[PATCH 0/3] repack --keep-unreachable

[Jeff King <peff@peff.net>]

On Sun, Jun 12, 2016 at 07:43:27PM -0600, Nasser Grainawi wrote:

> On Jun 12, 2016, at 4:13 PM, Jeff King <peff@peff.net> wrote:
> > 
> >    At GitHub we actually have a patch to `repack` that keeps all
> >    objects, reachable or not, in the pack, and use it for all of our
> >    automated maintenance. Since we don't drop objects at all, we can't
> >    ever have such a race. Aside from some pathological cases, it wastes
> >    much less space than you'd expect. We turn the flag off for special
> >    cases (e.g., somebody has rewound history and wants to expunge a
> >    sensitive object).
> > 
> >    I'm happy to share the "keep everything" patch if you're interested.
> 
> We have the same kind of patch actually (for the same reason), but
> back on the shell implementation of repack. It'd be great if you could
> share your modern version.

Here is a cleaned-up version of what we run at GitHub (so this is a
concept that has been exercised for a few years in production, but I had
to forward port the patches a bit; I _probably_ didn't introduce any
bugs. :) ).

The heavy lifting is done by the existing --keep-unreachable option to
pack-objects, which Junio added a long time ago[1] in support of a safer
"gc --auto". But it doesn't look like we ever documented or exercised
it, and "gc --auto" ended up using the loosen-unreachable strategy
instead. In fact, the rest of that series seems to have been dropped; I
couldn't find any discussion on the list explaining it, or why this one
patch was kept (so I don't think anybody upstream has ever used this
code, but as I said, we have been doing so for a few years, so I feel
confident in it).

  [1/3]: repack: document --unpack-unreachable option
  [2/3]: repack: add --keep-unreachable option
  [3/3]: repack: extend --keep-unreachable to loose objects

-Peff

[1] http://article.gmane.org/gmane.comp.version-control.git/58413