Re: [HACKERS] Checksums by default?

On 01/21/2017 05:51 PM, Stephen Frost wrote:
> * Petr Jelinek (petr.jelinek@2ndquadrant.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.
>

You make it look as if we swept that case under the carpet, despite 
there being quite a bit of relevant discussion in that thread.

We might argue how many deployments benefit from the Wal_level=minimal 
optimization (I'm sure some of our customers do benefit from it too), 
and whether it makes it 'common workload' or not.

It's trivial to construct a workload demonstrating pretty arbitrary 
performance advantage of the wal_level=minimal case. Hence no point in 
wasting time on demonstrating it, making the case rather irrelevant for 
the benchmarking.

Moreover, there are quite a few differences between enabling checksums 
by default, and switching to wal_level=minimal:

- There are reasonable workaround that give you wal_level=minimal back 
(UNLOGGED tables). And those work even when you actually need a standby, 
which is pretty common these days. With checksums there's nothing like that.

- You can switch between wal_level values by merely restarting the 
cluster, while checksums may only be enabled/disabled by initdb.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services