Re: Estimating total amount of shared memory required by postmaster

Alexey Klyukin <alexk@commandprompt.com> writes:
> We've recently come across the task of estimating the size of shared memory
> required for PostgreSQL to start.

> ...

> - Try to actually allocate the shared memory in a way postmaster does this
>   nowadays, if the process fails - analyze the error code to check whether the
>   failure is due to the shmmax or shmmall limits being too low. This would
>   need to be run as a separate process (not postmaster's child) to avoid
>   messing with the postmaster's own shared memory, which means that this would
>   be hard to implement as a user-callable stored function.

The results of such a test wouldn't be worth the electrons they're
written on anyway: you're ignoring the likelihood that two instances of
shared memory would overrun the kernel's SHMALL limit, when a single
instance would be fine.

Given that you can't do it in the context of a live installation, just
trying to start the postmaster and seeing if it works (same as initdb
does) seems as good as anything else.
        regards, tom lane