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Date: Mon, 5 Oct 2015 13:51:01 +0500
From: Roman Mamedov <rm@romanrm.net>
To: Erkki Seppala <flux-btrfs@inside.org>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: BTRFS as image store for KVM?
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On Mon, 05 Oct 2015 11:43:18 +0300
Erkki Seppala <flux-btrfs@inside.org> wrote:

> Lionel Bouton <lionel-subscription@bouton.name> writes:
>=20
> > 1/ AFAIK the kernel md RAID1 code behaves the same (last time I checked
> > you need 2 processes to read from 2 devices at once) and I've never seen
> > anyone arguing that the current md code is unstable.
>=20
> This indeed seems to be the case on my MD RAID1 HDD.

The key difference is that MD is smart enough to distribute reads to the le=
ast
loaded devices in a RAID (i.e. two processes reading from a two-HDD RAID1 w=
ill
~always use different HDDs, no matter what their PIDs are).

Another example of the differences was that in my limited testing I saw Btr=
fs
submit writes sequentially on RAID1: according to 'iostat', there were long
periods of only one drive writing, and then only the other one. The result =
was
the RAID's write performance ended up being lower than that of a single dri=
ve.

--=20
With respect,
Roman

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