linux-kernel - Re: BTRFS: Unbelievably slow with kvm/qemu

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Message-ID: <4C7AB645.5090804@wpkg.org>
Date:	Sun, 29 Aug 2010 21:34:29 +0200
From:	Tomasz Chmielewski <mangoo@...g.org>
To:	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org
CC:	hch@...radead.org, gg.mariotti@...il.com,
	"Justin P. Mattock" <justinmattock@...il.com>, mjt@....msk.ru,
	josef@...hat.com, tytso@....edu
Subject: Re: BTRFS: Unbelievably slow with kvm/qemu

Christoph Hellwig wrote:

> There are a lot of variables when using qemu.
>
> The most important one are:
>
>  - the cache mode on the device.  The default is cache=writethrough,
>    which is not quite optimal.  You generally do want to use cache=none
>    which uses O_DIRECT in qemu.
>  - if the backing image is sparse or not.
>  - if you use barrier - both in the host and the guest.

I noticed that when btrfs is mounted with default options, when writing 
i.e. 10 GB on the KVM guest using qcow2 image, 20 GB are written on the 
host (as measured with "iostat -m -p").


With ext4 (or btrfs mounted with nodatacow), 10 GB write on a guest 
produces 10 GB write on the host.


-- 
Tomasz Chmielewski
http://wpkg.org

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