Thursday, July 29, 2010

ASM Architecture

In ASM Architecture, Oracle database utilizes  a separate smaller database instance, which is installed  in a separate oracle home and created during database  setup. 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.

fig show difference between conventional storage and asm storage.

ASM Instance Background Processes 

There are at least two new background processes added for an ASM instance:
   - RBAL - coordinates rebalance activity for disk groups 
   - ORB0, ORB1… - These perform the actual rebalance data extent movements.
     There can be many of these at a time

Database Instance ASM Background Processes 

Any database instance that is using an ASM disk group will contain a background process called OSMB. The OSMB process is responsible for communicating with the ASM instance. A second additional background process, called RBAL (just like in the ASM Instance) performs a global open on ASM disks. A global open means that more than one database instance can be accessing the ASM disks at a time.

For certain database operations like file creation, ASM intervention is required and the database foreground connects directly to the ASM instance. Whenever a connection is made to the ASM instance, the OSMB process is started dynamically. Database instances are only allowed to connect to one ASM instance at a time, so they have at most one OSMB background process.

Disk Group:- A basic component of ASM is the disk group. ASM is configured by creating disk groups, which in turn database instances can use as the default location for files created in the database. Oracle provides SQL statements to create and manage disk groups, their contents, and their metadata.
For information regarding commands to administer ASM go to ASM Administration post from home menu. 

Allocation Units (AU):-ASM introduces the concept of an allocation unit (AU), the smallest contiguous disk space allocated by the ASM. The typical value for an AU is 1MB and is not user configurable. ASM does not allow physical blocks to be split across allocation units.

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