Secondary storage
State info that does not disappear
This is where all important state resides
- Persistent storage
Overview
Secondary storage reads and write in units of sectors (larger than bytes)
How do we modify a single byte?
- Read entire sector
- Modify byte
- Write back entire sector
Sector is unit of atomicity
- Sector write is always done completely
Magnetic Disk vs. Flash Memory
Cost vs. Performance
- Disk is cheaper, flash is faster
- Flash write performance degrades over time
Trends
Hard disk bandwidth and cost per bit improving exponentially
Seek time and rotational delay improving very slowly
- Requires moving physical objects
- Bottleneck