Messages in this thread |  | | | Date | Sun, 29 Aug 2010 20:14:41 -0400 | | From | Josef Bacik <> | | Subject | Re: BTRFS: Unbelievably slow with kvm/qemu |
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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,
Josef
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