1 minutes readFail Groups mirroring

• Mirroring is not implemented like RAID 1 arrays (where a disk is partnered with another disk)
• ASM does not mirror actual disks but does it’s mirroring at the extent level
• Mirroring occurs at the file extent level and these extents are distributed among several disks known as “partners”
• Partner disks will reside in one or more separate failure groups
• ASM automatically choses partners and limits the number of them to less than 10 (varies by RDBMS version) in order to contain the overall impact of multiple disk failures
• If a disk fails, then ASM updates its extent mapping such that reads will now occur on the surviving partners

          This is one example when ASM and the RDBMS communicate with each other

• If the disk cannot be onlined, it must be dropped. A new disk will be installed and ASM will copy the data back via a “rebalancing” operation. This happens automatically in the background