Methodology / worked examples
ArrayKeep Storage Planner guide
ArrayKeep connects a drive purchase question to a retention workload. First calculate a conservative usable array capacity, then model full data, daily changes, copies, and overhead. Keep the backup distinction visible throughout.
Understand layout capacity
The calculator uses common equal-drive estimates: RAID 0 uses all drives, RAID 1 uses one drive equivalent, RAID 5 uses count minus one, RAID 6 uses count minus two, and RAID 10 uses half the drives with an even count.
RAID 5 raw TB = (drive count - 1) x drive capacity
Four 8 TB drives in RAID 5 provide an illustrative 24 decimal TB before reserve and filesystem effects.
Convert raw space to a planning target
Apply a free-space reserve before comparing the array with a backup workload. Decimal TB and TiB are both shown because drive labels and operating-system displays use different unit conventions.
usable TB = raw TB x (1 - reserve / 100)
A 24 TB raw array with a 10% reserve leaves 21.6 decimal TB for this plan.
Model retention and copies
This simple model starts with one full backup, adds daily changes for the retention window, multiplies by independent copies, and adds backup overhead. Actual backup products may deduplicate, compress, or retain versions differently.
requirement = (full TB + daily GB x days / 1000) x copies x (1 + overhead / 100)
4 TB plus 50 GB/day for 30 days is 5.5 TB before copies and overhead.
Keep RAID separate from backup
A mirrored or parity array can lose data through deletion, malware, controller problems, theft, or correlated drive loss. Keep at least one independent copy and test restoration according to your risk and recovery needs.
- Treat the RAID result as capacity and availability planning.
- Treat the backup result as a workload estimate, not a recovery guarantee.
Limits and assumptions
- This simplified model assumes equal-capacity drives and does not verify NAS bays, drive support, filesystem behavior, snapshots, compression, or backup software.
- RAID improves availability or fault tolerance depending on layout, but it is not an independent backup and does not guarantee recovery.
- No live drive inventory, prices, affiliate catalog, or paid API is used.