QNAP NAS Storage: System Reserve, Snapshot Space, and Usable Capacity
QNAP QTS takes space in layers before your files: about 70 GB per storage pool, 1% of each RAID group for RAID metadata (up to 150 GB per group), and ext4 file system metadata. Guaranteed snapshot space is an option; set as a percentage, it defaults to 20% of the pool.
See exactly how much usable storage your configuration delivers.
QNAP QTS Storage Architecture
QNAP QTS organises storage into storage pools (the RAID groups) and volumes inside them, or into a static volume with no pool. QNAP publishes what each layer takes:
- Storage pool: about 70 GB per pool for storage configuration data.
- Static volume: about 20 GB per static volume.
- RAID metadata: 1% of each RAID group's total capacity, up to 150 GB per group.
- ext4 metadata: between 0.8% and 1.8% of a volume over 100 GB (larger volumes need less).
- Guaranteed snapshot space (optional): reserves pool space for snapshots; set as a percentage, the default is 20% of the pool. A static volume has no pool and does not support snapshots.
The snapshot reservation is the biggest single item when it is on, so set it to what your snapshot schedule needs, or leave it off if you do not use snapshots. The calculator counts it only on a storage pool, never on a static volume.
RAID Options on QNAP
- RAID 1. Exactly two disks; one disk's capacity is usable. Adding disks to a RAID 1 group converts it to RAID 5.
- RAID 5. (n − 1) × the smallest disk. Survives one failed disk.
- RAID 6. (n − 2) × the smallest disk. Survives two failed disks.
- RAID 10. (n ÷ 2) × the smallest disk, an even number of disks.
- RAID 0 / JBOD. No redundancy.
One QTS RAID 0, 5, 6 or 10 group holds at most 16 disks. QNAP can add disks to RAID 1, 5, 6, 50 and 60 groups while the pool stays online; each added disk must be at least as large as the smallest in the group. Growing by replacing disks with larger ones only adds space after every disk in the RAID group has been replaced and rebuilt.
A Worked Example
4 × 4 TB in RAID 5 gives 12 TB of layout. From that, QTS takes the RAID metadata (1% of the group), about 70 GB for the pool and the ext4 metadata, and guaranteed snapshot space if you turn it on. The RAID calculator shows each of these as its own line for your drives.
QNAP's usage alert defaults to 80% full and can be changed. It is an alert, not a limit; QNAP lists slower performance, failed snapshots or backups and refused writes as the risks once a pool is nearly full.
RAID keeps a NAS running through a drive failure. It is not a backup: deletion, ransomware, a failed rebuild or a dead NAS can still take everything, so keep a separate copy.
Frequently Asked Questions
Why does QNAP show less space than the drives add up to?
Three things: RAID parity (one disk in RAID 5, two in RAID 6), QTS's published reserves (about 70 GB per pool, 1% per RAID group, ext4 metadata), and guaranteed snapshot space if it is on. Your operating system may also count in binary units (TiB), which shows the same space as a smaller number; nothing is lost.
Does the calculator cover QuTS hero (ZFS)?
No. The QNAP option models QTS. QuTS hero uses ZFS with its own reserves, which the calculator does not model for QNAP.