Getting Started with Oracle Database 10g - Automatic Storage Management
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Oracle Database 10g introduces Automatic Storage Management (ASM), a service that provides management of disk drives. ASM can be used on a variety of configurations, including Oracle9i Database RAC installations. ASM is an alternative to the use of raw or cooked file systems and is part of Oracle's overall desire to make management of the Oracle Database 10g easier, overall.
ASM Features
ASM offers a number of features, including:
- Simplified daily administration
- The performance of raw disk I/O for all ASM files
- Compatibility with any type of disk configuration, be it just a bunch of disks (JBOD) or complex Storage Area Network (SAN)
- Use of a specific file-naming convention to name files, enforcing an enterprise-wide file-naming convention
- Prevention of the accidental deletion of files, since there is no file system interface and ASM is solely responsible for file management
- Load balancing of data across all ASM managed disk drives, which helps improve performance by removing disk hot spots
- Dynamic load balancing of disks as usage patterns change and when additional disks are added or removed
- Ability to mirror data on different disks to provide fault tolerance
- Support of vendor-supplied storage offerings and features
- Enhanced scalability over other disk-management techniques
ASM can work in concert with existing databases that use raw or cooked file systems. You can choose to leave existing file systems in place or move the database datafiles to ASM disks. Additionally, new database datafiles can be placed in either ASM disks or on the preexisting file systems. Databases can conceivably contain a mixture of file types, including raw, cooked, OMF, and ASM files (though the management of such a system would be more complex).
ASM makes adding and removing of disks easy. It will rebalance the data on the underlying disks as they are added and removed. This results in the best possible performance for your database.
ASM is not a replacement for your existing OS file systems. The disks assigned to ASM will not be visible from the OS. Thus, ASM is not going to be used for Oracle software installs, administrative directories, or the location of parameter files.
The ASM Instance
ASM starts with the ASM instance that is responsible for the management of the various disk groups and the associated files. If ASM is being used by a database then the ASM instance must be started before the Oracle database instance is started. The ASM instance has a very small footprint and does not consume much in the way of resources. The ASM instance will mount the disks, create an extent map, and then pass this information on to the Oracle database instance when it is started. Note that ASM has few real “run-time” responsibilities and that all I/O requests to the ASM file system are handled by the Oracle database itself. The only time that ASM gets involved during run time is if there is some disk configuration change. This would include file removal or creation, or addition or removal of a disk.
To create an ASM instance, create a parameter file called init_osm.ora. In the ASM parameter file, you need to set the instance_type parameter to a value of OSM. The instance_type parameter defines the instances as an ASM instance. Generally, other ASM-related parameters take on default values, which can be accepted. Once the parameter file is created, start the ASM instance, using the startup command. Shutting down the ASM instance is done via the shutdown command (normal, immediate, force, and abort commands are supported). Note that this will result in the shutdown of all database instances related to the ASM instance and that the mode used to shut down the ASM instance will be the same mode used to shut down the associated database instances.
Also, the Oracle Database 10g DBCA can be used to create, configure, and start up an ASM instance. If ASM is already in use, the DBCA will allow you to take advantage of it. If ASM is not installed, you will be given an opportunity to install it.
NOTE: The DBCA will give you an option to create an ASM instance, if one does not already exist! This includes configuration of the parameter file of that instance.
 This chapter is from Oracle Database 10g New Features, by Robert Freeman (McGraw-Hill/Osborne, 2004, ISBN: 0072229470). Check it out at your favorite bookstore today. Buy this book now.
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