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SubjectRe: BTRFS: Unbelievably slow with kvm/qemu
On Mon, Jul 12, 2010 at 03:34:44PM +0200, Giangiacomo Mariotti wrote:
> On Mon, Jul 12, 2010 at 9:09 AM, Michael Tokarev <> wrote:
> >
> > This looks quite similar to a problem with ext4 and O_SYNC which I
> > reported earlier but no one cared to answer (or read?) - there:
> >
> > (sent to qemu-devel and linux-fsdevel lists - Cc'd too).  You can
> > try a few other options, esp. cache=none and re-writing some guest
> > files to verify.
> >
> > /mjt
> >
> Either way, changing to cache=none I suspect wouldn't tell me much,
> because if it's as slow as before, it's still unusable and if instead
> it's even slower, well it'd be even more unusable, so I wouldn't be
> able to tell the difference. What I can say for certain is that with
> the exact same virtual hd file, same options, same system, but on an
> ext3 fs there's no problem at all, on a Btrfs is not just slower, it
> takes ages.

O_DIRECT support was just introduced recently, please try on the latest kernel
with the normal settings (which IIRC uses O_DIRECT), that should make things
suck alot less. Thanks,

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