From: Christoph Hellwig <hch@infradead.org>
To: NeilBrown <neilb@suse.de>
Cc: Dominique <dcouot@hotmail.com>,
linux-raid mailing list <linux-raid@vger.kernel.org>
Subject: Re: mdadm - level change from raid 1 to raid 5
Date: Mon, 3 Oct 2011 06:10:45 -0400 [thread overview]
Message-ID: <20111003101045.GA9026@infradead.org> (raw)
In-Reply-To: <20111003210744.45e637d7@notabene.brown>
On Mon, Oct 03, 2011 at 09:07:44PM +1100, NeilBrown wrote:
> The default for chunksize should be 512K I thought..
It is.
> I once saw a mathematical formula, but it was a function of the number of
> concurrent accesses and the average IO size - I think.
>
> Big is good for large streaming requests. Smaller is good for lots of random
> IO. Only way to know for sure is to measure your workload on different sizes.
>
> You can change it once the array is build, but it is a very slow operation as
> it has to move every block on every disk to somewhere else.
>
> mdadm -G /dev/md2 --chunk=32
FYI: For XFS I always get much better results using 32k chunk size, even
for simple streaming reads/writes. I haven't really tracked down why.
Also for any modern system I always have to massively increase the
stripe cache size.
next prev parent reply other threads:[~2011-10-03 10:10 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-09-30 18:31 mdadm - level change from raid 1 to raid 5 Dominique
2011-09-30 22:02 ` NeilBrown
2011-10-02 14:24 ` Dominique
2011-10-02 20:50 ` NeilBrown
2011-10-03 8:53 ` Dominique
2011-10-03 10:07 ` NeilBrown
2011-10-03 10:10 ` Christoph Hellwig [this message]
2011-10-03 10:52 ` Dominique
2011-10-05 1:18 ` NeilBrown
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