Some application assumptions feeding into this discussion:
- most data are written once
- read dominated workload (writes may be less than 10% or even less than 1%)
- while there is little locality, most requests refer to recent data (useful for power management)
Technology review
The following table compares the key characteristics of storage technologies. When possible, the values below focus on the raw potential of the underlying technology (ignoring packaging or other system issues). In other cases, I use representative numbers from current systems.
|
Capacity |
BW |
Read Latency |
Write Latency |
Cost |
Power Consumption |
|---|---|---|---|---|---|---|
Server Disk |
1TB |
100MB/s |
5ms |
5ms |
$0.2/GB |
10W idle - 15W active |
Low-end Disk |
200GB |
50MB/s |
5ms |
5ms |
$0.2/GB |
1W idle - 3W active |
Flash (NAND) |
128GB |
100MB/s |
20us |
200us |
$2/GB |
1W active |
DRAM (DIMM) |
4GB |
10GB/s |
60ns |
60ns |
$30/GB |
2W - 4W |
PCM |
? |
DRAM-like? |
100ns |
200ns |
disk-like? |
4W? |
- Why not use flash instead of RAM as the main storage mechanism?
- What is flash latency?
- SSDs:
- Current claims for x25-E: 75 µs read, 85 µs write
- Typical ONFI:
- Micron MT29H8G08ACAH1, 8, 16, 32 Gb
- Read 30 µs, Write 160 µs, Erase 3 ms
- Micron MT29H8G08ACAH1, 8, 16, 32 Gb
- SSDs:
- Can it be made a low enough so it doesn't impact RPC latency?
- What is the latency of typical flash packaging today? 100 µs?
- What is flash latency?
- Does flash offer advantages over disk as the backup mechanism?