If an oracle instance crashes abruptly, it may be possible that some shared memory segments are not released, if this happen you can use the ipcrm command to remove these segments.
The SHMMAX system setting allows you to increase the maximum size of a single shared memory segment.
To see how many oracle shared memory segments are used on your system
[oracle@dbbckp ~]$ ipcs
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 77266945 oracle 640 4096 0
0x00000000 77299714 oracle 640 4096 0
0xc8f5d818 77332483 oracle 640 4096 0
0x00000000 77004804 oracle 640 4096 0
0x00000000 77037573 oracle 640 4096 0
0x4975d098 77070342 oracle 640 4096 0
------ Semaphore Arrays --------
key semid owner perms nsems
0x56565656 688142 root 666 3
0x0000041e 786449 root 644 1
0x000003d4 819218 root 644 1
0x0000033d 851987 root 644 1
0xe5873228 233832468 oracle 640 125
0xe5873229 233865237 oracle 640 125
0xe587322a 233898006 oracle 640 125
0xe587322b 233930775 oracle 640 125
0xe587322c 233963544 oracle 640 125
0xe587322d 233996313 oracle 640 125
0xe587322e 234029082 oracle 640 125
0xe587322f 234061851 oracle 640 125
0xe5873230 234094620 oracle 640 125
0x66d899d4 233046045 oracle 640 125
------ Message Queues --------
key msqid owner perms used-bytes messages
0x000004d2 65537 root 666 0 0
But if your are running multiple oracle instances on the same server, this output doesn't tell you which instance is using which segments.
You can use the oracle sysresv utility to identify segments used by an instance (ORACLE_SID = DRTEST)
[oracle@dbbckp ~]$ sysresv
IPC Resources for ORACLE_SID "DRTEST" :
Shared Memory:
ID KEY
77004804 0x00000000
77037573 0x00000000
77070342 0x4975d098
Semaphores:
ID KEY
233046045 0x66d899d4
233078814 0x66d899d5
233111583 0x66d899d6
233144352 0x66d899d7
233177121 0x66d899d8
233209890 0x66d899d9
233242659 0x66d899da
233275428 0x66d899db
233308197 0x66d899dc
Oracle Instance alive for sid "DRTEST"
ipcrm -s <ID> for Semaphores
ipcrm -m <ID> for Shared Memory
