Re: repo.or.cz wishes?

From: Shawn O. Pearce <spearce@spearce.org>
Date: Aug 31, 2007, 21:09 UTC
Message-ID: <20070831210938.GC18160@spearce.org>
In-Reply-To: <20070829111345.GD29615@thunk.org>

Theodore Tso <tytso@mit.edu> wrote:

Show 14 quoted lines

> On Wed, Aug 29, 2007 at 12:15:23AM -0400, Shawn O. Pearce wrote:
> > > 
> > > git --git-dir=A gc
> > > ln A/.git/objects/pack/* B/.git/objects/pack
> > > git --git-dir=B gc --prune
> > > git --git-dir=A prune
> > 
> > No, it won't work.
> > 
> > The problem is that during the first `git --git-dir=A gc` call
> > you are deleting packfiles that may contain objects that B needs.
> > *poof*.  
> 
> But "git-gc" without the --prune doesn't delete any objects.

Yes, it does delete objects. Even without --prune. That is because git-gc is running `git-repack -a -d -l`. repack -a means repack all objects reachable from the current refs. The -d means delete the packfiles that existed when the repack started, as it is assumed that all needed (reachable) objects were copied into the new output packfile(s). The -d also means delete any loose objects that are now packed (git-prune-packed).

Yet there may be objects in A that A cannot reach anymore (deleted or rewound branch) but that B needs and B does not have a copy of. If these objects were in one of the prior packfiles of A and is not in the new packfile(s) of A then those objects are gone. *poof*.

Show 7 quoted lines

> So it
> should always be safe to use git-gc even if there are repositories
> that are relying on that repo's ODB.  It's only if you use git-gc
> --prune that you could get in troudble.  It might delete some
> packfiles containing objects needed by B, but only after consolidating
> all of the objects into a single packfile that contains all of the
> objects that had always been in A's ODB.

But when we repack we don't repack everything in A's ODB, we only
repack the things that A can reach.  If A cannot reach something
because a branch was rewound or deleted it won't survive the repack.
Then the repack is behaving like at least partially like gc --prune.

> So I don't see why this wouldn't work.

It only works if A cannot delete a branch or rewind a branch. In other words, once an object is stored in A's ODB it must always be reachable from A's refs.

-- 
Shawn.