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

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Date:	Mon, 30 Aug 2010 08:59:37 -0700
From:	"K. Richard Pixley" <rich@...r.com>
To:	Josef Bacik <josef@...hat.com>
CC:	Tomasz Chmielewski <mangoo@...g.org>, linux-kernel@...r.kernel.org,
	linux-btrfs@...r.kernel.org, hch@...radead.org,
	gg.mariotti@...il.com,
	"Justin P. Mattock" <justinmattock@...il.com>, mjt@....msk.ru,
	tytso@....edu
Subject: Re: BTRFS: Unbelievably slow with kvm/qemu

  On 8/29/10 17:14 , Josef Bacik wrote:
> On Sun, Aug 29, 2010 at 09:34:29PM +0200, Tomasz Chmielewski wrote:
>> 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
> Whoa 20gb?  That doesn't sound right, COW should just mean we get quite a bit of
> fragmentation, not write everything twice.  What exactly is qemu doing?  Thanks,
Make sure you build your file system with "mkfs.btrfs -m single -d 
single /dev/whatever".  You may well be writing duplicate copies of 
everything.

--rich
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