Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 5 - Preparing Shared Storage

ASM Disk Groups

ASM divides files 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 determine the default behavior.

Most installations will probably have two or more disk groups. The reasons for having multiple different disk groups include the following:

• To group disks of different manufacturers, different sizes or performance characteristics.
   

• To group disks with different external redundancy together; for example, JBOD (just a bunch of disks) would generally not be in the same disk group with disks from a RAID 1+0 or RAID5 configuration, but this is possible using ASM.
   

• To separate work and recovery areas for a given database.

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