Re: Let PostgreSQL's On Schedule checkpoint write buffer smooth spread cycle by tuning IsCheckpointOnSchedule?

On Wed, Dec 23, 2015 at 2:16 PM, Tomas Vondra
<tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>> Another point (which Jan Wieck made me think of) is that the optimal
>> behavior here likely depends on whether xlog and data are on the same
>> disk controller. If they aren't, the FPW spike and background writes
>> may not interact as much.
>
> I'm not sure what exactly you mean by "optimal behavior" here. Surely if you
> want to minimize interference between WAL and regular I/O, you'll do that.
>
> But I don't see what that has to do with the writes generated by the
> checkpoint? If we do much more writes at the beginning of the checkpoint
> (due to getting confused by FPW), and OS starts flushing that to disk
> because we exceed dirty_(background)_bytes, that surely interferes with
> reads (which is a major issue for queries).

Well, it's true that the checkpointer dirty page writes could
interfere with reads, but if you've also got lots of FPW-bloated WAL
records being written to the same disk at the same time, I would think
that'd be worse. No?

--
Robert Haas
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