Date: Wed, 27 May 2009 04:59:00 +1000 From: peterjeremy@optushome.com.au To: Artis Caune <artis.caune@gmail.com> Cc: Randy Bush <randy@psg.com>, freebsd-fs <freebsd-fs@freebsd.org> Subject: Re: raidz2 a bit big Message-ID: <20090526185900.GA98171@server.vk2pj.dyndns.org> In-Reply-To: <9e20d71e0905230537ibcaf852g1dc32b6ffc3a681d@mail.gmail.com> References: <m28wkon90c.wl%randy@psg.com> <9e20d71e0905230537ibcaf852g1dc32b6ffc3a681d@mail.gmail.com>
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On 2009-May-23 15:37:14 +0300, Artis Caune <artis.caune@gmail.com> wrote:
>2009/5/23 Randy Bush <randy@psg.com>:
>> a dozen 2tb drives in a raidz2
>Reads on such configurations are very slow.
Not really. Assuming each disk is capable of X IOPS and assuming
non-degraded mode, you can still issue (N-2)*X random reads/sec
because the parity stripes are not needed/used. (Compared to N*X
random reads/sec for a mirrored configuration). Degraded reads _are_
very slow because you need to read most of the spindles (I'm not sure
of the exact recovery mechanism for RAIDZ2 but it's probably close to
X random reads/sec).
Write performance _is_ poor - a write requires at least 2 (and maybe
3) physical reads and 3 physical writes. But you can still perform
multiple parallel writes across the RAIDZ2 set, giving you roughly
N*X/6 random writes/sec. (Compared to N*X/2 random writes/sec for
a mirrored configuration).
In general, you would not choose a RAIDZ{1,2} configuration where
write performance was an issue - the benefit you get is greater
storage utilisation.
>with 4 disks in raidz2 (total 3 raidz2 vdevs) you get 300 IOPS
Actually 600 reads/sec if all the sets are non-degraded. Unless you
are particularly worried by dual disk failures, you would probably be
better off using a 2+2 disk mirror configuration, rather than a 4-disk
RAIDZ2 setup.
--=20
Peter Jeremy
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