ASM required mirror free space to accommodate disk failures

Blogpost Dutch Prutser January 2 2013 03/01/2013

The summary of the blogpost

• REQUIRED_MIRROR_FREE_MB is the amount of free space required to restore redundancy after a failure that ASM can tolerate without affecting data availability. This amount depends on the redundancy level and the number of failure groups in the disk group. 
• Normal redundancy disk groups with at least three failure groups require an amount of free space that is equal to the size of a single failure group. Normal redundancy disk groups with only two failure groups require an amount of free space that is equal to the size of a single disk. 
• High redundancy disk groups with at least four failure groups require an amount of free space that is equal to the sum of the size of two failure groups. High redundancy disk groups with only three failure groups require an amount of free space that is equal to the sum of the size of two disks. 
• USABLE_FILE_MB is the amount of disk space available for storing user data. This amount depends on the total size of the disk group, the disk group redundancy and the amount of required free disk space that ASM needs to restore redundancy after a failure. USABLE_FILE_MB is calculated as follows:.
•  USABLE_FILE_MB = (FREE_MB – REQUIRED_MIRROR_FREE_MB) / [2|3]

Blogpost of Joel Goodman Monday, March 28, 2011 28/03/2011

Many ASM administrators have little or no experience with ASM mirroring, because they use External Redundancy for their diskgroups and ASM generally maintains one copy for each AU in this case. Tracking free space within a diskgroup in such a case is simple. Use the following details from view V$ASM_DISKGROUP to examine the redundancy, offline state and free space requirements for mirror recovery:

TOTAL_MB => indicates the total space in the diskgroup
FREE_MB => indicates the total free space  in the diskgroup
REQUIRED_MIRROR_FREE_MB => indicates free space required in the diskgroup to restore redundancy by copying allocation units as described above.
USABLE_FILE_MB => indicates how much of the FREE_MB may be safely used whilst leaving enough free space for mirror copy recovery in the case of disk failure
TYPE => indicates the redundancy attribute of the diskgroup.
OFFLINE_DISKS => indicates how many disks are offline in the diskgroup

Note: the LSDG command in the ASMCMD utility will provide the same information.

The crucial aspect of administering the diskgroups when ASM mirrors the AUs, is having enough free space to re-copy the AUs when a loss occurs. This is not a concern when external redundancy is used, but it is for normal and high redundancy, and requires that the ASM administrator be aware of the free space needs.