Re: BTRFS: Unbelievably slow with kvm/qemu

[Josef Bacik <josef@redhat.com>]

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,

Josef