Lecture 6 thoughts
We think a lot in computer science about costs: time cost, space cost, memory efficiency. And these costs fundamentally shape the kinds of solutions we build. But you may not think as often about financial costs! These also fundamentally shape the systems we build—in some ways more fundamentally—and the costs of storage technologies have changed at least as dramatically as their capacities and speeds.
Costs per megabyte of storage technologies (constant 2010 dollars)
| Year | Memory | Flash | Hard disk |
|---|---|---|---|
| ~1955 | $411,000,000.00 | $6,233.00 | |
| 1970 | $734,000.00 | $260.00 | |
| 1990 | $148.20 | $5.45 | |
| 2003 | $0.09 | $0.305 | $0.00132 |
| 2010 | $0.019 | $0.00244 | $0.000073 |
| ~2013 | $0.0047 | $0.00078 | $0.000038 |
Cost per byte relative to that of a hard disk in ~2013:
| Year | Memory | Flash | Hard disk |
|---|---|---|---|
| ~1955 | 11,000,000,000,000 | 160,000,000 | |
| 1970 | 19,000,000,000 | 6,800,000 | |
| 1990 | 3,900,000 | 140,000 | |
| 2003 | 2,400 | 8,000 | 35 |
| 2010 | 500 | 64 | 2 |
| ~2013 | 124 | 21 | 1 |
These are initial purchase costs & don’t include maintenance and power. DRAM uses more energy than hard disks, which use more than flash. (Prices from here and here. $1.00 in 1955 is $8.14 in 2010 dollars.)
Rough speeds of storage technologies
Type |
Latency |
Throughput |
Throughput |
|---|---|---|---|
Register |
0.5ns |
||
SRAM |
4ns |
||
DRAM |
60ns |
||
Flash (read) |
30µs |
250MB/s |
140MB/s |
Flash (write) |
300µs |
170MB/s |
14MB/s |
Hard disk |
~4-9ms |
58-96MB/s |
~1MB/s |
A typical recent disk might rotate at 15000 RPM. It would have ~4ms average seek time, ~2ms average rotational latency, and 58-96 MB/s maximum sustained transfer rate. (CS:APP2e)
To load 4 pages (that is, 4x4KB = 16KB) from disk takes ~8ms.