Mirror spaces are designed for increased performance and increased resiliency. Two-way mirror spaces can tolerate one disk failure and three-way mirror spaces can tolerate two disk failures. They are well suited to storing a broad range of data, from a general-purpose file share to a VHD library. When a mirror space is formatted with the Resilient File System (ReFS), Windows offers automatic data integrity maintenance. This is a layer of resiliency is above and beyond the resiliency achieved from maintaining multiple data copies to tolerate drive failure.
Mirror spaces require at least two disks (per tier if you use storage tiers) to protect you from a single disk failure. To protect a virtual disk from two simultaneous disk failures, you need at least three disks (per tier), with a minimum of five disks total in the pool to maintain pool metadata.