Why Oracle creates two shared memory segments instead of one?

From: Vsevolod Afanassiev <vafanassiev_at_yahoo.com>
Date: Thu, 25 Oct 2007 15:44:45 -0700
Message-ID: <1193352285.026233.151260@q5g2000prf.googlegroups.com>

I noticed that on some platforms Oracle always creates two shared memory
segments instead of one, even when both segments are much smaller than the
limit.

For example, on this Tru64 box the limit is set to 2 GB:

oracle > sysconfig -q ipc
ipc:

msg_max = 8192
msg_mnb = 16384
msg_mni = 64
msg_tql = 40
shm_max = 2147483640 <<<---- Limit set to 2 GB
shm_min = 1
shm_mni = 1024

Still Oracle creates two segments: one 8 MB, second 900 MB

oracle> ipcs -am

Shared Memory:

T      ID        KEY    MODE         OWNER    GROUP  CREATOR   CGROUP
NATTCH                SEGSZ   CPID   LPID   ATIME    DTIME    CTIME
m    1024     0x1e34 --rw-rw-rw-      root   daemon     root
daemon      4               524288 3679586 4104730 11:32:05 11:32:04
21:14:45
m       2          0 --rw-r-----    oracle      dba   oracle
dba      9              8388608 4090358 4115445  8:40:03  8:40:13
10:58:11
m       3 0x1c6492d0 --rw-r-----    oracle      dba   oracle
dba      9            947912704 4090358 4115445  8:40:03  8:40:13

10:58:11 Received on Thu Oct 25 2007 - 17:44:45 CDT