Oracle asm中 Usable_FILE_MB 和 REQUIRED_MIRROR_FREE_MB的含义
参考自： How to Calculate Usable_FILE_MB / REQUIRED_MIRROR_FREE_MB (文档 ID 1459611.1)
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是：
除了mirror之外，考虑到 “当一个disk 嗝屁之后，能对将来的new files 还原为原来的redundancy”这个情况 的剩余空间。
REQUIRED_MIRROR_FREE_MB
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:
Normal redundancy disk group with more than two 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.
High redundancy disk group with more than three failure groups
The value is the total raw space for all of the disks in the two largest failure groups.
Eg: normal redundancy
REQUIRED_MIRROR_FREE_MB is the size of 1 disk
DATA1 : REQUIRED_MIRROR_FREE_MB : 610352
The size of one of the disks in the DATA1 diskgroup - DISKA1 is 610352.