UnRAID NAS Storage: Parity Drive, Usable Capacity, and How It Differs from RAID
UnRAID is not RAID. It uses a dedicated parity drive rather than striped parity, which means usable capacity equals the sum of all data drives. But sequential read performance does not scale with drive count.
See exactly how much usable storage your configuration delivers.
How UnRAID Storage Works
UnRAID designates one (or optionally two) drives as parity drives. These parity drives must be at least as large as the largest data drive. All other drives are data drives, and each drive operates independently with its own filesystem.
Usable storage = sum of all data drives (excluding parity drives). With six 4TB drives (one parity), you have five 4TB data drives = approximately 18.2 TiB usable before filesystem overhead.
Single Parity vs Dual Parity
- Single parity (one parity drive): Protects against one drive failure. The parity drive must be ≥ the largest data drive.
- Dual parity (two parity drives): Protects against two simultaneous drive failures. Both parity drives must be ≥ the largest data drive.
Mixed Drive Sizes
UnRAID handles mixed drive sizes naturally. Unlike RAID 5/6 which limit all drives to the smallest drive's capacity. You can mix 2TB, 4TB, 6TB, and 8TB drives in the same array. Each drive's full capacity contributes to the usable total (minus parity drives).
The only constraint: parity drives must be ≥ the largest data drive. If you add a larger data drive without upgrading parity, UnRAID will prompt you to expand parity first.
No System Overhead
UnRAID's operating system runs from a USB boot drive. It does not consume space from the storage array. The RAID calculator applies 0 GB system overhead for UnRAID (vendor-published, confirmed in UnRAID documentation).
Filesystems on UnRAID
Each UnRAID data drive uses its own filesystem, independently formatted. XFS is the default (approximately 1% metadata overhead). Btrfs (4% overhead) and other Linux filesystems are supported. The pool array and cache drive (SSD) can use different filesystems.
Frequently Asked Questions
Is UnRAID as safe as RAID 5?
UnRAID with single parity tolerates one drive failure, the same as RAID 5. However, during a parity rebuild, UnRAID reads each entire drive. Which stresses all drives simultaneously. The risk of a second failure during rebuild is similar to RAID 5. Dual parity provides RAID 6 equivalent protection.
Can I add a drive to an existing UnRAID array?
Yes. UnRAID arrays can be expanded by adding drives at any time. New drives are assigned and parity is recalculated. This is one of UnRAID's key advantages. You can add a single drive without rebuilding the entire array or buying a matching set.
Why doesn't UnRAID read speed scale with drive count?
Because each file is stored on a single drive (no striping). A single file transfer reads from one drive at one drive's speed. RAID 5/6 and SHR stripe files across drives, so reads can use multiple drives in parallel. UnRAID trades read speed for flexibility.