so that when a disk fails, it can automatically reconstruct the contents of the failed disk to another disk in the diskgroup. The database should remain online if enough space is available. The amount of space required to ensure ASM can restore redundancy following disk failure is in the column REQUIRED_MIRROR_FREE_MB in the V$ASM_DISKGROUP view. The amount of free space that can be safely used in a disk group, taking mirroring into account, and yet be able to restore redundancy after a disk failure is in the column USABLE_FILE_MB in the V$ASM_DISKGROUP view. USABLE_FILE_MB should always be greater than zero. If USABLE_FILE_MB falls below zero, then more disks should be added to the disk group.
DOUBT:Why it needs to reconstruct the contents of failed disk to another disk in the diskgroup when it has the mirrored extentsbecause the data that was on the failed disk isnt mirrored anymore (no extra copies) and in order to ensure normal redundancy level, a new mirror copy of the data will be constructed
i dont understand when disk itself is failed how it can access it and reconstruct its content on another disk.from the still available, mirrored data