Calculate usable capacity of a RAID 1 mirrored array from disk count and size.
RAID 1 mirrors identical copies of data across all disks in the array, so usable capacity equals a single disk's size regardless of how many mirrors exist. The trade-off is maximum redundancy — all disks minus one can fail without data loss — at the cost of the lowest storage efficiency of any standard RAID level.
RAID 1 usable capacity
usable = disk_size (one disk's worth regardless of mirror count)
Storage efficiency
efficiency = 1/n × 100%
Yes. A 3-way or even 4-way mirror is possible — usable capacity stays at one disk's size, but fault tolerance increases to n-1 simultaneous failures.
Read throughput can improve because the controller can read from multiple mirrors in parallel. Write performance does not improve because every write must be committed to all mirrors.