Re: increasing the default WAL segment size

On Tue, Mar 21, 2017 at 8:10 PM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
>> We've already
>> created quite a few incompatibilities in this release, and I'm not
>> entirely eager to just keep cranking them out at top speed.
>
> That position would seem to imply that you're in favor of keeping the
> current default of 16MB, but that doesn't make sense given that you
> started this discussion advocating to make it larger. Changing your
> position is certainly fine, but it'd be good to be more clear if that's
> what you meant here or if you were just referring to the file naming
> scheme but you do still want to increase the default size.

To be honest, I'd sort of forgotten about the change which is the
nominal subject of this thread - I was more focused on the patch,
which makes it configurable. I was definitely initially in favor of
raising the value, but I got cold feet, a bit, when Alvaro pointed out
that going to 64MB would require a substantial increase in
min_wal_size. I'm not sure people with small installations will
appreciate seeing that value cranked up from 5 segments * 16MB = 80MB
to, say, 3 segments * 64MB = 192MB. That's an extra 100+ MB of space
that doesn't really do anything for you. And nobody's done any
benchmarking to see whether having only 3 segments is even a workable,
performant configuration, so maybe we'll end up with 5 * 64MB = 320MB
by default.

I'm a little worried that this whole question of changing the file
naming scheme is a diversion which will result in torpedoing any
chance of getting some kind of improvement here for v11. I don't
think the patch is all that far from being committable but it's not
going to get there if we start redesigning the world around it.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company