1. 1 Operating Systems Part VI: Mass- Storage Structure

2. 2 Disk Structure Platter Cylinder Track Sector

3. 3 Data Transfer Seek time: time to get to the track/cylinder containing the sector Rotational latency: time to get to the desired sector Disk bandwidth: number of bytes transferred per unit time (a.k.a. transfer rate)

4. 4 Disk Scheduling FCFS (First Come First Served) – Simplest to implement – Intrinsically fair – Does not provide the fastest service – Example 98,183,37,122,14,124,65,67 Assume start at cylinder 53

5. 5 Disk Scheduling FCFS (First Come First Served)

6. 6 Disk Scheduling SSTF (Shortest Seek Time First) – Selects the request with the minimum seek time from the current head position – Essentially a form of SJF – May cause starvation – Not optimal, but substantial improvement over FCFS

7. 7 Disk Scheduling SSTF (Shortest Seek Time First)

8. 8 Disk Scheduling SCAN Scheduling – Sometimes called the elevator algorithm – Starts at one end of disk and moves toward other end, servicing requests as it reaches each cylinder, until it gets to other end – Head continuously scans back and forth – Disadvantage: When head reverses direction, what happens to density of request in vicinity?

9. 9 Disk Scheduling SCAN Scheduling

10. 10 Disk Scheduling C-SCAN Scheduling – Variation of SCAN but returns to beginning of disk without servicing requests on return trip – Treats cylinder as circular list that wraps around from final cylinder to the first one

11. 11 Disk Scheduling C-SCAN Scheduling

12. 12 Disk Scheduling LOOK and C-LOOK Scheduling – Variations of SCAN and C-SCAN – Goes as far as final request in each direction then reverses course without going all the way to the end – Looks for request before continuing in a direction

13. 13 Disk Scheduling C-LOOK Scheduling

14. 14 Selection of Disk Scheduling Algorithm Depends heavily on number and type of request Can be heavily influenced by file allocation method (contiguous vs. linked/indexed) Location of directory (first/middle/last cylinder) -> better cached SSTF or LOOK reasonable default

15. 15 Disk Management Formatting – Low-level formatting (physical formatting) Fills disk with special data structure (header, data area, and trailer) for each sector (usually 512 bytes) -> usually done in factory Implements error-correcting code (ECC) which is updated with value calculated from all bytes in sector When sector is read, ECC is recalculated and compare with stored value -> mismatch = corruption

16. 16 Disk Management Formatting (continued) – Partitioning Divides disk into one or more groups of cylinders Tools: fdisk, Partition Magic, System Commander – Logical formatting Creation of file system (FAT/inode, free-space mapping, initial empty directory) Raw disk: large sequential array of disk blocks without any file system structure (used by some applications) -> bypasses directory structure, file names, space allocations, and other file services

17. 17 Disk Management Boot Block – Boot short for bootstrap – Boot program stored in ROM Fixed location and needs no initialization Cannot be infected by virus Problem: changing boot code requires changing ROM hardware chips – Solution: ROM -> boot disk (w/ boot partition)-> O/S kernel

18. 18 Disk Management Bad Blocks – Unavoidable due to moving parts and proximity of R/W head to platter surface – Data in bad blocks are usually lost – Handled manually in simple disks (IDE) Format scans for bad blocks and locks them away Chkdsk and scandisk do the same but used in normal operation – SCSI disks “smarter” in handling bad blocks Uses sector sparing or forwarding Sector slipping sometimes used in place of sparing

19. 19 Swap Space Goal: provide best throughput for virtual memory system Swap space use – Varies depending on O/S (e.g. entire process image, pages pushed out of MM swapped, etc.) – Some O/Ss allow multiple swap spaces – Safer to overestimate than to underestimate size Swap space location – Part of the file system (Windows) – Separate disk partition (normally raw device, e.g. Unix)

20. 20 RAID Structure Redundant Arrays of Inexpensive Disks Concept now extended to other devices such as tapes Redundancy solves problem of reliability – Duplicate each disk (mirroring/shadowing) Performance improvement via parallelism – Mirroring doubles read access times – Data striping: splitting data across multiple disks (bit- level, block-level, sector-level, etc.)

21. 21 RAID Levels Mirroring: high-reliability but expensive; striping: high-speed but does not improve reliability RAID levels provide a scheme that combines mirroring and striping – Cost-performance trade-offs (hence, levels) – Example: RAID 0 (block striping but without mirroring), RAID 1 (mirroring), RAID 0 + 1

22. 22 Disk Attachment Host-attached storage – May be IDE, SCSI, or RAID device Network-attached storage – Provides convenient way to share pool of storage and data – Consumes network bandwidth – More efficient if Storage Area Network (SAN) is implemented (using storage rather than networking protocols)

23. 23 Tertiary Storage Built with removable media Low-cost is defining characteristic Removable disks – Magnetic (e.g. floppy, zip drives, hot- swappable hard-disks) – Optical WORM (Write Once Read Many, e.g. CD-R) Phase-change (e.g. CD-RW) – Magneto-optical

24. 24 Tertiary Storage Tapes – Off-line: sits on shelves – Near-line: uses robotic arms -> between off- line and on-line (e.g. disks) Future technology – Holographic storage: records holographic photographs on special media – Micro-electronic mechanical systems (MEMS) -> produces small storage machines

25. 25 Tertiary Storage Performance Speed Reliability Cost