On Fri, Jun 08 2012 at 10:56am -0400, Benjamin S. Scarlet <scarlet@xxxxxxxxxxxx> wrote: > Is there or could there be support for writing to a snapshot > copy-on-write device from userspace? Are the bits contained therein > portable between two snapshot instances with the same sizes of origin > and copy-on-write devices? > > I tried a simple experiment and caused Very Bad Things to happen (a > kernel error claiming "BUG: Bad page state in process ..."), but I may > have done something bogus. Before I proceeded with further experiments, > I thought I'd ask what, if anything, ought to work. > > My ultimate goal is to reduce downtime while migrating a virtual guest > with its storage from one host to another. To that end what I'd like to > do is: > *) take a snapshot > *) transfer the frozen view of the snapshot while the guest continues to > run > *) stop the guest on the original host > *) transfer the deltas that have been written since the snapshot > *) start the guest on the new host > *) merge the deltas (assuming they weren't already merged during the > transfer, depending on how that was done) > > So please consider the following experiment, expressed mostly in terms > of lvm tools rather than dmsetup, just because this way I think I'm > giving myself fewer chances to mess up: > > 1) create an LVM volume A to play the role of the original volume. Fill > it up with some data. > 2) create an LVM snapshot of A, A-snap. > 3) create an LVM volume B to play the role of the destination volume, > the same size as A. > 4) copy (e.g. dd) from A into B, and simultaneously write some changes > into A-snap. > 5) create an LVM snapshot of B, B-snap. > 5.5) cross your fingers 'cause here I'm heading into the unknown (for > me) > 6) dmsetup suspend something (if any variant would work, could someone > tell me what?) > 7) copy bits from A-snap-cow to B-snap-cow > 8) dmsetup resume > 9) diff A-snap and B-snap and observe they're the same! (or so I would > like) > 10) proceed to use lvm to merge B-snap into B Your best bet on a starting point for this will be to use the new thin-provisioning target and its associated scalable snapshot support. As of the latest Linux 3.5, it provides the ability to get a snapshot of the metadata associated with snapshots, see this commit: http://git.kernel.org/linus/cc8394d86f045b86ff303d3c9e4ce47d97148951 This allows you to have access to the incremental differences between snapshots, etc. Joe Thornber showed an example of how to do incremental backup -- in terms of the thinp-test-suite (ruby) so there is a barrier to entry there but it is a start on more details: https://github.com/jthornber/thinp-test-suite/commit/4f126b48ebf73b392a507b3ccb58d12e28b81135 But the ability to merge the deltas from a snapshot into another snapshot hasn't been exposed as a thinp primitive yet. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
