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

[Kevin Wolf <kwolf@redhat.com>]

Am 12.07.2010 15:43, schrieb Josef Bacik:
> On Mon, Jul 12, 2010 at 03:34:44PM +0200, 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. 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. 

IIUC, he uses the default cache option of qemu, which is
cache=writethrough and maps to O_DSYNC without O_DIRECT. O_DIRECT would
only be used for cache=none.

Kevin