Re: Poor Software RAID-0 performance with 2.6.14.2

[Lars Roland <lroland@gmail.com>]

On 11/21/05, Neil Brown <neilb@suse.de> wrote:
> On Monday November 21, lroland@gmail.com wrote:
> > I have created a stripe across two 500Gb disks located on separate IDE
> > channels using:
> >
> > mdadm -Cv /dev/md0 -c32 -n2 -l0 /dev/hdb /dev/hdd
> >
> > the performance is awful on both kernel 2.6.12.5 and 2.6.14.2 (even
> > with hdparm and blockdev tuning), both bonnie++ and hdparm (included
> > below) shows a single disk operating faster than the stripe:
> >
> > ----
> > dkstorage01:~# hdparm -t /dev/md0
> > /dev/md0:
> >  Timing buffered disk reads:  182 MB in  3.01 seconds =  60.47 MB/sec
> >
> > dkstorage02:~# hdparm -t /dev/hdc1
> > /dev/hdc1:
> > Timing buffered disk reads:  184 MB in  3.02 seconds =  60.93 MB/sec
> > ----
>
> Could you try hdparm tests on the two drives in parallel?
>    hdparm -t /dev/hdb & hdparm -t /dev/hdd
>
> It could be that the controller doesn't handle parallel traffic very
> well.
>

hmm I should of cause have thought of this earlier - it does indeed
seam that the controller does not handle parallel traffic very well

-----------
dkstorage01:~# hdparm -t /dev/hdb
/dev/hdb:
 Timing buffered disk reads:  112 MB in  3.02 seconds =  37.09 MB/sec

dkstorage01:~# hdparm -t /dev/hdd
/dev/hdd:
 Timing buffered disk reads:  108 MB in  3.02 seconds =  35.76 MB/sec
-----------

Bonnie test shows the same picture.

> raid0 has essentially 0 cpu overhead.  It would be maybe a couple of
> hundred instructions which would be lost in the noise.  It just
> figures out which drive each request should go to, and directs it
> there.

Yeah so it is properly just a poor controller.


--
Lars Roland
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