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    SubjectRe: BTRFS: Unbelievably slow with kvm/qemu
    On 07/12/2010 12:09 AM, Michael Tokarev wrote:
    > 12.07.2010 09:24, 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.
    >
    > 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
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    >

    cool a solution... glad to see... no chance at a bisect with this?
    (getting this down too a commit or two makes things easier)

    Justin P. Mattock


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