Some clarification... Currently: We have a single "unRAID" array(*) and multiple user-defined "cache pools", or simply "pools".  Data devices in the unRAID array can be formatted with xfs, btrfs, or reiserfs file system. A pool can consist of a single slot, in which case you can select xfs or btrfs as the file system.  Multi-slot pools can only be btrfs.  What's unique about btrfs is that you can have a "raid-1" with an odd number of devices. With 6.12 release: