Re: better page-level checksums

On Tue, Jun 14, 2022 at 1:32 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> On Tue, Jun 14, 2022 at 1:22 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > I still am not clear on precisely what you are proposing here. I do
> > agree that there is significant bit space available in pd_flags and
> > that consuming some of it wouldn't be stupid, but that doesn't add up
> > to a proposal. Maybe the proposal is: figure out how many different
> > configurations there are for this new kind of page space, let's say N,
> > and then reserve ceil(log2(N)) bits from pd_flags to indicate which
> > one we've got.
>
> I'm just making a general point. Why wouldn't we start out with the
> assumption that we use some pd_flags bit space for this stuff?

Technically we don't already do that today, with the 16-bit checksums
that are stored in PageHeaderData.pd_checksum. But we do something
equivalent: low-level tools can still infer that checksums must not be
enabled on the page (really the cluster) indirectly in the event of a
0 checksum. A 0 value can reasonably be interpreted as a page from a
cluster without checksums (barring page corruption). This is basically
reasonable because our implementation of checksums is guaranteed to
not generate 0 as a valid checksum value.

While pg_filedump does not rely on the 0 checksum convention
currently, it doesn't really need to. When the user uses the -k option
to verify checksums in passing, pg_filedump can assume that checksums
must be enabled ("the user said they must be so expect it" is a
reasonable assumption at that point). This also depends on there being
only one approach to checksums.

--
Peter Geoghegan