Re: Linux Raid performance

[Mark Knecht <markknecht@gmail.com>]

On Fri, Apr 2, 2010 at 6:14 PM, Richard Scobie <richard@sauce.co.nz> wrote:
> Mark Knecht wrote:
>
>> Once all of that is in place then possibly more cores will help, but I
>> suspect even then it probably hard to use 4 billion CPU cycles/second
>> doing nothing but disk I/O. SATA controllers are all doing DMA so CPU
>> overhead is relatively *very* low.
>
> There is the RAID5/6 parity calculations to be considered on writes and this
> appears to be single threaded. There is an experimental multicore kernel
> option I believe, but recent discussion indicates there may be some problems
> with it.
>
> A very quick test on a box here on a Xeon E5440 (4 x 2.8GHz) and a SAS
> attached 16 x 750GB SATA md RAID6. The array is 72% full and probably quite
> fragmented and currently the system is idle.
>
> dd if=/dev/zero of=/mnt/storage/dump bs=1M count=20000
> 20000+0 records in
> 20000+0 records out
> 20971520000 bytes (21 GB) copied, 87.2374 s, 240 MB/s
>
> Looking at the outputs of vmstat 5 and mpstat -P ALL 5 during this, one core
> (probably doing parity generation) was around 7.56% idle and the other 3
> were around 88.5, 67.5 and 51.8% idle.
>
> The same test run when the system was commissioned and the array was empty,
> acheived 565MB/s writes.
>
> Regards,
>
> Richard

Richard,
   Good point. I was limited in my thinking to the sorts of arrays I
might use at home being no wider than 3, 4 or 5 disks. However for our
N-wide array as N approaches infinity so do the cycles required to run
it. I don think that applies to the OP but I don't know that.

   Thanks for making the point.

Cheers,
Mark