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Manage size of disk required in Oracle ASM

Manage size of disk required in Oracle ASM

V$ASM_DISKGROUP View help to manage the size of diskgroup required.

Reduced redundancy in Oracle ASM
Reduced redundancy means that one or more extents in the file are not mirrored at the expected level.
Example: High redundancy disk need 3 copies of file extent in failover groups but you have less than 3, you have two copies or less copies of the extent is called Reduce redundancy in ASM. Other causes of reduced redundancy files are disks running out of space or an insufficient number of failure groups.

V$ASM_DISKGROUP View
REQUIRED_MIRROR_FREE_MB indicates the amount of space required in a disk group to restore full redundancy without adding more space.
USABLE_FILE_MB indicates the amount of free space available for new files to restore redundancy after a disk failure
TOTAL_MB is the total usable capacity of a disk group in megabytes.
FREE_MB is the unused capacity of the disk group in megabytes.


SELECT name, type, total_mb, free_mb, required_mirror_free_mb, usable_file_mb FROM V$ASM_DISKGROUP;

 
NAME TYPE   TOTAL_MB FREE_MB REQUIRED_MIRROR_FREE_MB USABLE_FILE_MB
---- ------ -------- ------- ----------------------- --------------
DATA NORMAL 6144     3768    1024                    1372


Note: Negative Values of USABLE_FILE_MB
Negative value means you need to add more space in diskgroup.

Relation between USABLE_FILE_MB & FREE_MB & REQUIRED_MIRROR_FREE_MB
In case of Normal redundancy:
(FREE_MB – REQUIRED_MIRROR_FREE_MB) / 2 = USABLE_FILE_MB

Note:
–Depending on the value of FREE_MB, you may not be able to create new files.
–The next failure might result in files with reduced redundancy

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