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Re: [dm-devel] [PATCH RFC] dm snapshot: shared exception store

On Wed, 6 Aug 2008 15:14:50 -0400 (EDT)
Mikulas Patocka <mpatocka redhat com> wrote:

> Hi
> I looked at it.

Thanks! I didn't expect someone read the patch. I'll submit patches in
more proper manner next time.

> Alasdair had some concerns about the interface on the phone call. From my 
> point of view, the Fujita's interface is OK (using messages to manipulate 
> the snapshot storage and using targets to access the snapshots). Alasdair, 
> could you be pls. more specific about it?

Yeah, we can't use dmsetup create/destroy to create/delete
snapshots. We need something different.

I have no strong opinion about it. Whatever interface is fine by me as
long as it works.

> What I would propose to change in the upcoming redesign:
> - develop it as a separate target, not patch against dm-snapshot. The code 
> reuse from dm-snapshot is minimal, and keeping the old code around will 
> likely consume more coding time then the potential code reuse will save.

It's fine by me if the maintainer prefers it. Alasdair?

> - drop that limitation on maximum 64 snapshots. If we are going to 
> redesign it, we should design it without such a limit, so that we wouldn't 
> have to redesign it again (why we need more than 64 --- for example to 
> take periodic snapshots every few minutes to record system activity). The 
> limit on number of snapshots can be dropped if we index b-tree nodes by a 
> key that contains chunk number and range of snapshot numbers where this 
> applies.

Unfortunately it's the limitation of the current b-tree
format. As far as I know, there is no code that we can use, which
supports unlimited and writable snapshot.

> - do some cache for metadata, don't read the b-tree from the root node 
> from disk all the time.

The current code already does.

> Ideally the cache should be integrated with page 
> cache so that it's size would tune automatically (I'm not sure if it's 
> possible to cleanly code it, though).

Agreed. The current code invents the own cache code. I don't like it
but there is no other option.

> - the b-tree is good structure, I'd create log-structured filesystem to 
> hold the b-tree. The advantage is that it will require less 
> synchronization overhead in clustering. Also, log-structured filesystem 
> will bring you crash recovery (with minimum coding overhead) and it has 
> very good write performance.

A log-structured filesystem is pretty complex. Even though we don't
need a complete log-structured filesystem, it's still too complex,

A copy-on-Write manner to update the b-tree on disk (as some of the
latest file systems do) is a possible option. Another option is using
journaling as I wrote.

> - deleting the snapshot --- this needs to walk the whole b-tree --- it is 
> slow. Keeping another b-tree of chunks belonging to the given snapshot 
> would be overkill. I think the best solution would be to split the device 
> into large areas and use per-snapshot bitmap that says if the snapshot has 
> some exceptions allocated in the pertaining area (similar to the 
> dirty-bitmap of raid1). For short lived snapshots this will save walking 
> the b-tree. For long-lived snapshots there is no help to speed it up... 
> But delete performance is not that critical anyway because deleting can be 
> done asynchronously without user waiting for it.

Yeah, it would be nice to delete a snapshot really quickly but it's
not a must.

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