ASM Mirroring

ASM can provide RAID like protection for your data. ASM provides redundancy via failure groups, essentially when you create a disk group you assign disks that are members of the disk group to a particular failure group. ASM will then mirror the data between these failure groups. The idea being you keep disks that are dependent on the same hardware in the same failure group, so failure of a particular component does not impact the availability of your disk group.

ASM actually mirrors at the extent level. When an extent is allocated there is the concept of a primary extent and a secondary extent, essentially a primary copy and a mirror copy and these are allocated within different failure groups. The above diagram shows primary extents in purple and secondary extents in red (each square represents an extent).

By default, ASM always reads the primary copy of an extent. At first I thought this was a big limitation and would reduce the I/O bandwidth available, but of course ASM mirroring ensures the primary copies are spread across the failure groups so that the I/O is spread over the maximum number of drives.

Frits Hoogland pointed out to me this may still be less efficient than traditional mirroring in that ASM has no chance of optimising a read based on which underlying physical device has the closest disk head position.

To be fair, Oracle themselves to state that external redundancy is preferred unless you have particular requirements that can only be provided with a software RAID solution. Oh and yeah, preferred_read_failure_groups seem like a real win for extended clusters if that is your bag.

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