Re: [Qemu-block] [Qemu-devel] [PATCH RFC 1/1] block/rbd: increase dynamically the image size

[Stefano Garzarella]

On Thu, Apr 11, 2019 at 01:06:49PM -0400, Jason Dillaman wrote:
> On Thu, Apr 11, 2019 at 9:02 AM Stefano Garzarella <address@hidden> wrote:
> >
> > On Thu, Apr 11, 2019 at 08:35:44AM -0400, Jason Dillaman wrote:
> > > On Thu, Apr 11, 2019 at 7:00 AM Stefano Garzarella <address@hidden> wrote:
> > > >
> > > > RBD APIs don't allow us to write more than the size set with 
> > > > rbd_create()
> > > > or rbd_resize().
> > > > In order to support growing images (eg. qcow2), we resize the image
> > > > before RW operations that exceed the current size.
> > >
> > > What's the use-case for storing qcow2 images within a RBD image? RBD
> > > images are already thinly provisioned, they support snapshots, they
> > > can form a parent/child linked image hierarchy.
> > >
> >
> > Hi Jason,
> > I understand your point of view, maybe one use case could be if you have
> > a qcow2 image and you want to put it in the rdb pool without convert it.
> >
> > I'm going through this BZ [1] and I'll ask if they have other
> > use cases in mind.
> 
> Assuming no good use-cases, perhaps it would just be better to make
> the qemu-img error messages more clear.
> 

Hi Jason,
I asked about use-cases and they want to use qcow2 on rbd in order to
take advantage of these qcow2 features [1]: external snapshots,
Copy-on-write, and optional compression and encryption.

Maybe the more interesting are external snapshots and Copy-on-write,
since encryption can be achieved with LUKS and rbd should support
compression for a specified pool.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1171007#c13

Cheers,
Stefano