On Tue, Apr 12, 2016 at 06:03:02PM -0700, Junio C Hamano wrote: > > So we'd probably need a "treev2" object type that gives room for an > > algorithm byte (or we'd have to try to shove it into the mode, but since > > old versions won't know the new algorithm anyway, I don't think it > > solves that much...). Or you can just define for the whole tree object > > (either implicit in its type, or in a header) that it always uses > > algorithm X. > > This will hurt the performance a lot during the transition period as > it no longer will be possible to rely on "most of the time a fine > grained commit changes only a small part of the tree, and we can > cheaply avoid descending into trees that haven't changed because we > can tell that the corresponding tree objects in the pre- and post- > trees have the same object name" optimization. But we cannot avoid > it. Yeah. I'd hope in general that there would be a single commit that does the transition, and we'd only pay it when doing diffs across the boundary. And even then, I think a local-only cache of aliases could mitigate the worst of it. > > 7. After a while, flip the config option on. Hopefully the readers > > from steps 1-5 have percolated to the masses by then, and it's not > > a horrible flag day. > > > > We're basically on step 0 right now. I'm sure I'm missing some > > subtleties in there, too. > > One subtlety is that 7. "not a flag day" may not be a good thing. > > There has to be a section of a history that spans the transition, > set of commits and trees that have pointers to both kinds of object > names. The narrower such a section of the history, the more > pleasant to use the result of the transition would be. > > Different projects that can have their own flag days at their own > pace is a good thing, so the above observation does not invalidate > your transition plan, though. Good point. I do think projects would do well to have a moment where they switch to the new format, and don't freely intermingle. We could possibly do some magic there to help things out. For example, if we are building on a commit that is sha-2, we automatically use more sha-2 objects to point to them. And then the "flag day" for a project is simply that somebody pushes to "master" using sha-2, and everybody else's git (which learned long ago to speak the new algorithm) just picks it up. Of course that's not exactly a flag day for projects that branch from old history for bugfixes. But it might be close enough. -Peff -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html