Il 26/09/2013 09:58, Stefan Hajnoczi ha scritto: > On Wed, Sep 25, 2013 at 02:38:36PM +0800, Chunyan Liu wrote: >> Btrfs has terrible performance when hosting VM images, even more when the >> guest in those VM are also using btrfs as file system. >> One way to mitigate this bad performance would be to turn off COW >> attributes on VM files (since having copy on write for this kind of data is >> not useful). We could improve qemu-img to ensure they flag newly created >> images as "nocow". For those who want to use Copy-on-write (for >> snapshotting, to share snapshots across VM, etc..) could be able to change >> this behaviour by 'chattr', either globally or per VM. > > The full implications of the NOCOW attribute aren't clear to me. Does > it really mean the file cannot be snapshotted? Or is it purely a data > integrity issue where overwriting data in-place puts that data at risk > in case of hardware/power failure? > >> I wonder could we add a patch to improve qemu-img create, to set 'nocow' >> flag by default on newly created images? > > I think that would be fine. It's a ioctl(FS_IOC_SETFLAGS, FS_NOCOW_FL) > call so not even too btrfs-specific.
I'm not sure... I have some questions: 1) Does btrfs cow mean that one could run with cache=unsafe, for example? If we create the image with nocow, this would not be true. 2) Does ZFS have the same problem? In other words, could this just be considered a btrfs bug? Paolo
