Disks (see Fig. 6)

• Management and ordering of disk access requests is important:
  • Huge speed gap between memory and disk
  • Disk throughput is extremely sensitive to
    • Request order $\Longrightarrow$ Disk Scheduling
    • Placement of data on the disk $\Longrightarrow$ file system design
  • Disk scheduler must be aware of disk geometry
• Disk management issues
  • Formatting
    • Physical: divide the blank slate into sectors identified by headers containing such information as sector number; sector interleaving
    • Logical: marking bad blocks; partitioning (optional) and writing a blank directory on disk; installing file allocation tables, and other relevant information (file system initialization)
  • Reliability
    • disk interleaving or striping
    • RAIDs (Redundant Array of Inexpensive Disks): various levels, e.g., level 0 is disk striping)
  • Controller caches newer disks have on-disk caches (128KB 512KB)

Figure 6: Disk Structure.