8.1 Files and Other Persistent Storage

Persistence

• Data stored in memory disappears on power failure or reboot
• The OS should provide a way to store data longer term

Access

• Provide a means to reference stored objects by name
• Objects could also be accessed by contents

Files

• Blend persistence and access

Common forms of persistent storage

• File - Array of bytes
• Table - Array of typed data fields
• Persistent Object - Serialized data structure from a programming context

Common access services

• Hierarchical directories mapping names to objects

• Indexes providing access based on contents

  8.2 Storage Technology

• The recent history of persistent storage has been dominated by spinning hard drives
• Solid-state drives have begun to replace spinning drives

Disk Drive

• Store data in 512 byte blobs called sectors (other sizes are possible)
• Conceptually, an array of sectors
• The processor can request that an ordered group of sectors be transferred to RAM or vice versa

Disk drive performance

• Linear reads are much faster than random reads
• Head seeking takes
• Rotational latency (waiting for the sector to arrive under the head) also adds to latency
• Reads should be queued on the drive to allow the drive to make correct performance decisions

SSDs

• Do not experience mechanical seeks or rotational latency
• Less penalty for random reads
• Generally lower latency and high bandwidth than spinning drives
• Currently more expensive per byte