Hardware and procurement¶
Decided¶
- Start with a Spectra Stack, not a Cube. The first library doubles as the instrument that measures the constants every throughput figure depends on.
- A few petabytes to start, replicated, while distribution and encoding mature. The eventual store is petabytes of radiology and pathology imaging.
- Three sites with 3-way replication.
Library facts¶
Supplied by Spectra for the Cube; figures for the Stack are still needed.
| Spectra Cube | |
|---|---|
| Slots | 50–1,670, in steps of 10 |
| Native capacity (LTO-10) | Up to 66.8 PB, which is 40 TB per slot and implies PA media |
| Drives | Up to 30 half-height or 16 full-height |
| Throughput | Up to 32.4 TB/hr native (30 half-height LTO-10 drives, i.e. 300 MB/s each) |
| Form | 42U standalone frame |
| Compute | None: a management controller only (LumOS REST API) |
Sizing, 3-way replication across three sites¶
About 3.43× raw per usable byte (three copies × RS(28,4) inside each volume):
| Usable | Per site, PA 40 TB cartridges | Per site, LA 30 TB cartridges |
|---|---|---|
| 2 PB | 57 cartridges, 70 slots | 76 cartridges, 80 slots |
| 3 PB | 86 cartridges, 90 slots | 114 cartridges, 120 slots |
| 5 PB | 143 cartridges, 150 slots | 191 cartridges, 200 slots |
Throughput scales with drive count only up to the robot: about 14 drives per site on scattered work. Laying data out so it is read together is worth 4.55× and costs nothing.
Media: LA or PA¶
PA holds 1.33× per cartridge and per licensed slot, so it is cheaper per TB whenever its price premium is under 33 %. Above about 132 PB logical, LA can't reach the target within four frames at all. Proposed: PA, unless write-once media is offered only in LA for the cohorts that need it, or the premium exceeds 33 %.
Each site also needs a host¶
The library has no compute. Each site needs a server to run the Cresco agent, the storage software and the spool. Proposed: a modest 1U machine with 8–16 cores, 64–128 GB RAM, a SAS HBA (or FC, or a Spectra Swarm 40 GbE bridge), and 2–4 TB of NVMe spool. The spool size depends on LTO-10's minimum streaming speed, which is not yet known.
Questions for Spectra¶
- Stack: slots per module, drive bays per module, maximum modules, and whether one robot serves the whole stack
- The guaranteed native, uncompressed, decimal capacity of the PA part number, not "up to"
- Whether LTO-10 is offered half-height
- Write-once LTO-10 availability and price premium, for both LA and PA
- Is Capacity-on-Demand a licence key on installed slots, or a field hardware install?
- Can drives be added after purchase?
- Load/unload cycle ratings for cartridges and drives, in writing
- Mount cycle breakdown as a distribution: pick, load, locate, rewind to start (is it mandatory?), unload
- Minimum sustained streaming speed before the drive stalls ("shoe-shines")
- SCSI Persistent Reservation support, for multi-host fencing
- Robot exchange rate
- Itemised pricing, and whether a minimal unit (1–2 drives, 40–80 slots) can be bought as a rebuild target or extra failure domain