Re: BTRFS as image store for KVM?

[Roman Mamedov <rm@romanrm.net>]

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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:
> 
> > 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.
> 
> 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 least
loaded devices in a RAID (i.e. two processes reading from a two-HDD RAID1 will
~always use different HDDs, no matter what their PIDs are).

Another example of the differences was that in my limited testing I saw Btrfs
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 drive.

-- 
With respect,
Roman

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