Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 8 - RAC Administration

Oracle Disk Manager

Many of the file system/volume manager vendors have incorporated the ODM interface. Well known examples include, Veritas Database Edition (Advanced Cluster for Oracle Database 10g RAC) and Polyserve Matrix Server for Oracle 10g RAC. Several ODM semantics have been included in the DAFS v1.0 Protocol Specifications offered by Network Appliances.

Oracle Database 10g ASM (Automated Storage Management)

ASM divides files into 1 MB extents and spreads the extents for each file evenly across all of the disks in a disk group. ASM does not use a mathematical function to track the placement of each extent; it uses pointers to record extent location. This allows ASM to move individual extents of a file when the disk group configuration changes without having to move all extents to adhere to a formula based on the number of disks.

As said above, for normal datafiles, ASM uses 1 megabyte extents. For 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. The DBA decides at creation time whether or not to use fine-grained striping. File-type specific templates in the disk group determine the default behavior.