Re: [Qemu-devel] [PATCH] qemu-img: set nocow flag to new file

[Stefan Hajnoczi <stefanha@redhat.com>]

On Tue, Dec 10, 2013 at 10:23:41PM +0000, Alex Bennée wrote:
> 
> stefanha@redhat.com writes:
> 
> > On Mon, Nov 18, 2013 at 12:54:59PM +0800, Chunyan Liu wrote:
> >> 2013/11/15 Stefan Hajnoczi <stefanha@gmail.com>
> >> 
> >> > On Thu, Nov 14, 2013 at 04:15:28PM +0800, Chunyan Liu wrote:
> >> > > Set NOCOW flag to newly created images to solve performance issues on
> >> > btrfs.
> <snip>
> >> > This should be optional and I'm not sure it should be the default.
> >> >
> >> > Rationale: If you're on btrfs you probably expect the copy-on-write and
> >> > snapshot features of the file system.  We shouldn't silently disable
> >> > that unless the user asks for it.
> <snip>
> >
> > 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.
> >
> > Please make it a .bdrv_create() option which is off by default to avoid
> > breaking existing users' workflows/scripts.  The result should be
> > something like:
> >
> > $ qemu-img create test.img 8G # file has NOCOW cleared
> > $ qemu-img create -o nocow=on test.img 8G # file has NOCOW set
> 
> I agree we shouldn't break existing work flows. I wonder if it would OK
> for qemu-img to issue a warning (when not --quiet) when it detects
> creation of an image on a partition where performance may not be as
> expected due to COW behaviour.

A warning could help or at least prompt users to consider switching to
nocow.

Stefan