Re: Lowering the default wal_blocksize to 4K

On Wed, Oct 11, 2023 at 4:28 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> That leaves only the segments where a record starts exactly on the
> first usable byte of a segment, which is why I was trying to think of
> a way to cover that case too. I suggested we could notice and insert
> a new record at that place. But Andres suggests it would be too
> expensive and not worth worrying about.

Hmm. Even in that case, xl_prev has to match. It's not like it's the
wild west. Sure, it's not nearly as good of a cross-check, but it's
something. It seems to me that it's not worth worrying very much about
xlp_seg_size or xlp_blcksz changing undetected in that scenario - if
you're doing that kind of advanced magic, you need to be careful
enough to not mess it up, and if we still cross-check once per
checkpoint cycle that's pretty good. I do worry a bit about the sysid
changing under us, though. It's not that hard to get your WAL archives
mixed up, and it'd be nice to catch that right away.

--
Robert Haas
EDB: http://www.enterprisedb.com