From: Junio C Hamano <gitster@pobox.com>
Date: Wed, 06 Jul 2011 23:22:23 GMT
Subject: Re: generation numbers
Message-ID: <7viprfhu4w.fsf@alter.siamese.dyndns.org>
In-Reply-To: <20110706181200.GD17978@sigill.intra.peff.net>

Jeff King <peff@peff.net> writes:

> Yeah, that would be fine. With a sorted list of binary sha1s and 32-bit
> generation numbers, you're talking about 24 bytes per commit. Or a 6
> megabyte cache for linux-2.6.
>
> You'd probably want to be a little clever with updates. If I have
> calculated the generation number of every commit, and then do "git
> commit; git tag --contains HEAD", you probably don't want to rewrite the
> entire cache. You could probably journal a fixed number of entries in an
> unsorted file (or even in a parallel directory structure to loose
> objects), and then periodically write out the whole sorted list when the
> journal gets too big. Or choose a more clever data structure that can do
> in-place updates.

As to the low level implementation detail I agree everything you said, but
I have been wondering how the generation number should intereact with
grafts and replaces. It certainly would be safest whenever you change
grafts (which should be a rare event anyway).