backward diffs in knits?

[Robert Collins]

On Tue, 2007-04-10 at 09:54 -0500, John Arbash Meinel wrote:
> 
> 
> I think that in the common case of files growing, backwards deltas are
> slightly smaller, and it optimizes for extracting the TIP revision,
> which helps for things like "bzr checkout", "bzr update", and "bzr
> diff".
> 
> 
> When I was discussing this stuff with Martin and Robert, I didn't
> think
> the idea was to actually store backwards deltas in Knits, but a
> different storage format.

Well there are several discussions going on concurrently with no
particular urgency:
 - a long term wishful-thinking idea for lockless repositories to
increase concurrency. The current thinking on this is that of large blob
files with built in indexes and potentially heavily optimised for size -
so deltas could be out-of-ancestry-but-within-the-blob etc.
 - continued incremental improvements to knits - can we get more
performance without changing the disk format.
 - things like aarons proposal for multi-parent deltas,  or this thread
on backwards diffs: things that we can do by changing the knit format
but dont require radical changes to the system or have long dependency
chains that would require significant work.

> I don't think backwards deltas should be generated "on-the-fly",
> because
> of the potential for a new corruption to lose old data.

I'm not sure what you mean here, in particular you can always perform a
check of all data after the pull, if and only if we have altered
representation.

> However, it could make sense for an "archive" command. Where you
> packaged up a bunch of older revisions to make them more dense, and
> sort
> of put them off to the side. They should still be accessible, but
> maybe
> they wouldn't be in the default search every time.

I see this as a variation on 'pack' - is that what you were thinking of?


> And I can say that bzr.dev is 54MB for .bzr/ and 11MB for working
> files
> (including .pyc, etc). So I'm curious if we can do better than 5x.

How much is inventory ? :).


-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 189 bytes
Desc: This is a digitally signed message part
Url : https://lists.ubuntu.com/archives/bazaar/attachments/20070411/c7afb7fc/attachment.pgp