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Wednesday, 11 July 2012

An Introduction to Automatic Storage Management (ASM)

Automatic Storage Management (ASM) is a new feature introduced in Oracle Database 10g that provides the database administrator with a simple storage management interface that is consistent across all server and storage platforms. ASM provides the performance of sync I/O with the easy management of a file system.
Key Features of ASM
– Ease of Maintenance
– Stripes files rather than logical volumes
– Enables online disk reconfiguration and dynamic rebalancing
– Provides adjustable rebalancing speed
– Provides file based redundancy
– Supports only Oracle files
Why ASM?
To find answer to this question look at its storage management feature and benefit of ASM. It clearly states why we need ASM
Stripping/Mirroring: ASM includes striping and mirroring to provide balanced and secure storage. The level of redundancy and the granularity of the striping can be controlled using templates. The new ASM functionality can be used in combination with existing raw and cooked file systems, along with OMF and manually managed files.
Direct I/O: By making use of Direct I/O, higher cache hit ratio can be achieved. Buffered I/O uses most important resources like CPU and memory. In case of buffered I/O Oracle blocks are cached both in the SGA and in the file system buffer cache.
Buffered I/O fills up the file system cache with Oracle Data, where as using the Direct I/O allows non-Oracle data to be cached in the file system much more efficiently.
Benefits of ASM
One unique benefit of ASM is that mirroring is applied on file basis, rather than on a volume basis. Therefore, the same disk group can contain a combination of files protected by mirroring, or not protected at all.
The ASM supports data files, log files, control files, archive logs, Recovery Manager (RMAN) backup sets, and other Oracle database file types.
The ASM supports Real Application Clusters (RAC) and eliminates the need for a cluster Logical Volume Manager or a cluster file system.
One of the flexible feature of ASM is it does not eliminate any existing database functionality which uses non ASM files. Existing database are able to operate as they always have been. New files may be created as ASM files, while existing ones are administered in the old way or can be migrated to ASM.
ASM Functionality
The ASM functionality is controlled by an ASM instance. The main components of ASM are disk groups, each of which comprise of several physical disks that are controlled as a single unit. The physical disks are known as ASM disks, while the files that reside on the disks are know as ASM files.
The ASM divides a file into pieces and spreads them evenly across all the disks. The ASM uses an index technique to track the placement of each piece. Traditional striping techniques use mathematical functions to stripe complete logical volumes. The ASM includes mirroring protection without the need to purchase a third-party Logical Volume Manager.
Any single ASM file is contained in only one disk group. However, a disk group may contain files belonging to several databases, and a single database may use storage from multiple disk groups. ASM files are always spread across all ASM disks in the disk group. The ASM disks are partitioned in allocation units (AU) of on megabyte each. An AU is the smallest contiguous disk space that ASM allocates. The ASM does not allow physical blocks to split across AUs.
Note: ASM is shipped with the database and available as part of base code set and there is no need to go through a separate installation in the custom tree installation. It is available in both the Enterprise Edition and Standard Edition installations.
The ASM Instance
The ASM instance is a separate instance, that is mounted but never has a database associated with it. You will only create one ASM instance per node. The Oracle Database Configuration Assistant (DBCA) can be used to create the ASM instance or you can choose to create the ASM instance manually.


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