I/O Performance

• Filesystems
  • Cache, RAM, Disk, Network, …
• Persistance

Storage Hierarchy

1. Registers, 1KiB, 1 cycle time
2. Cache, (L1, L2, L3), 16MiB, 1-2 cycles
3. RAM, 32 GiB
4. Flash/Hard Disk, 4 TiB (secondary storage)
5. Tapes (optical, non optical), Network (tertiary storage), 4 EiB

Performance Metrics

• Utilization (fraction of I/O rate capacity being used)

• Latency (delay between issuing I/O request and completing it)

• Throughput (rate of I/O completions per second)

• Classic disk algorithms use CPU rarely (since at the time, relatively, hard disks were somewhat faster than CPUs)

• Modern disk algorithms use CPU more frequently (since now, CPUs are significantly faster than disks)

Hard Disks

Overhead of doing disk I/O:

• Seek time: 8 ms (for I/O head/arm to hit track)
• Rotational latency