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

[Daniel P. Berrange]

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.

Daniel
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