I have a hosting product which consists of qemu-kvm virtual machines backed by LVM2 logical volumes as virtual drives, accessed either locally or over iscsi. I'm thinking of migrating in time to a distributed block store, such as Ceph's rbd or Sheepdog. 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've seen Ceph's snapshot support, and in particular the rbd snapshot support, which lets me make read-only clones of a rados block device. What I'm after is not quite the same as writeable snapshots, as I'd also like to be able to offer the user the ability to delete the original block device independently of the clone, potentially before the clone itself is deleted, so the clone is properly independent of the source apart from some shared blocks. (If I stored my images as files in a local btrfs filesystem, I could get exactly the behaviour I'm imagining by cloning the image file.) 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. Best wishes, Chris. -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
