Re: BTRFS: Unbelievably slow with kvm/qemu

["Justin P. Mattock" <justinmattock@gmail.com>]

On 07/11/2010 10:24 PM, Giangiacomo Mariotti wrote:
> Hi, is it a known problem how much slow is Btrfs with kvm/qemu(meaning
> that the image kvm/qemu uses as the hd is on a partition formatted
> with Btrfs, not that the fs used by the hd inside the kvm environment
> is Btrfs, in fact inside kvm the / partition is formatted with ext3)?
> I haven't written down the exact numbers, because I forgot, but while
> I was trying to make it work, after I noticed how much longer than
> usual it was taking to just install the system, I took a look at iotop
> and it was reporting a write speed of the kvm process of approximately
> 3M/s, while the Btrfs kernel thread had an approximately write speed
> of 7K/s! Just formatting the partitions during the debian installation
> took minutes. When the actual installation of the distro started I had
> to stop it, because it was taking hours! The iotop results made me
> think that the problem could be Btrfs, but, to be sure that it wasn't
> instead a kvm/qemu problem, I cut/pasted the same virtual hd on an
> ext3 fs and started kvm with the same parameters as before. The
> installation of debian inside kvm this time went smoothly and fast,
> like normally it does. I've been using Btrfs for some time now and
> while it has never been a speed champion(and I guess it's not supposed
> to be one and I don't even really care that much about it), I've never
> had any noticeable performance problem before and it has always been
> quite stable. In this test case though, it seems to be doing very bad.
>
> cheers
>

not sure with butter filesystems.. but, what is the last good kernel? 
are you able to bisect?

Justin P. Mattock