Messages in this thread | | | Date | Mon, 12 Jul 2010 17:40:50 +0400 | From | Michael Tokarev <> | Subject | Re: BTRFS: Unbelievably slow with kvm/qemu |
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Giangiacomo Mariotti wrote: > On Mon, Jul 12, 2010 at 9:09 AM, Michael Tokarev <mjt@tls.msk.ru> 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
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