Re: Lowering the default wal_blocksize to 4K

Hi,

On 2023-10-10 21:30:44 +0200, Matthias van de Meent wrote:
> On Tue, 10 Oct 2023 at 06:14, Andres Freund <andres@anarazel.de> wrote:
> > 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.
> 
> Hmm. I don't think we should remove those checks, as I can see people
> that would want to change their XLog block size with e.g.
> pg_reset_wal.

I don't think that's something we need to address in every physical
segment. For one, there's no option to do so. But more importantly, if they
don't change the xlog block size, we'll just accept random WAL as well. If
somebody goes to the trouble of writing a custom tool, they can live with the
consequences of that potentially causing breakage. Particularly if the checks
wouldn't meaningfully prevent that anyway.


> After that we'll only have the system ID left from the extended
> header, which we could store across 2 pages in the (current) alignment
> losses of xlp_rem_len - even pages the upper half, uneven pages the
> lower half of the ID. This should allow for enough integrity checks
> without further increasing the size of XLogPageHeader in most
> installations.

I doubt that that's a good idea - what if there's just a single page in a
segment? And there aren't earlier segments? That's not a rare case, IME.

Greetings,

Andres Freund