Re: [Qemu-devel] [PATCH] block: Add bdrv_forbid_ext_snapshots.

[Kevin Wolf]

Am 26.09.2013 um 15:35 hat Benoît Canet geschrieben:
> Le Thursday 26 Sep 2013 à 13:43:19 (+0200), Kevin Wolf a écrit :
> > Am 26.09.2013 um 04:01 hat Jeff Cody geschrieben:
> > > On Wed, Sep 25, 2013 at 04:23:22PM +0200, Benoît Canet wrote:
> > > > Drivers having a bs->file where set to recurse the call to their child.
> > > > Protocol and drivers designed to be on the bottom of the stack where 
> > > > set to allow
> > > > snapshots.
> > > > Future protocols like quorum where creating snapshots does not make 
> > > > sense
> > > > without block filters will be set to forbid snapshots.
> > > > 
> > > > Signed-off-by: Benoit Canet <address@hidden>
> > 
> > > > diff --git a/block.c b/block.c
> > > > index 4a98250..ff296df 100644
> > > > --- a/block.c
> > > > +++ b/block.c
> > > > @@ -4651,3 +4651,30 @@ int bdrv_amend_options(BlockDriverState *bs, 
> > > > QEMUOptionParameter *options)
> > > >      }
> > > >      return bs->drv->bdrv_amend_options(bs, options);
> > > >  }
> > > > +
> > > > +bool bdrv_is_ext_snapshot_forbidden(BlockDriverState *bs)
> > > > +{
> > > 
> > > I think either:
> > > A) Name this function bdrv_forbid_ext_snapshots(), or
> > > B) Name the BlockDriver function ptr to .bdrv_is_ext_snapshot_forbidden
> > > 
> > > The idea being that this function and the BlockDriver function ptr
> > > should have the same name (e.g. bdrv_has_zero_init, and
> > > bs->drv->bdrv_has_zero_init, etc..)
> > 
> > Yes, I agree, some consistent naming is desirable. I don't think
> > bdrv_forbid_ext_snapshots() is a good name, because it implies that
> > calling this function is what forbids the snapshot (i.e. an action
> > similar to adding a migration blocker), whereas in fact it just checks
> > whether snapshots are forbidden.
> > 
> > How about bdrv_ext_snapshot_allowed(), which avoid double negations when
> > we check for "not forbidden"? Or perhaps even bdrv_check_ext_snapshot(),
> > which would be a more generic name that could be extended to the
> > three-way distinction we intended to have in the end:
> > 
> > - External snapshots are forbidden
> > - May snapshot, but below this BDS (ask bs->file; this is for filters)
> > - Do the snapshot here
> 
> Whould .bdrv_check_ext_snapshot being NULL imply "- Do the snapshot here" as
> Jeff suggested ?

That would probably be the most convenient option.

Kevin