ASUSTOR NAS Storage: ADM, RAID Options, and Why Overhead Varies
ASUSTOR has not published a system reserve figure for ADM, so the RAID calculator gives ASUSTOR the layout maths only: the space each RAID level leaves from your drives, before ADM's own system and file system overhead.
See exactly how much usable storage your configuration delivers.
What the calculator does for ASUSTOR
Choose ASUSTOR (ADM) in the RAID calculator and it works out the layout: how much of your drives each RAID level leaves for data. It does not subtract a ASUSTOR system or file system reserve, because ASUSTOR has not published one we can cite, and an estimate shown as a vendor figure would be guesswork. The capacity ADM shows after you create the volume will be somewhat lower than the layout figure.
The calculator also does not know which expansion steps ADM allows. When you plan adding or replacing drives, it shows the arithmetic and asks you to check that your ASUSTOR model supports the step.
RAID levels on ASUSTOR
- RAID 1. A mirror: with two drives, one drive's capacity is usable.
- RAID 5. (n − 1) × the smallest drive. Three drives minimum. Survives one failed drive.
- RAID 6. (n − 2) × the smallest drive. Four drives minimum. Survives two failed drives.
- RAID 10. (n ÷ 2) × the smallest drive, with an even number of drives, four minimum. Survives one failed drive, and up to one per mirror pair.
- RAID 0 / JBOD. No redundancy: one failed drive loses the data on it (RAID 0: all of it).
The formulas assume identical drives and give layout capacity before ADM's own overhead. With mixed sizes, classic RAID counts every drive at the size of the smallest.
Finding the real figure on your NAS
- Create the storage pool and volume in ADM.
- Compare the capacity ADM reports with the calculator's layout figure for the same drives and RAID level.
- The difference is ADM's system and file system overhead on your model and firmware.
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 doesn't the RAID calculator subtract a ASUSTOR system reserve?
The calculator only subtracts reserves a vendor publishes and we can cite: Synology's per-drive system partitions and file system metadata, QNAP's pool, RAID and ext4 reserves, and the OpenZFS reserve. ASUSTOR has not published a figure, so the result is the layout capacity and the page says so.