[OpenIndiana-discuss] RAIDZ performance

[Reginald Beardsley]

Wow!  I'm also deeply embarrassed for not having looked at the source myself before posting.  I should have.

FWIW A 10x performance hit for double parity instead of single parity is probably a code tuning or algorithm issue.

Have Fun!
Reg

--- On Thu, 2/21/13, Sašo Kiselkov <skiselkov.ml at gmail.com> wrote:

> From: Sašo Kiselkov <skiselkov.ml at gmail.com>
> Subject: Re: [OpenIndiana-discuss] RAIDZ performance
> To: openindiana-discuss at openindiana.org
> Date: Thursday, February 21, 2013, 2:08 PM
> On 02/21/2013 07:27 PM, Timothy
> Coalson wrote:
> > I think last time this was asked, the consensus was
> that the implementation
> > was based on linear feedback shift registers and xor,
> which happens to be a
> > reed-solomon code (not as clear on this part, but what
> matters is what it
> > is, not what it isn't).  Regardless, from reading
> the source previously, I
> > am fairly sure it operates bytewise, with xor for first
> syndrome (parity),
> > and LFSR and then xor for the other syndromes.
> > 
> > See
> > http://openindiana.org/pipermail/openindiana-discuss/2012-October/010419.html
> 
> I tore out the parity calculations for raidz1 and raidz2
> (attached) from
> vdev_raidz.c and here are the results:
> 
>  ("5 1 32 1000000" below means 1000000 iterations over a
> 5-drive
>   raidz-1 at 32k per data drive; 4 data drives * 32k =
> 128k block)
> $ for ((I=0; $I < 2 ; I=$I + 1 )); do time ./raidz_test 5
> 1 32 1000000 &
> done
> real    0m32.045s
> user    0m32.336s
> sys     0m0.015s
> 
> real    0m32.372s
> user    0m32.486s
> sys     0m0.017s
> 
> So combined raidz1 throughput is:
> 128 * 1024 * 1000000 / 2^30 / 32 * 2 = 7.6293 GB/s
> 
>  ("4 2 64 1000000" below means 1000000 iterations over a
> 4-drive
>   raidz-2 at 64k per data drive; 2 data drives * 64k =
> 128k block)
> RAIDZ2:
> for ((I=0; $I < 2 ; I=$I + 1 )); do time ./raidz_test 4 2
> 64 1000000 & done
> real    3m3.040s
> user    3m0.920s
> sys     0m0.078s
> 
> real    3m3.082s
> user    3m1.092s
> sys     0m0.058s
> 
> So combined raidz2 throughput is:
> 128 * 1024 * 1000000 / 2^30 / 183 * 2 = 1.3341 GB/s
> 
> Next comes the factor of reduced data spindle count. A
> 4-drive raidz1
> will contain 3 data spindles, while a 4-drive raidz2 will
> only contain 2
> data spindles. Fewer spindles = less raw throughput.
> 
> I think we can thus conclude that the performance drop
> Reginald is
> seeing is entirely expected.
> 
> Cheers,
> --
> Saso
> 
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