Re: [libvirt] [PATCH] add --nocow option to vol-create and vol-clone

On Thu, Dec 05, 2013 at 11:16:42AM +0000, Daniel P. Berrange wrote: > On Thu, Dec 05, 2013 at 06:35:12PM +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 is to turn off COW attributes on VM files (since having copy on > > write for this kind of data is not useful). > > > > According to 'chattr' manpage, NOCOW could be set to new or empty file only on > > btrfs, so this patch tries to add a --nocow option to vol-create functions and > > vol-clone function, so that users could have a chance to set NOCOW to a new > > volume if that happens to create on a btrfs like file system. > > What effect / impact does setting this flag have from a functional > POV ? Why would we not just unconditonally enable it on btrfs so > it was fast "out of the box" ? I'm loathe to add a btrfs-specific > flag to our public API. Quoting from the qemu-devel thread on the same subject: > When the NOCOW attribute is set on a file, reflink copying (aka > file-level snapshots) do not work: > > $ cp --reflink test.img test-snapshot.img > > This produces EINVAL. > > It is a regression if qemu-img create suddenly starts breaking this > standard btrfs feature for existing users. So as with QEMU, I don't think libvirt can do something which could break existing users of brtfs in this way. So this would have to be an opt-in of some kind. We already have a way to express "features" for storage volumes in the XML description. We could use this to express a 'nocow' feature. This is preferrable to an API flag, since this would let a user query XML for an existing volume to discover if it had 'nocow' or not.