Re: POSIX shared memory support

Chris, et al,

(commit-fest consensus discussion)
* Chris Marcellino wrote:
> In case you haven't had enough, here is another version of the code
> to make Postgres use POSIX shared memory. Along with the issues that
> have already been addressed, this version ensures that orphaned
> backends are not in the database when restarting Postgres by using a
> single 1 byte SysV segment to see who is attached to the segment
> using shmctl/IPC_STAT/nattach.

This really feels like a deal-breaker to me. My first reaction to this
patch, honestly, is that it's being justified for all the wrong reasons.
Changing to POSIX shm seems like a reasonable goal in general, provided
it can do what we need, but doing it to work around silly defaults
doesn't really work for me. If the real issue you have is with the SysV
limits then I'd suggest you bring that up with the kernel/distribution
folks to get them to use something more sane.

Looking around a bit, it looks like it's already being addressed in some
places, for example Solaris 10 apparently uses 1/4th of memory, while
Centos 5 uses 4GB. Suse also uses a larger default, from what I
understand. Supporting this effort to get it raised on various
platforms and distributions seems like a much better approach.

Additionally, it strikes me that there *is* a limit on POSIX shared
memory too, generally half of ram on the systems I've looked at, but
there's no guarentee that'll always be the default or that half of ram
will always be enough for us. So, even with this change, the problem
isn't completely 'solved'.

Finding a way for POSIX shm to do what we need, including Tom's
concerns, without depending on SvsV shm as a crutch work around, would
make this change much more reasonable and could be justified as moving
to a well defined POSIX standard, and means we may be able to support
platforms which either are new and don't implement SysV but just POSIX,
or cases where SysV is being actively depreceated. Neither of which is
possible if we're stuck with using it in some cases.

Thanks,

Stephen