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SubjectRe: 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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