Re: Disappointing RAID10 Performance

[Tomasz Chmielewski <mangoo@wpkg.org>]

adfas asd wrote:
> I was hoping to get better performance with RAID10 than from the raw disks, but that's turned out to not be the case.  Experimenting with the readahead buffer I get these bandwidths with the following command:
> # time dd if={somelarge}.iso of=/dev/null bs={readahead size}
> 
> /dev/sd?
>  1024 71.3 MB/s
>  2048 71.2 MB/s
>  4096 77.7 MB/s
>  8192 69.4 MB/s
>  16384 76.6 MB/s
> 
> /dev/md2
>  1024  67.1
>  2048  69.1
>  4096  75.7
>  8192  64.9
>  16384 69.0
> 
> Using RAID10offset2 on 2 WD 2TB drives, and always the same input file.
> 
> Why would RAID10 performance be -poorer-? 

If you only use your RAID-10 array for a single "dd if=bigfile 
of=/dev/null" then yes, it does not give you much over mirroring.

If you start using your drives for two "dd if=bigfile[12] of=/dev/null" 
at the same time, you will notice the difference.



-- 
Tomasz Chmielewski
http://wpkg.org