Re: Partitioned packs

On Tue, 3 Apr 2007, Linus Torvalds wrote:

Side note: I think that there *are* cases where partitioned packs can do better, but I think that in order to do better you should

 - partition by "recency", ie put objects that are not reachable from any 
   recent point in older packs.

 - make sure that the "packed_git" list is always sorted so that the older 
   data packs are at the end.

and that should actually speed up many loads, just because the recent objects are all in one pack, and because it's smaller, that pack can be looked up a bit faster.

On the other hand, the power of a log(n) function like a binary search is that lookup in a big pack that is four times the size of four smaller packs is really not all that much more expensive, so the advantage is probably pretty small.

And for things that need old objects (and "git blame" does obviously very much tend to fall into that category), any partitioning is likely to be bad.

So I think partitioning is valid, but my suspicion is that you'd want to partition for *other* reasons than highest performance. Better reasons to have multiple packs:

 - just because you haven't repacked ;)
 - to keep "git repack" times down by marking old big packs as "keep" once 
   they get big enough (the space advantage of packing eventually flattens 
   out, so there's no real overwhelming reason to repack old stuff if you 
   have "enough")
 - filesystem and pack-file limitations (ie the 2**31 limit)

but I doubt performance is ever going to be a really compelling one.

You can obviously always optimize for some very *particular* load by packing optimally for just that one (keep exactly the objects you need in one particular pack, don't even touch any other packs), but I don't think any load is *so* special that you shouldn't think of other loads.

			Linus