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. The amount of space displayed in this column takes the effects of mirroring into account. The value is computed as follows:
For a normal redundancy disk group, 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.
For a high redundancy disk group, the value is the total raw space for all of the disks in the two largest failure groups.