2013/9/26 Stefan Hajnoczi <stefa...@gmail.com> > 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?
Yes, I think so. The benefits brought by COW: data integrity and convenient snapshot, would be disappears. 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. > > OK. I'll prepare the patch. Thanks. Regards, Chunyan > Stefan > >
