Oracle Tips by Burleson

Oracle10g Grid Computing with RAC
Chapter 5 - Preparing Shared Storage

Load balancing (SAME)

Oracle has been promoting the concept of SAME – Stripe and Mirror Everything, which aimed at using the full I/O capacity of the all the disks. Many database users have implemented “SAME sets”, each of about 4 mirrored disk pairs. However, the SAME functionality does not completely eliminate the disk imbalances. There are different types of files where the IO activity is not uniform, and the configuration of SAME is not the cure for all IO ills.

ASM goes a step forward, in balancing the IO activity. Rebalancing distributes file data evenly across all the disks of the ASM Disk Group. ASM automatically rebalances a disk group whenever disks are added or dropped. ASM ensures that a file is evenly spread across all disks in a disk group when the file is allocated, so rebalancing is not required except when the storage configuration changes. With the I/O balanced whenever files are allocated and whenever the storage configuration changes, you never need to search for hot spots in a disk group and manually move data to restore a balanced I/O load. It also allows the addition of disks online. There are extra background processes that conduct the rebalancing activity in a non-intrusive way.

Thus, the ASM makes manual load balancing unnecessary, because the dynamic rebalancing overcomes the major drawback of SAME. When new disks are added to a disk group, rebalancing happens automatically without an outage.

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