From: Jonathan Nieder <jrnieder@gmail.com>
Date: Mon, 16 Jul 2018 19:19:23 GMT
Subject: Re: [PATCH] gc: do not warn about too many loose objects
Message-ID: <20180716191923.GC11513@aiede.svl.corp.google.com>
In-Reply-To: <20180716191505.857-1-newren@gmail.com>

Hi,

Elijah Newren wrote:

> The basic problem here, at least for us, is that gc has enough
> information to know it could expunge some objects, but because of how
> it is structured in terms of several substeps (reflog expiration,
> repack, prune), the information is lost between the steps and it
> instead writes them out as unreachable objects.  If we could prune (or
> avoid exploding) loose objects that are only reachable from reflog
> entries that we are expiring, then the problem goes away for us.

My understanding is that exploding the objects is intentional behavior,
to avoid a race where objects are newly referenced while they are being
pruned.

I am not a fan of that behavior.  It's still racy.  But when we've
brought it up in the past, the consensus seems to have been that it's
better than nothing.  Documentation/technical/hash-function-transition.txt
section "Loose objects and unreachable objects" describes a way to
eliminate the race.

Thanks and hope that helps,
Jonathan