1U Backup Appliance -- purpose-built backup target, not a primary-storage device. Everything about its spec sheet is tuned for "the thing a real share mirrors to," not for serving a demanding client directly.
8 disks, 100TB usable, RAID6, 20000 IOPS, 8GB cache, 50% built-in redundancy chance -- the highest built-in redundancy of any NAS-category device, at the cost of the lowest IOPS/cache numbers among the three NAS tiers.
Cheaper per-TB than the RAID tiers by deliberately trading away IOPS/cache for it -- exactly the right trade for a device whose whole job is sitting there holding a synced copy, not answering live client traffic.
The natural home for a `backupOfNasDeviceId` mirror share: point another NAS's client share at this device as its backup target, and the disaster-recovery tick keeps `usedBlocks` and virtual files in sync between them whenever both ends are powered, linked, and addressed.
Tip: a mirror only protects you if it's actually current -- the sync status shown next to a mirrored share (Synced / Stale / Never synced yet) is the thing to check before assuming a fault won't cost you data, not just the presence of a configured backup target.
See also: the Game Systems manual's Data Management section for backup mirrors and disaster recovery.