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Date: Fri, 29 Jan 2010 11:10:55 -0500
From: Steve Polyack <korvus@comcast.net>
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To: Bob Friesenhahn <bfriesen@simple.dallas.tx.us>
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Cc: freebsd-fs@freebsd.org
Subject: Re: Reading from ZFS mirror 2x slower than expected?
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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