Oracle ASM Fast Mirror Resync

What is ASM Fast Mirror Resync and How It Works

Oracle ASM Fast Mirror Resync is a new feature introduced in 11g,to restore the redundancy faster after a transient disk path failure.

What is Transient Failures

Any problems that make a failure group temporarily unavailable are considered transient failures. transient failures can be caused by disk path malfunctions, such as
cable failures, host bus adapter failures, controller failures, or disk power supply interruptions.

How Oracle ASM Fast Mirror Resync Works

Oracle ASM fast resync keeps track of pending changes to extents on an offline disk during a disk outage. The extents are resynced when the disk is brought back online.
The feature is supported by disk_repair_time attribute of the Diskgroup. ASM keeps track of changed extents until disk_repair_time and apply the changes if the disk is back online with in this period else it drops the disk and initiates rebalance operation.
As you can see the rebalance operation is only initiated, if the disk is dropped after disk_repair_time. Without Oracle ASM Fast Mirror Resync feature the rebalance is done once a disk is dropped and again once the disk is added back to the diskgroup and therefore with OFMR it saves quite some IO resulting in performance improvement and if the disk is back online with in disk_repair_time, only changes need to be applied, resulting faster synchronization.

How to Find the Disk_Repair_Time Attribute Setting for a Diskgroup

ASMCMD> lsattr -l -G TEST_DATA disk_repair_time
Name Value
disk_repair_time 3.6h <= 3.6 H is default

How to Change the Disk_Repair_Time Attribute Setting for a Diskgroup

ASMCMD> setattr -G TEST_DATA disk_repair_time 90M
OR
SQL>ALTER DISKGROUP data SET ATTRIBUTE 'disk_repair_time' = '4.5h'
Note:- You need compatible.rdbms aand compatible.asm ttribute minimum 11.1.0.0.0 to set disk_repair_time attribute.
You can specify the value of disk_repair_time in Minutes of Hours (M or H). If no unit specified the default is Hours.

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