Setting shared_buffers in postgresql.conf does not seem to take effect

Question

We're on CentOS release 6.6, PostgreSQL version 8.4.20. (Yes, this is not bleeding edge.)

In postgresql.conf, we have:

shared_buffers = 4096MB

kernel shm values are set nice and high:

[root@green data]# sysctl -a | grep shm
kernel.shmmax = 15922077696
kernel.shmall = 3887226
kernel.shmmni = 4096
kernel.shm_rmid_forced = 0

We have plenty of memory:

[root@green data]# free
             total       used       free     shared    buffers     cached
Mem:      31097812   30474972     622840    2873672    1961088   20565360
-/+ buffers/cache:    7948524   23149288
Swap:      1959920      93852    1866068

Yet the value for shared_buffers reported by pg_settings is only 512MB, not the 4GB set in postgresql.conf:

postgres=# select name, setting, min_val, max_val, context from  
pg_settings where name='shared_buffers';
      name      | setting | min_val |  max_val   |  context   
----------------+---------+---------+------------+------------
 shared_buffers | 524288  | 16      | 1073741823 | postmaster

Yes, we've done a full restart, and SHOW config_file confirms that I've edited the right postgresql.conf.

My very great thanks to anyone who can provide insight into this mystery.

Answer 1

The canonical unit for shared_buffers is pages of 8kB, so the actual memory allocated in bytes is:

524288 * 8192 = 4294967296 or 4096*1024*1024 as requested.

You can also check the size of the segment of memory with ipcs -m

Answer 2

What @Daniel explained so accurately becomes rather obvious when you add the respective column to your query:

SELECT name, setting, unit, min_val, max_val, context
FROM   pg_settings WHERE name = 'shared_buffers';

Or just:

SELECT * FROM pg_settings WHERE name = 'shared_buffers';

Consider the project guidelines for your "non-bleeding-edge" (a.k.a. outdated and unsupported) Postgres version.

Answer 3

got the same issue and when I ran the following query:

select * from pg_settings 

I got sourcefile pointing to postgresql.auto.conf, so removed the file as only shared_buffers value was set in the file and restarted

select *  from pg_settings where name='shared_buffers';

-[ RECORD 1 ]---+-------------------------------------------------------------
name            | shared_buffers
sourcefile      | /u02/pgsql/data/postgresql.conf