The V$ASM_DISKGROUP view contains the following columns that contain information to help you manage capacity:
REQUIRED_MIRROR_FREE_MB indicates the amount of space that must be available in a disk group to restore full redundancy after the worst failure that can be tolerated by the disk group without adding additional storage. This requirement ensures that there are sufficient failure groups to restore redundancy. Also, this worst failure refers to a permanent failure where the disks must be dropped, not the case where the disks go offline and then back online.
The amount of space displayed in this column takes the effects of mirroring into account. The value is computed as follows:
A normal redundancy disk group with more than two REGULAR failure groups
The value is the total raw space for all of the disks in the largest failure group. The largest failure group is the one with the largest total raw capacity. For example, if each disk is in its own failure group, then the value would be the size of the largest capacity disk.
A high redundancy disk group with more than three REGULAR failure groups
The value is the total raw space for all of the disks in the two largest failure groups.
USABLE_FILE_MB indicates the amount of free space, adjusted for mirroring, that is available for new files to restore redundancy after a disk failure. USABLE_FILE_MB is computed by subtracting REQUIRED_MIRROR_FREE_MB from the total free space in the disk group and then adjusting the value for mirroring. For example, in a normal redundancy disk group where by default the mirrored files use disk space equal to twice their size, if 4 GB of actual usable file space remains, then USABLE_FILE_MB equals roughly 2 GB. You can then add a file that is up to 2 GB. For information about negative values of USABLE_FILE_MB, refer to "Negative Values of USABLE_FILE_MB".
TOTAL_MB is the total usable capacity of a disk group in megabytes. The calculations for data in this column take the disk header overhead into consideration. The disk header overhead depends on the number of Oracle ASM disks and Oracle ASM files. This value is typically about 1% of the total raw storage capacity. For example, if the total LUN capacity provisioned for Oracle ASM is 100 GB, then the value in the TOTAL_MB column would be about 99 GB.
FREE_MB is the unused capacity of the disk group in megabytes, without considering any data imbalance. There may be situations where the value in the FREE_MB column shows unused capacity but because one Oracle ASM disk is full, database writes fail because of the imbalance in the disk group. Ensure that you initiate a manual rebalance to force even data distribution which results in an accurate presentation of the values in the FREE_MB column.
The results from the following query show capacity metrics for a normal redundancy disk group that consists of six 1 GB (1024 MB) disks, each in its own failure group:
SQL> 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
The REQUIRED_MIRROR_FREE_MB column shows that 1 GB of extra capacity must be available to restore full redundancy after one or more disks fail. The first three numeric columns in the query results are raw numbers. That is, they do not take redundancy into account. Only the last column is adjusted for normal redundancy. In the query output example for the data disk group, the calculation is as follows:
The V$ASM_DISKGROUP view contains the following columns that contain information to help you manage capacity:
REQUIRED_MIRROR_FREE_MB indicates the amount of space that must be available in a disk group to restore full redundancy after the worst failure that can be tolerated by the disk group without adding additional storage. This requirement ensures that there are sufficient failure groups to restore redundancy. Also, this worst failure refers to a permanent failure where the disks must be dropped, not the case where the disks go offline and then back online.
The amount of space displayed in this column takes the effects of mirroring into account. The value is computed as follows:
A normal redundancy disk group with more than two REGULAR failure groups
The value is the total raw space for all of the disks in the largest failure group. The largest failure group is the one with the largest total raw capacity. For example, if each disk is in its own failure group, then the value would be the size of the largest capacity disk.
A high redundancy disk group with more than three REGULAR failure groups
The value is the total raw space for all of the disks in the two largest failure groups.
USABLE_FILE_MB indicates the amount of free space, adjusted for mirroring, that is available for new files to restore redundancy after a disk failure. USABLE_FILE_MB is computed by subtracting REQUIRED_MIRROR_FREE_MB from the total free space in the disk group and then adjusting the value for mirroring. For example, in a normal redundancy disk group where by default the mirrored files use disk space equal to twice their size, if 4 GB of actual usable file space remains, then USABLE_FILE_MB equals roughly 2 GB. You can then add a file that is up to 2 GB. For information about negative values of USABLE_FILE_MB, refer to "Negative Values of USABLE_FILE_MB".
TOTAL_MB is the total usable capacity of a disk group in megabytes. The calculations for data in this column take the disk header overhead into consideration. The disk header overhead depends on the number of Oracle ASM disks and Oracle ASM files. This value is typically about 1% of the total raw storage capacity. For example, if the total LUN capacity provisioned for Oracle ASM is 100 GB, then the value in the TOTAL_MB column would be about 99 GB.
FREE_MB is the unused capacity of the disk group in megabytes, without considering any data imbalance. There may be situations where the value in the FREE_MB column shows unused capacity but because one Oracle ASM disk is full, database writes fail because of the imbalance in the disk group. Ensure that you initiate a manual rebalance to force even data distribution which results in an accurate presentation of the values in the FREE_MB column.
The results from the following query show capacity metrics for a normal redundancy disk group that consists of six 1 GB (1024 MB) disks, each in its own failure group:
SQL> 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 1372The REQUIRED_MIRROR_FREE_MB column shows that 1 GB of extra capacity must be available to restore full redundancy after one or more disks fail. The first three numeric columns in the query results are raw numbers. That is, they do not take redundancy into account. Only the last column is adjusted for normal redundancy. In the query output example for the data disk group, the calculation is as follows:
(FREE_MB - REQUIRED_MIRROR_FREE_MB) / 2 = USABLE_FILE_MB
(3768 - 1024) / 2 = 2744 / 2 = 1372
for further more information, review the link mentioned below: https://docs.oracle.com/database/121/OSTMG/GUID-CF644399-17BF-4A2B-B01A-8F11B90A5267.htm#OSTMG94125