case (i.e. 32MB/second). This seems to indicate that
prefetch is entirely disabled if the file has ever been read before.
However, there is a new wrinkle in that the second cpio completes
twice as fast with prefetch disabled even though 'zpool iostat'
indicates the same consistent throughput. The difference goes away if
I tripple the number of files.
With 3000 8.2MB files:
Doing initial (unmount/mount) 'cpio -C 131072 -o > /dev/null'
14443520 blocks
real 3m41.61s
user 0m0.44s
sys 0m8.12s
Doing second 'cpio -C 131072 -o > /dev/null'
14443520 blocks
real 1m50.12s
user 0m0.42s
sys 0m7.21s
Now if I increase the number of files to 9000 8.2MB files:
Doing initial (unmount/mount) 'cpio -C 131072 -o > /dev/null'
144000768 blocks
real 35m51.47s
user 0m4.46s
sys 1m20.11s
Doing second 'cpio -C 131072 -o > /dev/null'
144000768 blocks
real 35m22.41s
user 0m4.40s
sys 1m14.22s
Notice that with 3X the files, the throughput is dramatically reduced
and the time is the same for both cases.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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