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

On Sun, 7 Dec 2008 22:41:19 -0500 (EST)
Mikulas Patocka <mpatocka 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/


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.

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?

(*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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