Re: Repacking many disconnected blobs

From: Linus Torvalds <torvalds@osdl.org>
Date: Jun 14, 2006, 18:52 UTC
Message-ID: <Pine.LNX.4.64.0606141146310.5498@g5.osdl.org>
In-Reply-To: <Pine.LNX.4.64.0606141113130.5498@g5.osdl.org>

On Wed, 14 Jun 2006, Linus Torvalds wrote:

Show 5 quoted lines

> 
> 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.

Actually, thinking about it more, the way you do things, shuffling probably won't even help.

Why? Because you'll obviously have multiple files, and even if each file were to be sorted "correctly", the access patterns from any global standpoint won't really matter, becase you'd probably bounce back and forth in the pack-file anyway.

So if anything, I would say

 - just dump them into the packfile in whatever order is most convenient

 - if you know that later phases will go through the objects and actually 
   use them (as opposed to just building trees out of their SHA1 values) 
   in some particular order, _that_ might be the ordering to use.

 - in many ways, getting good delta chains is _much_ more important, since 
   "git repack -a -d" will re-use good deltas from a previous pack, but 
   will _not_ care about any ordering in the old pack. As well as 
   obviously improving the size of the temporary pack-files anyway.

I'll pontificate more if I can think of any other cases that might matter.

		Linus