Re: mdadm raid1 read performance

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On Thu, May 05, 2011 at 09:45:38AM +1000, NeilBrown wrote:
> On Thu, 5 May 2011 00:08:59 +0100 Liam Kurmos <quantum.leaf@xxxxxxxxx> wrote:
> 
> > as a separate question, what should be the theoretical performance of raid5?
> 
> x(N-1)
> 
> So a 4 drive RAID5 should read at 3 time the speed of a single drive.

Actually, theoretically, it should be more than that for reading, more like N minus
some overhead. In a raid5 stripe of 4 disks, when reading you do not read
the checksum block, and thus you should be able to have all 4 drives
occupied with reading real data. Some benchmarks back this up, 
http://home.comcast.net/~jpiszcz/20080329-raid/
http://blog.jamponi.net/2008/07/raid56-and-10-benchmarks-on-26255_10.html
The latter reports a 3.44 times performance for raid5 reads with 4
disks, significantly over the N-1 = 3.0 mark.

For writing, you are correct with the N-1 formular.

best regards
keld
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