Re: WAL insert delay settings

From: Stephen Frost <sfrost(at)snowman(dot)net>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: WAL insert delay settings
Date: 2019-02-15 13:50:03
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* Andres Freund (andres(at)anarazel(dot)de) wrote:
> On 2019-02-14 11:02:24 -0500, Stephen Frost wrote:
> > On Thu, Feb 14, 2019 at 10:15 Peter Eisentraut <
> > peter(dot)eisentraut(at)2ndquadrant(dot)com> wrote:
> > > On 14/02/2019 11:03, Tomas Vondra wrote:
> > > > But if you add extra sleep() calls somewhere (say because there's also
> > > > limit on WAL throughput), it will affect how fast VACUUM works in
> > > > general. Yet it'll continue with the cost-based throttling, but it will
> > > > never reach the limits. Say you do another 20ms sleep somewhere.
> > > > Suddenly it means it only does 25 rounds/second, and the actual write
> > > > limit drops to 4 MB/s.
> > >
> > > I think at a first approximation, you probably don't want to add WAL
> > > delays to vacuum jobs, since they are already slowed down, so the rate
> > > of WAL they produce might not be your first problem. The problem is
> > > more things like CREATE INDEX CONCURRENTLY that run at full speed.
> > >
> > > That leads to an alternative idea of expanding the existing cost-based
> > > vacuum delay system to other commands.
> > >
> > > We could even enhance the cost system by taking WAL into account as an
> > > additional factor.
> >
> > This is really what I was thinking- let’s not have multiple independent
> > ways of slowing down maintenance and similar jobs to reduce their impact on
> > I/o to the heap and to WAL.
> I think that's a bad idea. Both because the current vacuum code is
> *terrible* if you desire higher rates because both CPU and IO time
> aren't taken into account. And it's extremely hard to control. And it
> seems entirely valuable to be able to limit the amount of WAL generated
> for replication, but still try go get the rest of the work done as
> quickly as reasonably possible wrt local IO.

I'm all for making improvements to the vacuum code and making it easier
to control.

I don't buy off on the argument that there is some way to segregate the
local I/O question from the WAL when we're talking about these kinds of
operations (VACUUM, CREATE INDEX, CLUSTER, etc) on logged relations, nor
do I think we do our users a service by giving them independent knobs
for both that will undoubtably end up making it more difficult to
understand and control what's going on overall.

Even here, it seems, you're arguing that the existing approach for
VACUUM is hard to control; wouldn't adding another set of knobs for
controlling the amount of WAL generated by VACUUM make that worse? I
have a hard time seeing how it wouldn't.



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