Rockstor is a BTRFS based system, not designed nor purposed for the use of mdraid.
While mdraid can be put under or even on top of BTRFS, that is against the ideology of Rockstor.
BTRFS is not built for speed. If I/O performance is what you’re guaging your interest on, you will likely be disappointed.
However, it is very good at allowing users to achieve some level of redundancy regardless of what disks they have available. (IE: If one has 3 1Tb drives, a 500Gb and a 750Gb, you can still achieve a redundant RAID with 2624Mb of usable space.)
Is also supports CoW and snapshot capability.
Think something more like ZFS, but lacking ZFS’ confusion about pool expansion, which requires creating VDEVs consisting of one or more disks, and adding them to the pool - Whereas BTRFS can arbitrarily add or remove disks from a pool (assuming sufficient space after removal).
BTRFS also less confusion about redundancy. AFAIK ZFS requires multiple devices per VDEV in order to achieve redundancy for that VDEV, regardless of the pool layout - whereas with BTRFS in an appropriate RAID format, any disk can be lost without incurring data loss)