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

[Daniel P. Berrange]

On Wed, Dec 11, 2013 at 09:29:54AM +0100, Stefan Hajnoczi wrote:
> On Tue, Dec 10, 2013 at 10:23:41PM +0000, Alex Bennée wrote:
> > 
> > address@hidden writes:
> > 
> > > On Mon, Nov 18, 2013 at 12:54:59PM +0800, Chunyan Liu wrote:
> > >> 2013/11/15 Stefan Hajnoczi <address@hidden>
> > >> 
> > >> > 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.

IMHO such warnings are not nice - if a user does not wish to use
the 'nocow' option because they want to keep the ability to use
file label snapshots they should not be subjected to a warning
message forever more.

I suggest this is something to document in the man page instead.

Daniel
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