Re: Checksums by default?

* Petr Jelinek (petr(dot)jelinek(at)2ndquadrant(dot)com) wrote:
> On 21/01/17 17:31, Stephen Frost wrote:
> > This is just changing the *default*, not requiring checksums to always
> > be enabled. We do not hold the same standards for our defaults as we do
> > for always-enabled code, for clear reasons- not every situation is the
> > same and that's why we have defaults that people can change.
>
> I can buy that. If it's possible to turn checksums off without
> recreating data directory then I think it would be okay to have default on.

I'm glad to hear that.

> >> The change of wal_level was supported by benchmark, I think it's
> >> reasonable to ask for this to be as well.
> >
> > No, it wasn't, it was that people felt the cases where changing
> > wal_level would seriously hurt performance didn't out-weigh the value of
> > making the change to the default.
>
> Really?

Yes.

> https://www.postgresql.org/message-id/d34ce5b5-131f-66ce-f7c5-eb406dbe026f@2ndquadrant.com

From the above link:

> So while it'd be trivial to construct workloads demonstrating the
> optimizations in wal_level=minimal (e.g. initial loads doing CREATE
> TABLE + COPY + CREATE INDEX in a single transaction), but that would be
> mostly irrelevant I guess.

> Instead, I've decided to run regular pgbench TPC-B-like workload on a
> bunch of different scales, and measure throughput + some xlog stats with
> each of the three wal_level options.

In other words, there was no performance testing of the cases where
wal_level=minimal (the old default) optimizations would have been
compared against wal_level > minimal.

I'm quite sure that the performance numbers for the CREATE TABLE + COPY
case with wal_level=minimal would have been *far* better than for
wal_level > minimal.

That case was entirely punted on as "mostly irrelevant" even though
there are known production environments where those optimizations make a
huge difference. Those are OLAP cases though, and not nearly enough
folks around here seem to care one bit about them, which I continue to
be disappointed by.

Even so, I *did* agree with the change to the default of wal_level,
based on an understanding of its value and that users could change to
wal_level=minimal if they wished to, just as I am arguing that same
thing here when it comes to checksums.

Thanks!

Stephen