From: Jeff King <peff@peff.net>
Date: Thu, 02 Sep 2010 15:58:13 GMT
Subject: Re: git pack/unpack over bittorrent - works!
Message-ID: <20100902155810.GB14508@sigill.intra.peff.net>
In-Reply-To: <AANLkTikBnKQJmgOms2wK1+6fCLtHWiWkhuCVMN7kKLXP@mail.gmail.com>

On Thu, Sep 02, 2010 at 04:42:55PM +0100, Luke Kenneth Casson Leighton wrote:

> >>  it seems to be.  as long as each version of git produces the exact
> >> same pack object, off of the command "git pack-objects --all --stdout
> >> --thin {ref}<  {objref}"
> >
> > This is not guaranteed.
> [...]
> * under what circumstances is it not guaranteed?  and, crucially, is
> it necessary to care?   i.e. if someone does a shallow git clone, i
> couldn't give a stuff.

pack-objects will reuse previously found deltas. So the deltas you have
in your existing packs matter. The deltas you have in your existing
packs depend on many things. At least:

  1. Options you used when packing (e.g., --depth and --window).

  2. Probably exactly _when_ you packed. You could find a good delta
     from A to B. Later, object C comes into existence, and would
     provide a better delta base for B. I don't think we will ever try A
     against C, unless --no-reuse-delta is set.

     You have a different pack than somebody who packed after A, B, and
     C all existed.

     In practice, this tends not to happen much because the best deltas
     are usually going backwards in time to a previous version. But it
     can happen.

-Peff