what is an ASM INSTANCE in oracle

[srihari]

hai
can any one tell me what is an ASM instance in oracle and how to remove that manually.

[techguru]

Automated Storage Management (ASM) is a file management system introduced in Oracle10g. This feature removes the need to have a file system or a volume manager for managing the database files and eliminates the complexity of balancing data across disks and controllers manually. Instead, the ASM stripes data across the disks automatically, thereby maximizing throughput and minimizing the risk of data loss. The ASM also maintains the data striping as disks are added/removed and re-balances the input/output load dynamically.

ASM doesn't have to be installed in order to install an Oracle database. To use ASM files, there must be at least one ASM instance configured and started prior to starting a database instance that uses ASM files. As part of the ASM instance startup procedure, the various disk groups and their files are identified. The ASM instance mounts the disks, and then creates an extent map, which is passed to the database instance. The database instance itself is responsible for any actual input/output operations.

The ASM instance is only involved during the creation or deletion of files and when disk configurations change (such as dropping or adding a disk). When these types of changes occur, the ASM instance automatically rebalances the disks and provides the necessary information to refresh the extent map in the SGA of the database instance. Of course, this process requires that the ASM instance run concurrently with the database instance, and only shut down after the database instance is closed.

The impact of the ASM instance on performance of the database instance is minimal. The former does not process transactions affecting the individual database objects; therefore, the average SGA allocation needed by the instance is no more than 64MB. Unless the server's memory is already at the maximum recommended operating system/DBMS allocation, 64MB should have no impact on the memory available for the database instance.

http://ist.uwaterloo.ca/~baumbach/ORACLE10g/asm.html

[madhusri]

Automatic Storage Management

Spare Capacity Recommendation for ASM Normal and High Redundancy Disk Groups
ASM does not require hot spare drives to protect against disk failures, but it does require spare capacity. You must have sufficient spare capacity in your disk group to handle the largest failure you are willing to tolerate. After a disk fails, the reconstruction process depletes the space capacity and as a result, you may not have enough space to successfully create new files. The following guidelines will ensure that you have sufficient space to create files even if you suffer a disk failure.
In a normal redundancy disk group, you should have enough free space in your disk group to tolerate the loss of disks in one failure group.
In a high redundancy disk group, you should have enough free space to cope with the loss of two failure groups.

[Ralph]

That's a very detailed explanation on ASM INSTANCE in oracle. Can you tell me the commands that are related to the usage of ASM INSTANCE and ASM files in oracle.