Re: reftable [v4]: new ref storage format

[Shawn Pearce <spearce@spearce.org>]

On Mon, Jul 31, 2017 at 4:43 PM, Shawn Pearce <spearce@spearce.org> wrote:
> On Mon, Jul 31, 2017 at 12:42 PM, Junio C Hamano <gitster@pobox.com> wrote:
>>
>> As a block cannot be longer than 16MB, allocating uint32 to a
>> restart offset may be a bit overkill.  I do not know if it is worth
>> attempting to pack 1/3 more restart entries in a block by using
>> uint24, though.
>
> I don't think its worth the annoyance in code that has to deal with
> this field. An example 87k ref repository with a 4k block size has ~9
> restarts per block and ~19 bytes of padding. Using uint24 saves us 9
> bytes, yet the average ref here costs 27 bytes. We aren't likely to
> fill the block with another ref, or another restart point.

I thought about this more. We can fit an additional ref per block in
some cases. It saves ~20 KiB for one of my example repositories if
restart_offset is a uint24.

Given that readers have to deal with these being unaligned loads
anyway, its not significantly harder to work with uint24 vs. uint32.
So I've changed the definition to be uint24.