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

[Jason Dillaman]

On Sun, Apr 14, 2019 at 9:20 AM Stefano Garzarella <address@hidden> wrote:
>
> 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,

Copy-on-write is natively supported by RBD. The concept of external
snapshots seems similar to just automating the process of creating a
new copy-on-write image. Compression is also supported by Ceph on the
cluster side by recent releases.

> 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



-- 
Jason