Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 5 - Preparing Shared Storage

Mirroring

Storage Array or Hardware provides mirroring of disks or disk partitions protection against the possible loss of disks and disruption to the storage availability. With hardware mirroring, the operating system sees only one device and performs only one write. The hardware is responsible for ensuring that the write is directed to all mirrors and is logically atomic.

Mirroring can also be implemented at host level by using the volume management software. When implemented at host level, with software mirroring, there is the possibility of a system failure occurring after a write has completed on one side of the mirror, but before it has reached the other(s). Therefore, to ensure that mirror consistency can be re-established reasonably quickly after a crash, software mirroring maintains a dirty region log on disk. This identifies the mirror regions that have been written to recently, and thus might be inconsistent in the event of a system failure. The maintenance of a dirty region log adds a major performance overhead to software mirroring. In the past, Oracle has worked with some LVM vendors to eliminate the need for dirty region logs on mirrored Oracle Database files, but this facility is only available on some platforms and introduces extra complexity.

When the hardware mirroring is not available, ASM provides the best possible mirroring. No dirty region logs are needed, as Oracle manages to restore consistency between the mirrors using its own redo log data.

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