Re: BTRFS: Unbelievably slow with kvm/qemu

[K. Richard Pixley]

 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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