Stable storage is tangible
- We spend a lot of time in software in the purely-electronic realm
- Everything’s moving at the basically the speed of light
- But light is energy
- Stable storage makes data available independent of energy
- Materials science, mechanical engineering: stuff
Magnetic tape
- Format: Magnetic polarity on a flexible, wound medium
- Sequential storage only
- Offline, archival
- Very low cost (around 0.6¢/GiB)
- Very low speed
- Good longevity (30 years)
Hard disk
- Format: Magnetic polarity on a rigid, spinning medium
- Sequential and random-access storage
- Different performance properties
- Low cost (around 2¢/GiB)
- Low speed
- Medium-grade longevity (3–5 years in constant use)
Solid-state memory
- Format: Floating-gate transistors
- “The charge contained in [a floating-gate metal–oxide–semiconductor field-effect transistor] remains unchanged for…typically longer than 10 years.”
- Sequential and random-access storage
- Medium-low cost (around 15¢/GiB)
- Good speed
- Medium-grade longevity (a few years after exhaustion)
Glass?!
Geometry and physics
SSD vs. HDD throughput
| Random read | Random write | Sequential read | Sequential write | |
|---|---|---|---|---|
| Seagate Savio 15K.3 HDD | 2 MB/s | 2 MB/s | 223 MB/s | 223 MB/s |
| Samsung 840 Pro SSD | 103 MB/s | 287 MB/s | 421 MB/s | 384 MB/s |
| Seagate 600 SSD | 84 MB/s | 252 MB/s | 424 MB/s | 374 MB/s |
| Intel 335 SSD | 39 MB/s | 222 MB/s | 344 MB/s | 354 MB/s |
From “Operating Systems: Three Easy Pieces”, Arpaci-Dusseau and Arpaci-Dusseau
Latency
- L1 cache: 0.5 ns
- DRAM: 100 ns
- SSD: 150,000 ns
- HDD: 15,000,000 ns