Re: Lowering the default wal_blocksize to 4K

On Thu, Oct 12, 2023 at 9:05 AM Robert Haas <robertmhaas@gmail.com> wrote:
> But if we do want to keep those cross-checks, why not take what Thomas
> proposed a little further and move all of xlp_sysid, xlp_seg_size, and
> xlp_xlog_blcksz into XLOG_CHECKPOINT_REDO? Then long and short page
> headers would become identical.

FTR that's exactly what I was trying to say.

> We'd lose the ability to recheck those
> values for every new segment, but it seems quite unlikely that any of
> these values would change in the middle of replay. If they did, would
> xl_prev and xl_crc be sufficient to catch that? I think Andres says in
> a later email that they would be, and I think I'm inclined to agree.
> False xl_prev matches don't seem especially unlikely, but xl_crc seems
> like it should be effective.

Right, it is strong enough, and covers the common case where a record
crosses the segment boundary.

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.