Re: Replication/backup defaults

On 01/03/2017 11:56 PM, Tomas Vondra wrote:
> Hi,
>
...
> I'll push results for larger ones once those tests complete (possibly
> tomorrow).
>

I just pushed additional results (from the additional scales) to the git
repositories. On the larger (16/32-cores) machine with 2x e5-2620, the
results look like this

scale minimal replica logical
-----------------------------------------------------
100 23968 24393 24393
1000 23412 23656 23794
10000 5283 5320 5197

and on the smaller one (i5-2500k with 4 cores) I got this:

scale minimal replica logical
-----------------------------------------------------
50 5884 5896 5873
400 5324 5342 5478
1000 5341 5439 5425

The scales were chosen so that the smallest one fits into shared
buffers, the medium exceeds shared buffers but still fits into RAM, and
the largest scale exceeds RAM.

The results seem to confirm that for this workload (regular pgbench),
there's very little difference between the different WAL levels.
Actually, the 'replica' seems a tad faster than 'minimal', but the
difference may be easily due to noise.

I've also looked at the amount of WAL actually produced, by doing
pgbench runs throttled to the same throughput, and counting the number
of archived WAL segments & running pg_xlogdump. Interestingly enough,
those two metrics differ quite a bit - for example for scale 1000 (on
the 32-core machine), the 2h runs produced these number of WAL segments:

minimal: 5515 (88.2GB)
replica: 5587 (89.4GB)
logical: 6058 (96.9GB)

so 'replica' adds ~1.3% and 'logical' ~9.8%. But per pg_xlogdump, the
WAL amounts are only 73.3GB, 73.9GB and 74.4GB - a difference of only
~1.5% between minimal and logical. The values are also much lower than
raw WAL size, so I assume it's because pg_xlogdump ignores some extra
overhead, present in the segments. Moreover, the sequential nature of
WAL writes means even the +10% is not a big deal (unless it results in
saturating the bandwidth, but running on >90% is a bad idea anyway).

My conclusion from these results is that using 'wal_level=replica' by
default seems fine. Perhaps even wal_level=logical would be OK, but
that's probably a too big step for 10.0.

Any ideas how to construct a plausible workload where the differences
are significantly larger? Running the tests on non-SSD storage might
also be useful.

regards

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