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TECH: Unix Semaphores and Shared Memory Explained (Doc ID 15566.1)

Last updated on SEPTEMBER 20, 2023

Applies to:

Oracle Database - Enterprise Edition - Version to [Release 8.1.7 to 12.1]
Gen 1 Exadata Cloud at Customer (Oracle Exadata Database Cloud Machine) - Version N/A and later
Oracle Cloud Infrastructure - Database Service - Version N/A and later
Oracle Database Cloud Exadata Service - Version N/A and later
Oracle Database Exadata Express Cloud Service - Version N/A and later
Generic UNIX


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. This paper primarily discusses the process Oracle goes through to
allocate shared memory and semaphores at instance startup. Other important
points unrelated to startup as well as some troubleshooting information will
be touched upon.


Understanding Oracle and Shared Memory/Semaphores.

IMPORTANT: This note applies to Oracle7 onwards but it is important to note that from onwards,
the SGA can be created in more than one shared memory segment.
See Note 1399908.1 for further details. 


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