Re: adding new 32-bit on-disk (unsigned) timestamp formats (was: [PATCH v5 02/17] pack-mtimes: support reading .mtimes files)

On 5/25/2022 5:11 AM, Ævar Arnfjörð Bjarmason wrote:

I must say that I really don't like this part of the format. Is it
really necessary to optimize the storage space here in a way that leaves
open questions about future time_t compatibility, and having to
introduce the first use of unsigned 32 bit timestamps to git's codebase?

The commit-graph file format uses unsigned 34-bit timestamps (packed
with 30-bit topological levels in the CDAT chunk), so this "not-64-bit
signed timestamps" thing is something we've done before.
 

Yes, this is its own self-contained format, so we don't *need* time_t
here, but it's also really handy if we can eventually consistently use
64 time_t everywhere and not worry about any compatibility issues, or
unsigned v.s. signed, or to create our own little ext4-like signed 32
bit timestamp format.

We can also use a new file format version when it is necessary. We
have a lot of time to add that detail without overly complicating the
format right now.

If we really are trying to micro-optimize storage space here I'm willing
to bet that this is still a bad/premature optimization. There's much
better ways to store this sort of data in a compact way if that's the
concern. E.g. you'd store a 64 bit "base" timestamp in the header for
the first entry, and have smaller (signed) "delta" timestamps storing
offsets from that "base" timestamp.

This is a good idea for a v2 format when that is necessary.

Thanks,
-Stolee