ASM & RDBMS Interactions: Finding the ASM Instance

I thought it would be worthwhile having a couple of blog posts on how the RDBMS instance interacts with the ASM instance in various situations.

So I guess we should start at the beginning and ask how the RDBMS actually knows where to contact the ASM instance that it is storing its files in at all?

When an ASM instance is started it registers with the Cluster Synchronisation Services (CSS) daemon running as occsd.bin. This is part of Oracle Clusterware, but the occsd.bin runs in a single instance configuration as well.

Whenever an RDBMS instance needs for the first time to access files that reside on ASM storage and the RDBMS has not needed prior access to the ASM instance, the RDBMS needs to find the connection details for the ASM instance.

These connection details are NOT stored anywhere and you do not need to configure them. The RDBMS instance contacts CSS and gets passed the connect string for the ASM instance. The RDBMS then connects as sysdba to the ASM instance using OS authentication.

It’s actually the RDBMS’s ASMB background process that connects to the ASM instance and remains connected while there are ASM files open. Termination of this ASMB background process is fatal for the instance.


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