Re: patch to allow disable of WAL recycling

On 2018-07-15 20:32:39 -0400, Robert Haas wrote:
> On Thu, Jul 5, 2018 at 4:39 PM, Andres Freund <andres@anarazel.de> wrote:
> > This is formulated *WAY* too positive. It'll have dramatic *NEGATIVE*
> > performance impact of non COW filesystems, and very likely even negative
> > impacts in a number of COWed scenarios (when there's enough memory to
> > keep all WAL files in memory).
> >
> > I still think that fixing this another way would be preferrable. This'll
> > be too much of a magic knob that depends on the fs, hardware and
> > workload.
> 
> I tend to agree with you, but unless we have a pretty good idea what
> that other way would be, I think we should probably accept the patch.

I don't think I've argued against that - I just want there to be
sufficient caveats to make clear it's going to hurt on very common OS &
FS combinations.


> I think part of the problem here is that whether a WAL segment is
> likely to be cached depends on a host of factors which we don't track
> very carefully, like whether it's been streamed or decoded recently.
> If we knew when that a particular WAL segment hadn't been accessed for
> any purpose in 10+ minutes, it would probably be fairly safe to guess
> that it's no longer in cache; if we knew that it had been accessed <15
> seconds ago, that it is probably still in cache.  But we have no idea.

True. Additionally we don't know whether, even if cold cache,
re-initializing files isn't worse performance-wise than recycling files.

Greetings,

Andres Freund