Re: Cloning rados block devices

[Gregory Farnum <gregf@hq.newdream.net>]

On Sun, Jan 23, 2011 at 6:07 AM, Chris Webb <chris@arachsys.com> wrote:
> One feature I would really like to be able to export to users is an ability
> to make copy-on-write clones of virtual hard drives, in a Ceph context
> generating a new rbd image from an existing one, or from a snapshot of an
> existing image if that's easier.
> ....
> I don't see any mention of a feature like this on the Ceph roadmap, and I'm
> not familiar enough with the internal design yet to know whether this is an
> easy extension given the book-keeping already in place for snapshots, or
> whether what I'm proposing is much harder. Is anyone working on this sort of
> thing already, or does the feature even already exist and I've failed to
> find it? If not, I'd be very interested in any thoughts on how difficult
> this would be to implement given the infrastructure that is already in
> place.
We've discussed similar things, but this isn't on the roadmap and I
don't think anything like it is either. There are a few problems with
simply re-using the existing snapshot mechanism. First is that it
doesn't support branching snapshots at all, and this is a hard enough
problem that we've talked about doing it for other reasons in the past
and always gone with alternative solutions. (It's not impossible,
though.) The second is that right now, all versions of an object are
stored together, on the same OSD. Which makes it pretty likely that
you'd get a lot of people cloning, say, your Ubuntu base image and
modifying the same 16 blocks, and you end up with one completely full
OSD and a fairly empty cluster. (There are mechanisms in RADOS to deal
with overloaded OSDs, but this issue of uneven distribution is one
that I would worry about even so.)

So with that said, if I were going to implement copy-on-write RBD
images, I'd probably do so in the RBD layer rather than via the RADOS
commands. Yehuda would have a better idea of how to deal with this
than I do, but I'd probably modify the header to store an index
indicating the blocks contained in the parent image and which blocks
in that range have been written to. Then set up the child image as its
own image (with its own header and rados naming scheme, etc) and
whenever one block does get written to, copy the object from the
parent image to the child's space and mark it as written in the
header. I'm not sure how this would impact performance, but presumably
most writes would be in areas of the disk not contained in the parent
image, and I don't think it would be too difficult to implement. This
wouldn't be as space-efficient as cloning for small changes like a
config file (since it would modify the whole block, which defaults to
4MB), but I bet it's better than storing 3000 installs of an Ubuntu
LTS release.
-Greg