Re: Lowering the default wal_blocksize to 4K

Hi,

On 2023-10-09 23:16:30 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > There's an alternative approach we could take, which is to write in 4KB
> > increments, while keeping 8KB pages. With the current format that's not
> > obviously a bad idea. But given there aren't really advantages in 8KB WAL
> > pages, it seems we should just go for 4KB?
> 
> Seems like that's doubling the overhead of WAL page headers.  Do we need
> to try to skinny those down?

I think the overhead is small, and we are wasting so much space in other
places, that I am not worried about the proportional increase page header
space usage at this point, particularly compared to saving in overall write
rate and increase in TPS. There's other areas we can save much more space, if
we want to focus on that.

I was thinking we should perhaps do the opposite, namely getting rid of short
page headers. The overhead in the "byte position" <-> LSN conversion due to
the differing space is worse than the gain. Or do something inbetween - having
the system ID in the header adds a useful crosscheck, but I'm far less
convinced that having segment and block size in there, as 32bit numbers no
less, is worthwhile. After all, if the system id matches, it's not likely that
the xlog block or segment size differ.

Greetings,

Andres Freund