Clear Shared Memory And Semaphore (ORA-27121)

What Is Shared Memory and Semaphores

Shared memory and semaphores are two important resources for an Oracle instance on UNIX.
An instance cannot start if it is unable to allocate what it needs.

Shared memory is a memory region that can be shared between different processes. Oracle uses shared memory for implementing the SGA, which needs to be visible to all database sessions.  Shared memory is also used in the implementation of the SQL*Net V1 Fast driver as a means of communicating between the application and shadow process. Each shadow process stores its PGA in a shared memory segment. In the latter two cases, Oracle allocates the shared memory dynamically as opposed to the allocation of the SGA, which occurs at instance STARTUP.

Semaphores can be thought of as flags. They are either on or off state.  A process can turn ON, the flag or turn it OFF. If the flag is already on, processes who try to turn on the flag will sleep until the flag is off.  Upon awakening, the process will reattempt to turn the flag on, possibly succeeding or possibly sleeping again.  Such behaviour allows semaphores to be used in implementing a post-wait driver - a system where processes can wait for events (i.e. wait on turning on a semaphore) and post events (i.e. turning of a semaphore).  This mechanism is used by Oracle to maintain concurrency control over the SGA, since it is writeable by all processes attached. Also, for the same reasons, use of the Fast Driver requires additional semaphores.  However, these semaphores will be allocated dynamically instead of at instance STARTUP.

Configuring Shared Memory and Semaphores

http://docs.oracle.com/cd/E19575-01/821-0182/abdco/index.html

Clear Shared Memory And Semaphore (HP-UX)

Sometimes it happens that after a crash or after an abnormal shutdown the oracle database instance does not release the shared memory or semaphores.

Error
##################################################
SQL> startup
ORA-27121: unable to determine size of shared memory segment
Error 13: Permission denied
##################################################

Cause
SHUTDOWN ABORT
CLUSTER FAILOVER
KILL PMON

Action
Shared Memory:
# ipcs -pmb | egrep 'oracle|orar12'

m  997425174 0xf5039ea8 --rw-rw----     oracle       dba
m     163864 0x4c17eb4c --rw-------        oracle       dba
m     786457 0x00000000 --rw-rw----      orar12      dba

Clear Shared Memory

ipcrm -m <SharedMemory ID>

Semaphores:
# ipcs -s |egrep 'oracle|orar12'

s   10321961 0xd9da4e50 --ra-------    oracle       dba
s   77168682 0x709a05d8 --ra-ra----    oracle       dba
s    2162731 0xd9da4ff0 --ra-------       orar12      dba

Clear Semaphores
ipcrm -s <SemaphoreID>

Allocate More Semaphores in KERNEL Parameter File

If Oracle Database creates more segments for the instance than the number of protection keys, then increase the value of the shmmax kernel parameter.


How To identify the Semaphores for a particular Oracle Instance
Get the shared memory and semaphore IDs

In Oracle 8i
$ export ORACLE_SID=ORCL
$ svrmgrl
SVRMGR> connect internal
SVRMGR> oradebug ipc
Information written to trace file.
SVRMGR> exit


In 9i & Above

$ export LD_LIBRARY_PATH=$ORACLE_HOME/lib
$ cd $ORACLE_HOME/bin
$ sysresv
$ sysresv ORCL

IPC Resources for ORACLE_SID "ORCL" :
Shared Memory:
ID              KEY
324468764       0x0f73169c
Semaphores:
ID              KEY
22085675        0x13c503f0
Oracle Instance alive for sid "ORCL"


Check the Semaphores ID and compare with above

ps -ef | grep ORCL| grep LOCAL

  oracle 14848     1  0 07:11:10 ?         2:05 oracleORCL (LOCAL=NO)

SQL> oradebug setospid 14848
SQL> oradebug ipc

In udump location

List of Semaphore ID as below

Number of semaphore sets:        = 1
Semaphore identifiers:           = 1
Semaphore List=
35684397
-------------- system semaphore information -------------
IPC status from /dev/kmem as of Tue Oct  1 14:21:45 2013
T         ID     KEY        MODE        OWNER     GROUP   CREATOR    CGROUP NSEMS   OTIME    CTIME
Semaphores:
s          0 0x4f20037e --ra-------      root      root      root      root     1 14:20:40 19:48:16
s          1 0x412011f3 --ra-ra-ra-      root      root      root      root     1 16:32:00 19:48:17