Re: Checksums by default?

On 21/01/17 17:51, Stephen Frost wrote:
> * 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.

Which is random usecase very few people do on regular basis. Checksums
affect *everybody*.

What the benchmarks gave us is a way to do informed decision for common
use. All I am asking for here is to be able to do informed decision as
well.

--
Petr Jelinek http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services