Tape libraries are sold by slots, licensed by slots and limited by drives, and those are three different numbers. Get them wrong and you either pay for slots you never fill or buy a library that cannot finish the weekly full before Monday. This guide works through the arithmetic in five steps, then applies it to a 200 TB estate on a Quantum Scalar i6.
View the data behind this chart
| LTO-9, all licensed | LTO-9, vaulted | LTO-10, vaulted | |
|---|---|---|---|
| Cartridges per full | 12 | 12 | 6 |
| Slots in use | 236 | 236 | 134 |
| Licensed slots | 250 | 100 | 75 |
| i6 modules for slots | 3 | 3 | 2 |
| Drives, 24 h + restore | 7 | 7 | 7 |
| i6 modules overall | 3 | 3 | 3 |
Three numbers, not one
Physical slots are the cartridge positions the library chassis has. Licensed slots are the ones you pay to use for backup; on Quantum’s Scalar range, capacity-on-demand licensing unlocks them in 25-slot steps on the i3 and i6 and 100-slot steps on the i6000 and i7 RAPTOR. Drives decide how fast the library reads and writes, and each chassis module has a fixed number of drive bays.
Plan on native cartridge capacity throughout. Vendors quote compressed capacity at 2.5:1, but backup data that the backup software has already compressed or encrypted will not shrink again on tape.
Step 1: cartridges per backup set
Divide the size of each backup set by the native capacity of one cartridge and round up. LTO-9 holds 18 TB native, LTO-10 (PA) 40 TB, LTO-10 (LA) 30 TB. By capacity alone a 200 TB full needs 12 LTO-9 cartridges (11.1 rounded up) or 5 LTO-10 (PA) cartridges. One more floor applies: a tape is written by one drive at a time, so a full written by several drives in parallel needs at least one cartridge per drive. Written by six drives, each drive writes 33.3 TB: two LTO-9 cartridges each (12 in all) or one LTO-10 (PA) cartridge each (6 in all).
Check how your backup software uses tapes. Many media-pool settings start each job on a fresh cartridge, which wastes the tail of the last tape in every set but keeps sets separable for export. If yours appends jobs to partly used tapes, the count falls; size on the fresh-tape assumption unless you know otherwise.
Step 2: which sets live in the library
Not every cartridge in your retention policy needs a slot. Split them three ways: the current cycle that must stay online for restores (typically the recent fulls and incrementals), the sets kept offline inside the library in a vault partition, and the sets that leave the building. Only the first two need slots. For the retention side of the policy itself, see backup retention policy and GFS retention explained.
Step 3: scratch and cleaning cartridges
Add a scratch pool, the blank cartridges new jobs write to before old ones expire. One week of new writes is a sensible floor. Add one or two cleaning cartridges. These need licensed slots like any other online tape.
Step 4: licensed versus physical slots
Online cartridges, scratch and cleaning tapes need licensed slots. Tapes in a Quantum Active Vault partition do not: Quantum builds vault partitions from unlicensed slots. So the physical library must hold everything that stays in the building, but you license only the working set. On a capacity-on-demand library that is the difference between licensing the whole chassis and licensing a fraction of it. Active Vault is itself an optional feature; on the Scalar i6000 and i7 RAPTOR an export-redirect policy fills the vault automatically, while on the i3 and i6 tapes are moved in from the library GUI.
View the data behind this chart
| Layer | Detail |
|---|---|
| Weekly fulls | 4 × 12 cartridges = 48 licensed slots |
| Daily incrementals | 24 cartridges, six a week for 4 weeks |
| Scratch + cleaning | 18 scratch + 2 cleaning = 20 licensed slots |
| Active Vault: monthlies | 144 cartridges in unlicensed slots |
| Offsite: yearlies | 36 cartridges, no slots needed |
Step 5: drives from the backup window
Drive-hours per full are the full size divided by drive speed. Full-height LTO-9 and LTO-10 drives write 400 MB/s native, about 1.44 TB an hour; half-height LTO-9 writes 300 MB/s. Divide drive-hours by your window and round up, then add one drive so restores can run while a backup is in progress. If you license EDLM media scanning, add its dedicated scanning drives too.
Finally check the drive bays. A Scalar i6 module holds up to three full-height drives and 100 slots, so the module count is whichever is larger: slots divided by 100, or drives divided by three.
Worked example: 200 TB on a Scalar i6
Our arithmetic, not a Quantum sizing: a 200 TB weekly full, nightly incrementals of 4% (8 TB, one cartridge each, six a week), four weekly fulls and four weeks of incrementals kept online, twelve monthly fulls retained, three yearly fulls offsite, and a 24-hour window for the weekly full.
- •Online cycle on LTO-9: 4 fulls × 12 cartridges = 48, plus 24 incrementals = 72 cartridges.
- •Scratch and cleaning: one week of writes (12 + 6) = 18, plus 2 cleaning = 20. Working set: 92 slots, so 100 licensed in 25-slot steps.
- •Monthly fulls: 12 × 12 = 144 cartridges. In licensed slots that would take the licence to 250; in an Active Vault partition they use unlicensed slots and the licence stays at 100.
- •Yearly fulls: 3 × 12 = 36 cartridges offsite, needing no slots.
- •Physical slots: 92 + 144 = 236, so three 100-slot Scalar i6 modules (18U).
- •Drives: 200 TB at 400 MB/s is about 139 drive-hours, so 6 drives for a 24-hour window (23 hours at full native speed with no headroom; real designs allow for source throughput and tape loads), plus 1 for restores = 7. Three modules hold up to 9.
What LTO-10 changes
Rerun the same policy on 40 TB LTO-10 (PA) cartridges. Written by six drives, a 200 TB full takes 6 cartridges, one per drive. The online cycle falls to 48 cartridges (24 for fulls, 24 incrementals), scratch and cleaning to 14 (6 + 6 + 2), so the working set is 62 slots (75 licensed). The monthly vault falls to 72 cartridges and physical slots to 134, which fits two modules. But the drive count does not change, because LTO-10 writes at the same 400 MB/s native as full-height LTO-9: seven drives still need a third module’s drive bays, unless a 48-hour window cuts the backup drives to three, four with the restore drive, which two modules’ six bays hold.
And LTO-10 drives cannot read LTO-9 or older tapes, so if your archive includes earlier cartridges keep LTO-9 drives in the library, often in their own partition, until those tapes expire. See LTO drives and media.