Re: [PATCH 05/24] midx: implement `DISP` chunk

From: Junio C Hamano <gitster@pobox.com>
Date: Dec 13, 2023, 19:20 UTC
Message-ID: <xmqqwmthnc31.fsf@gitster.g>
In-Reply-To: <ZXn33HVBFqidAThn@nand.local>

Taylor Blau <me@ttaylorr.com> writes:

Show 11 quoted lines

>   - Turn any cross-pack deltas (which are stored as OFS_DELTAs) into
>     REF_DELTAs. We already do this today when reusing an OFS_DELTA
>     without `--delta-base-offset` enabled, so it's not a huge stretch to
>     do the same for cross-pack deltas even when `--delta-base-offset` is
>     enabled.
>
>     This would work, but would obviously result in larger-than-necessary
>     packs, as we in theory *could* represent these cross-pack deltas by
>     patching an existing OFS_DELTA. But it's not clear how much that
>     would matter in practice. I suspect it would have a lot to do with
>     how you pack your repository in the first place.

I have to wonder if the cost of additional computation to see when is safe to allow cross-pack delta can be kept reasonably low, as you'd need to prove that you are not introducing a cycle by doing so. Compared to that, spending a dozen or so bytes for the offset for rare cases of having to express such a cross-pack delta pair would be much less worrisome to me.

> Thanks for the terrific suggestion. I'll clean up the results of trying
> to implement it, and share it with the list soon (ideally before the end
> of this week, after which I'm on vacation until the new year).

Good to hear that you'll be recharging soon.  Enjoy.