[Qemu-devel] Re: BTRFS: Unbelievably slow with kvm/qemu

[Michael Tokarev]

Giangiacomo Mariotti wrote:
> On Mon, Jul 12, 2010 at 9:09 AM, Michael Tokarev <address@hidden> 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:
>> http://permalink.gmane.org/gmane.linux.file-systems/42758
>> (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.

Actually it's not that simple.

>     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.

It is exactly the same with ext4 vs ext3.  But only on metadata-intensitive
operations (for qcow2 image).  Once you allocate space, it becomes fast,
and _especially_ fast with cache=none.  Actually, it looks like O_SYNC
(default cache mode) is _slower_ on ext4 than O_DIRECT (cache=none).

(And yes, I know O_DIRECT does NOT imply O_SYNC and vise versa).

/mjt