Date: Fri, 29 Jan 2010 11:10:55 -0500 From: Steve Polyack <korvus@comcast.net> To: Bob Friesenhahn <bfriesen@simple.dallas.tx.us> Cc: freebsd-fs@freebsd.org Subject: Re: Reading from ZFS mirror 2x slower than expected? Message-ID: <4B63088F.9030407@comcast.net> In-Reply-To: <alpine.GSO.2.01.1001281350210.17824@freddy.simplesystems.org> References: <4B619485.8080106@eenet.ee> <alpine.GSO.2.01.1001281350210.17824@freddy.simplesystems.org>
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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
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