Re: Repacking many disconnected blobs

[Linus Torvalds <torvalds@osdl.org>]

On Wed, 14 Jun 2006, Keith Packard wrote:

> On Wed, 2006-06-14 at 08:53 -0700, Linus Torvalds wrote:
> 
> >  - You can list the objects with "most important first" order first, if 
> >    you can.  That will improve locality later (the packing will try to 
> >    generate the pack so that the order you gave the objects in will be a 
> >    rough order of the resul - the first objects will be together at the 
> >    beginning, the last objects will be at the end)
> 
> I take every ,v file and construct blobs for every revision. If I
> understand this correctly, I should be shuffling the revisions so I send
> the latest revision of every file first, then the next-latest revision.
> It would be somewhat easier to just send the whole list of revisions for
> the first file and then move to the next file, but if shuffling is what
> I want, I'll do that.

You don't _need_ to shuffle. As mentioned, it will only affect the 
location of the data in the pack-file, which in turn will mostly matter 
as an IO pattern thing, not anything really fundamental.  If the pack-file 
ends up caching well, the IO patterns obviously will never matter.

Eventually, after the whole import has finished, and you do the final 
repack, that one will do things in "recency order" (or "global 
reachability order" if you prefer), which means that all the objects in 
the final pack will be sorted by how "close" they are to the top-of-tree. 

And that will happen regardless of what the intermediate ordering has 
been.

So if shuffling is inconvenient, just don't do it.

On the other hand, if you know that you generated the blobs "oldest to 
newest", just print them in the reverse order when you end up repacking, 
and you're all done (if you just save the info into some array before you 
repack, just walk the array backwards).

			Linus