PostgreSQL's share_buffer calculation using shmget() versus kernel.shmmax

[Mel Llaguno <mllaguno@xxxxxxxxxxxx>]

All,

I'm wondering about how postgresql calculates the value for shared buffers as I see some discrepancies with what the following script provides versus what is recommended in the pgctl.log when the database fails to start.

#!/bin/bash

# simple shmsetup script

page_size=`getconf PAGE_SIZE`

phys_pages=`getconf _PHYS_PAGES`

shmall=`expr $phys_pages / 2`

shmmax=`expr $shmall \* $page_size` 

echo kernel.shmmax = $shmmax

echo kernel.shmall = $shmall

Any pointers which would explain these differences would be greatly appreciate. Ultimately, I'd like to calculate the expected kernel.shmmax which matches postgresql's shmget() call.

TIA,

Mel