The database instance behavior and the ab_.dat file

Some people still think that the ASM is which performs all IO activity of our database, I have seen several DBAs worried about to use ASM because of  the database will have a new "IO Layer", that is absolutely false because the ASM instance perform a few IO tasks, the most IO task are performed by the database instance and that is the goal of this article, to explain how the ASM instance and database instance work together in order to perform the IO operations. Through this article we will talk about the File extent Map, the ab_<ASM SID>.dat file, the importance of the Shared pool when we are using ASM, and so on.  The most important thing here is that you (the reader) understand how the IO operations are performed when you are using ASM.

What is the "ab_<ASM SID>.dat" file? This file is used by the database instance in order to messages ASM instance. When the database instance needs to send a message  to the ASM instance, the database instance reads this file in order to find out the enough information for getting connected to ASM instance. This file is in $ORACLE_HOME/dbs. If this file doesn't exist the database will not be able to connect to ASM instance and you will receive an error. This file is important because  is involved in the database instance work.

http://www.toadworld.com/platforms/oracle/w/wiki/10978.the-database-instance-behavior-and-the-ab-asm-sid-dat-file.aspx

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