ITFN 2601 Introduction to Operating Systems

1. ITFN 2601Introduction to Operating Systems Lecture 21 Disk Access

2. Agenda • Disc Types • Magnetic • RAID • Optical • Read/Write Scheduling • Error Handling • Stable Read/Writes

3. Magnetic Disks • Cylinders • Ring Number • Heads • Which Platter to Read • Sectors • Rotational Location

4. Disk Hardware

5. RAID • Redundant Array of Inexpensive Disks • Allows for the amalgamation of many drives • Eliminates danger of a “Single Large Expensive Disk” • Built-in error mechanism • Built-in recovery mechanism • No Error/Recovery in RAID-0 and RAID-2

6. RAID Levels • 6 Levels • RAID-0: Write-Striping • RAID-1: Write-Mirror • RAID-2: Byte-Striping • RAID-3: Byte-Striping w/Parity • RAID-4: Write-Striping w/Parity • RAID-5: Write-Striping w/Parity-scattering

7. RAID 0 • Each write is issued to a different drive • Less wait for write-completion • Prefers drives being equivalent

8. RAID 1 • Writes are issued as in RAID 0 • Writes are identically issued to backup-discs • Highly size dependent

9. RAID 2 • As RAID 0, except write is broken into bytes • Normal writes are in block-sized chunks • Each byte is issued to a different drive

10. RAID 3 • Writes are broken and issued as RAID 2 • One disc is “Parity” • XOR of the bytes being written on that pass • One bit per byte

11. RAID 4 • As RAID 0 • Each parity bit represents one strip • Parity will have n bits, where n is the number of drives (minus the parity drive)

12. RAID 5 • Based on RAID 4 • If the Parity drive fails, all recovery data is lost! • Staggers Parity across drives • If any drive is lost, it’s data is recovered from Parity

13. Optical Devices • Optical Disks (LaserDisc) • 30cm diameter • Compact Disc • Audio [IS-10149; Red Book] (1980) • Computer [CD-ROM; Yellow Book] (1984)

14. Physical Construction • Data is encoded in Pits/Lands • Pit – On • Land – Off

15. Red/Yellow Specifications • Symbols • 8bits->14bits, Encoding & Error Correction • Frames • 42 Symbols (24 data bytes; 18 ECC) • Sectors • 98 Frames • 2048 data bytes • 288 ECC

16. CD-R/RW • Standard computer CD is “CD-ROM” • Disc is extruded plastic • CD-Recordable [Orange Book] (1989) • Disc has dye that is activated by light • CD-ReWritable • Dye has two states (transparent & opaque) • Drive has three lasers

17. DVD • Digital Video Disc (or Versatile) • Spiral is smaller • Pits/Lands are smaller • “Smaller” laser • Holds up to 4.7G (vs 650M) • 4 types • Single/Double Sided • Single/Double Layered

18. Low Level Formatting • Performed by the manufacturer • Dictates the structure of the drive • Writes each track with information about the track • Skips over “bad” sectors

19. Disk Format • Moving the head is slow • Disc would have to spin back • Cylinder Skew • Transfer rate may be slow • Interleaving sector #s

20. Disk Arm Scheduling • Rotation and Head movement are bad • First Come First Serve • Shortest Seek First • Identical to FIFO/SJF Process Scheduling • Elevator Algorithm • Keep moving in one direction • Turn around at end of requests

21. Error Handling • Bad sectors • Each drive has n sectors per track • Only m are available for use (m < n) • The rest are “backup” • If a sector has repeated read/write errors • Copy everything into the next backup sector • Mark the original as “bad”

22. Stable Storage • If an error occurs during a write … • Old data is gone • New data isn’t there  • Stable storage ensures data integrity • Relies on the fact that errors are “uncommon” • Like RAID 1

23. Stable Writes • Stable Writes • Write the data on main-disc • Repeat until the write is successful • Write the data on secondary-disc • Stable Reads • Read from the main-disc • Repeat some number of times • Read from secondary-disc (if unsuccessful)

24. Crash Recovery • Valid data always exists • May not have last write, but is not invalid • Usually acceptable