[dm-devel] ANNOUNCE: consolidated patches for shared snapshots

[Mikulas Patocka]

On Mon, 8 Dec 2008, FUJITA Tomonori wrote:

> On Sun, 7 Dec 2008 22:41:19 -0500 (EST)
> Mikulas Patocka <mpatocka at redhat.com> wrote:
> 
> > I released new patches for shared snapshots. The functionality is the same 
> > as in previous releases, but the two snapshot exception stores, one 
> > created by Fujita Tomonori and one by me, are consolidated and common code 
> > is shared.
> > 
> > The patches are at:
> > http://people.redhat.com/mpatocka/patches/kernel/new-snapshots/
> 
> Thanks,
> 
> As I said before, I have no preference about how to integrate new
> snapshot implementations into the existing code. I'm happy to follow
> the maintainer, Alasdair?
> 
> 
> > The patches don't alter existing snapshot implementation, they contain 
> > just new files (they alter only Kconfig and Makefile to make the new files 
> > compilable).
> > 
> > The new architecture is as follows:
> > 
> > The module dm-multisnapshot.ko contains common code for both exception 
> > stores. It has basically processing and queuing IOs and attaching 
> > snapshots and interfacing with device mapper.
> 
> I've not closely looked at your dm-multisnapshot but after a quick
> look, I'm not sure about the number of exception I/Os (except for
> metadata I/Os) with an origin write.

When writable snapshots will be implemented in my exception store, there 
will be a situation when the code will need to perform more copies. My 
implmentation stores snapshot ID ranges into the btree. For example, if I 
have snapshots 0-10. Someone wrote to snapshot 5. Now someone writes to 
the origin at this chunk. Two copies need to be performed, one for 
snapshots 0-4 and one for snapshots 6-10.

Kcopyd can perform at most 8 copies efficiently (one read + many 
concurrent writes).

That's why there is this reset_query/query_next_remap/add_next_remap loop.

reset_query resets the search state in the snapshot store, 
query_next_remap asks for next remapping to be done and add_next_remap 
inserts the remapping into the b-tree. It continues to loop until it fills 
all 8 kcopyd slots or until query_next_remap reports that there is nothing 
more to do.

For your implementation, the loop executes just once, because you use 
bitmap of snapshot IDs and you process all the snapshots in one call.
 
> For example, I create five snapshots and remove the 1 and 3 snapshots
> (*1). Now I have 0, 2, 4 snapshots. Then when I update the origin, how
> many exception I/Os dm-multisnapshot needs to perform?

For your implementation, just one. For my implementation, also just one, 
because I don't resuse snapshot IDs --- so when I implement it, I can just 
insert one entry with range 0-4. More entries need to be inserted when 
someone wrote to the snapshots in the middle of the range.

> (*1) I know dm-multisnapshot doesn't support deleting a snapshot for
> now, but I expect that it will support it in the future.
> 
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Mikulas