[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Old Index]
Re: Help with low raid5 performance
On Mon, 15 Nov 2010 21:07:07 +0100
Peter Kerwien <peter%kerwien.homeip.net@localhost> wrote:
> On 11/15/2010 08:50 PM, Thor Lancelot Simon wrote:
> > On Mon, Nov 15, 2010 at 06:40:52PM +0100, Peter Kerwien wrote:
> >> Interesting! So just moving the wedge a whole stripe was the trick?
> > You really want filesystem data blocks to be stripe-aligned. Play
> > with it a little and see how it works for you.
> > Thor
> Thanks, but I don't want to play with it. I would like to understand
> what I perhaps did wrong, and how to correct it.
Sorry that I'm chiming in a bit late at this point, after Ian
Clark already pointed out what is most likely the culprit...
In your initial config:
> START layout
> 64 1 1 5
this says that the stripe width is 64 blocks... With 2 data blocks and
1 parity block in each stripe, that gives you a total of 128 blocks of
data in a stripe.
When you did this:
> # gpt show raid0
> 64 1953546015 1 GPT part - NetBSD UFS/UFS2
You basically aligned the partition on the half-stripe which, I
believe, ends up in having a whole bunch of the filesystem aligned on
half-stripes. E.g. every 64K write you do ends up straddling two stripes,
causing the read-modify-write small-write penalty for every 64K write.
If you re-align that partition to be 128 blocks from the start of the
RAID set, it should perform significantly better.
I hope this helps some, and thanks to Ian Clark for catching the issue
that I'd missed when I first glanced at your post.
Main Index |
Thread Index |