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:
