On 2017-02-06 16:06:25 -0500, Tom Lane wrote:
> Andres Freund <and...@anarazel.de> writes:
> > On 2017-02-06 15:39:10 -0500, Peter Eisentraut wrote:
> >> On 2/6/17 6:28 AM, Konstantin Knizhnik wrote:
> >>> I wonder why do we prohibit now configuration of Postgres without mmap?
> >> It's not really prohibited, but it's not something that people generally
> >> need, and we want to keep the number of configuration variations low.
> > I think that was a fairly bad call. Making it hard to use anything but
> > mmap (on mmap supporting platforms) caused a fair bit of trouble and
> > performance regressions on several platforms by now (freebsd reported it
> > fairly quickly, and now aix), all to avoid a trivial amount of code and
> > one guc.
> > FWIW, there's a patch somewhere in the archive making it configurable.
> Clearly we should do something, but I'm not sure that a GUC is the right
> answer; far too few people would set it correctly.  I think it might be
> better to have the per-platform "template" files decide whether to set

Well, sysv shmem will be less "comfortable" to use on those platforms
too. And you'll usually only hit the performance problems on bigger
installations. I don't think it'll be an improvement if after an upgrade
postgres doesn't work anymore because people have gotten used to not
having to configure sys shmem.

I suspect a better solution would be to have a list GUC with a platform
dependant default (i.e. sysv, anonymous on freebsd/aix; the other way
round on linux). At startup we'd then try those in order.



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