Reading from ZFS mirror 2x slower than expected?
Steve Polyack
korvus at comcast.net
Fri Jan 29 16:26:10 UTC 2010
On 01/28/10 14:54, Bob Friesenhahn wrote:
> On Thu, 28 Jan 2010, Urmas Lett wrote:
>
>> While reading from simple zfs mirror 'dd if=zfs.test of=/dev/null
>> bs=512k' systat -vm shows individual disks only 50% busy:
>
> How big is the file 'zfs.test'? How was it created? If it was
> created from /dev/zero then its content is bogus. You need to test
> with non-zero data.
>
> It may be that zfs's prefetch is not working adequately. With perfect
> prefetch, the read rate should be doubled.
>
I can also vouch that the mirror performance is less than stellar.
Writes are also about 50% slower than single-disk performance, although
this may be expected due to checksums / other ZFS overhead. I can try
to provide more details when I get time.
Steve
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