Re: optimising a push by fetching objects from nearby repos

From: Junio C Hamano <gitster@pobox.com>
Date: May 11, 2014, 18:04 UTC
Message-ID: <xmqqbnv4ur7t.fsf@gitster.dls.corp.google.com>
In-Reply-To: <536F08C5.3010705@gmail.com>

Sitaram Chamarty <sitaramc@gmail.com> writes:

Show 10 quoted lines

> But what I was looking for was validation from git.git folks of the idea
> of replicating what "git clone -l" does, for an *existing* repo.
>
> For example, I'm assuming that bringing in only the objects -- without
> any of the refs pointing to them, making them all dangling objects --
> will still allow the optimisation to occur (i.e., git will still say "oh
> yeah I have these objects, even if they're dangling so I won't ask for
> them from the pusher" and not "oh these are dangling objects; so I don't
> recognise them from this perspective -- you'll have to send me those
> again").

So here is an educated guess by a git.git folk. I haven't read the codepath for some time, so I may be missing some details:

 - The set of objects sent over the wire in "push" direction is
   determined by the receiving end listing what it has to the
   sending end, and then the sending end excluding what the
   receiving end told that it already has.

 - The receiving end tells the sending end what it has by showing
   the names of its refs and their values.

Having otherwise dangling objects in your object store alone will not make them reachable from the refs shown to the sending end. But there is another trick the receiving end employes.

 - The receiving end also includes the refs and their values that
   appear in the repository it borrows objects from its alternate
   repositories, when it tells what objects it already has to the
   sending end.

So what you "assumed" is not entirely correct---bringing in only the objects will not give you any optimization.

But because we infer from the location of the object store (i.e. "objects" directory) where the refs that point at these borrowed objects exist (i.e. in "../refs" relative to that "objects" directory) in order to make sure that we do not have to say "oh these are dangling but we know their history is not broken", we still get the same optimisation.

At least, that is the theory ;-)