Applies to:

Oracle Server - Enterprise Edition - Version: 9.2.0.6 to 11.2.0.3 - Release: 9.2 to 11.2
z*OBSOLETE: Microsoft Windows Server 2003
***Checked for relevance on 19-Dec-2011***

Symptoms

ORA-27100 is reported during a manual startup of the database .

Restarting the service, the same error is encountered.
Rebooting the server does not help as well (because that also restarts the service).

Changes

The parameter sga_max_size was increased, e.g. from 2GB to 3GB.

Cause

Changing the parameter sga_max_size (from OEM or directly on the SQL prompt) will make the change in the spfile.

ORA-27100 is received as a result of changes made to parameters affecting the SGA size.
The service was created to start with an spfile.
When the service is restarted, it will use the spfile and the error occurs due to the SGA parameter change.

Test Case:

The actual root cause is in fact the limitation of physical memory.
When the service is not restarted but the instance is stopped and restarted after increasing sga_max_size, the next errors can be observed:

Solution

To be able to start the instance/database again, please use the next steps:

C:\> sqlplus / as sysdba

Problem Description:
==================== You are trying to start an Oracle instance when ORA-27100 is reported.ORA-27100: shared memory realm already exists Cause: Tried to start duplicate instances, or tried to restart an instance that had not been properly shutdown Action: Use a different instance name, or cleanup the failed instance's SGA The instance does not come up successfully and instance recovery is required.
However, instance recovery cannot complete because the instance is requiring
too much memory and will not start.Bringing the database up in stages reveals ORA-27102 during a startup nomount.Solution Description:
===================== The size of the SGA must be reduced so the instance can startup or the new
software must be removed. To determine if the problem is memory related:1. Start svrmgr30 or connect as the internal SYSDBA user.SVRMGR> Connect as internal/userpassword@SID
SVRMGR> shutdown immediate
SVRMGR> startup nomount pfile=C:\ORANT\DATABASE\initSID.oraor with 9i and 10gsqlplus /nolog
SQL> connect / as sysdba
SQL> shutdown immediate
SQL> startup nomount pfile='C:\ORANT\DATABASE\initiSID.ora'ORA-27102 will be reported if the problem is memory related. The instance
cannot start. Proceed to step 2.2. shutdown immediateReduce one or more of the following initSID.ora parameter values using a text
editor:   shared_pool_size   (affects the library and data dictionary cache)
db_block_buffers   (affects the database buffer cache)
log_buffer         (affects the redo log buffer area)3. Return to Server Manager or SQL*Plus and try to start the instance again.SVRMGR> startup nomount pfile=C:\ORANT\DATABASE\initSID.oraor SQL>  startup nomount pfile='C:\ORANT\DATABASE\initSID.ora'If the instance startup still fails with ORA-27102 repeat steps 1-3
until the instance starts. Once it starts proceed to step 4.4. At the Server Manager prompt or in SQL*Plus issue:alter database mount
alter database openIf you are on a unix machine, you can clean up from last shutdown.1) Delete $ORACLE_HOME/dbs/sgadef<sid>.dbf files (only applicable forversons prior to 8.1.x).2) Delete $ORACLE_HOME/dbs/lk<sid> files3) If more than one instance on this box, need to identify only thesemaphores and shared memory associtated with that instance.a) SVRMGR>connect internalb) SVRMGR>oradebug ipc  (This will list all shared memory and semaphoresfor this instance)or on 9i and 10ga) sqlplus /nologb) connect / as sysdbac) oradebug ipc4) Remove shared memory and semaphores for this instance.a) ipcs (Lists all Interprocesses)b) ipcrm -m <id> (Removes Shared Memory Processes)c) ipcrm -s <id> (Removes Semaphores)After these steps the database should be able to startup without
any errors and without reducing the size of the SGA.Explanation:
============ ORA-27100 is received as a result of changes made to initSID.ora parameters
affecting the SGA size, or a new software was recently installed on the
database server which requires additional memory.Reducing the size of the SGA allows the instance to start and recover. The
database can then be mounted and opened successfully.References:
=========== Administrators Guide
Reference Guide
Concepts Guide