[dm-devel] access to snapshot deltas

[Benjamin S. Scarlet]

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