Two-way mirror: 2. Three-way mirror: 5.

A Storage Spaces mirror layout keeps multiple identical copies of every piece of data spread across separate physical disks so that the loss of a disk does not lose data. A two-way mirror keeps two copies of the data, and the minimum configuration that can hold two independent copies across separate physical disks is two disks; this layout tolerates the loss of a single disk. A three-way mirror keeps three copies of the data and is designed to tolerate the simultaneous loss of two disks, but Storage Spaces does not implement this with the naive minimum of three disks; to be able to actually rebuild and re-protect data after a failure (and to spread the write load so that a single physical disk failure does not create a data-loss window while resiliency is temporarily reduced), Microsoft ' s documented minimum requirement for a three-way mirror space is five physical disks. This is a specific, documented number that differs from the pattern one might expect by simply extending the two-way mirror ' s minimum (two disks for two copies) to three copies (which would suggest three disks); the extra disks in the three-way case exist to give Storage Spaces the flexibility it needs to safely place and later re-mirror three copies of every slab of data across the pool. Administrators planning capacity for a three-way mirror pool on a JBOD enclosure must provision at least five disks, not three, before the layout can even be created.