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    SubjectRe: BTRFS: Unbelievably slow with kvm/qemu
      On 8/29/10 17:14 , Josef Bacik wrote:
    > On Sun, Aug 29, 2010 at 09:34:29PM +0200, Tomasz Chmielewski wrote:
    >> Christoph Hellwig wrote:
    >>> There are a lot of variables when using qemu.
    >>> The most important one are:
    >>> - the cache mode on the device. The default is cache=writethrough,
    >>> which is not quite optimal. You generally do want to use cache=none
    >>> which uses O_DIRECT in qemu.
    >>> - if the backing image is sparse or not.
    >>> - if you use barrier - both in the host and the guest.
    >> I noticed that when btrfs is mounted with default options, when writing
    >> i.e. 10 GB on the KVM guest using qcow2 image, 20 GB are written on the
    >> host (as measured with "iostat -m -p").
    >> With ext4 (or btrfs mounted with nodatacow), 10 GB write on a guest
    >> produces 10 GB write on the host
    > Whoa 20gb? That doesn't sound right, COW should just mean we get quite a bit of
    > fragmentation, not write everything twice. What exactly is qemu doing? Thanks,
    Make sure you build your file system with "mkfs.btrfs -m single -d
    single /dev/whatever". You may well be writing duplicate copies of


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      Last update: 2010-08-30 18:11    [W:0.033 / U:55.376 seconds]
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