ASM disk groups

ASM employs several basic concepts. ASM operates on disk groups which is a collection of disks defined by ASM and transparent to the interface. Within the groups, it creates ASM datafiles. Therefore the main building blocks of ASM are:
•ASM disks
•ASM disk groups
•ASM files
•ASM templates
You can run the list data groups (lsdg) ASM command from a bash shell script to quickly see the disk space usage:
#!/bin/bash
. /home/oracle/set_oraenv
sid="+ASM1"
echo "Check Space on "$sid
export ORACLE_SID=$sid
asmcmd << EOF
lsdg
EOF
Let's explore these ASM concepts in detail.
Components of the ASM instance
There are several components within an ASM instance.
ASM disk groups
A disk group is basically one or more ASM disks that are managed as a single logical unit. Any data-structure stored in an ASM disk group is totally contained within that disk group, or self-contained. A database using ASM disks doesn't have to be shutdown in order for a disk to be added or dropped. ASM rebalances the spread of data to ensure an even I/O load to all disks in a disk group when the disk group configuration changes.

We mentioned that any single ASM file is self-contained in a single ASM disk group. However, an ASM disk group can contain files belonging to several databases, and a single database can use storage from multiple ASM disk groups. You can specify a disk group as the default disk group for files created in a database by specifying the disk group in file destination initialization parameters.

ASM divides the datafiles into 1MB extents and spreads the extents for each file evenly across all of the disks in a disk group. ASM uses pointers to record extent location instead of using a mathematical function to track the placement of each extent. When the disk group configuration changes, ASM moves individual extents of a file rather than having to move all extents to adhere to a formula based on the number of disks.

For files, such as log files, that require low latency, ASM provides fine-grained (128k) striping to allow larger I/Os to be split and processed in parallel by multiple disks. At file creation time, you can decide whether or not to use fine-grained striping. File type specific templates in the disk group