Wednesday, June 29, 2011

resizing SHMMAX

Anyone who has used Postgres and changed their memory settings to "greater than tiny" has surely run into this problem. (I'm sure this applies to other databases too - I believe I've run into this in Oracle also). The problem is the kernel won't let Postgres take as much shared memory as it is requesting.

postgres@mwrynnix:~$ service postgresql restart
 * Restarting PostgreSQL 9.0 database server                                                                                                                                                                    * The PostgreSQL server failed to start. Please check the log output:
4e0b32b0.2cd4.1(0) 2011-06-29 10:12:01.406 EDT @: FATAL:  could not create shared memory segment: Invalid argument
4e0b32b0.2cd4.2(0) 2011-06-29 10:12:01.406 EDT @: DETAIL:  Failed system call was shmget(key=5432001, size=139411456, 03600).
4e0b32b0.2cd4.3(0) 2011-06-29 10:12:01.406 EDT @: HINT:  This error usually means that PostgreSQL's request for a shared memory segment exceeded your kernel's SHMMAX parameter.  You can either reduce the request size or reconfigure the kernel with larger SHMMAX.  To reduce the request size (currently 139411456 bytes), reduce PostgreSQL's shared_buffers parameter (currently 16384) and/or its max_connections parameter (currently 104).
        If the request size is already small, it's possible that it is less than your kernel's SHMMIN parameter, in which case raising the request size or reconfiguring SHMMIN is called for.
        The PostgreSQL documentation contains more information about shared memory configuration.

(In case you didn't realize startup didn't work, it tells you [fail] at the end there.)

This page explains it all -

I would just like to add that in addition to changing the conf file (mentioned on that page), you can modify the file /proc/sys/kernel/shmmax to set it to the value you want NOW. You'll still need to change the .conf to "persist" the change when you reboot. Example below - changing 32 MB shmmax to 256MB:

root@mwrynnix:~# cat /proc/sys/kernel/shmmax
root@mwrynnix:~# echo 268435456 > /proc/sys/kernel/shmmax
root@mwrynnix:~# cat /proc/sys/kernel/shmmax

And now we start up Postgres...

root@mwrynnix:~# service postgresql start
 * Starting PostgreSQL 9.0 database server                                                                                                                                                              [ OK ]

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