 Two-way mirror: 2

 Three-way mirror: 5

In the Administering Windows Server Hybrid Core Infrastructure materials, the Storage Spaces guidance explains that mirror layouts keep multiple copies of each slab of data across different physical drives in a pool. A two-way mirror maintains two copies, so the pool must contain at least two physical disks to place each copy on a separate disk. This provides protection from a single disk failure with continuous availability. For higher resiliency, a three-way mirror maintains three copies of every write. The Windows Server implementation requires a minimum of five physical disks in the pool to create a three-way mirror virtual disk. The extra drives beyond the three copies are needed to satisfy the layout/columning rules used by Storage Spaces for performance striping and to ensure there is sufficient distribution to sustain a failure and still have capacity and parallelism for repair operations. The curriculum emphasizes that mirror spaces are recommended for workloads needing fast writes and quick recovery, while parity spaces trade performance for capacity efficiency. Therefore, when planning a JBOD-backed storage pool on a single Windows Server, you must allocate at least 2 disks for a two-way mirror and at least 5 disks for a three-way mirror to meet the platform’s resiliency and layout requirements.