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 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
