ASM Concepts

ASM also offers the benefits of mirroring and striping. An advantage of ASM over the conventional methods is the file based storage reliability policy. Hence, the same disk group can have a combination of files protected by mirroring or no protection. ASM does not manage binaries, alert logs, trace files, or password files.

ASM Architecture

For the ASM architecture, Oracle Database 10g utilizes a separate smaller database instance,  which is installed in a separate oracle home and created during database set up. An ASM instance manages the metadata that is needed to make ASM files available to regular database instances. ASM instance and database instances have access to a common set of disks called disk groups.

Database instances communicate with an ASM instance only to get information about the layout of these files and access the contents of ASM files directly. To create a database that uses storage managed by ASM, the ASM instance needs to be started.

An ASM instance has two new background process types, one for coordinating the disk group rebalance activity and one for data extent movements (ORB0, ORB1, etc.) Each database instance using ASM has two new background processes called OSMB and RBAL. In database instances, OSMB connects to foreground processes.  In ASM instances, RBAL performs global calls to open the disks in disk groups. Using these connections, periodic messages are exchanged to update statistics and to verify that both instances are running healthy.