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Operating Systems

Operating Systems. Certificate Program in Software Development CSE-TC and CSIM, AIT September--November, 2003. Objectives introduce issues such as disk scheduling, formatting, and swap space management. 13. Secondary Storage (S&G, Ch. 13). Contents. 1. A Hard Disk (Again)

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Operating Systems

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  1. Operating Systems Certificate Program in Software DevelopmentCSE-TC and CSIM, AITSeptember--November, 2003 • Objectives • introduce issues such as disk scheduling, formatting, and swap space management 13. Secondary Storage(S&G, Ch. 13)

  2. Contents 1. A Hard Disk (Again) 2. Disk Scheduling 3. Disk Formatting 4. Boot Blocks 5. Swap Space Management 6. Disk Reliability

  3. 1. A Hard Disk (Again) Fig. 2.5, p.33 spindle track t actuator sector s read-writehead : cylinder c arm platter rotation continued

  4. The Logical View: • one-dimensional array of logical blocks • a block is the smallest transfer unit(typically 512-1024 bytes) • Mapping to the hardware: • for each cylinder (outer to inner) • start at first sector of outermost track • go round the track • move to the next track on the cylinder

  5. Mapping Issues • Avoiding defective sectors: • use substitutes from elsewhere on the disk • The number of sectors varies per track • less sectors per track in the centre • Zones • a zone is a collection of tracks whose sectors/track are the same

  6. 2. Disk Scheduling • Fast access times requires: • fast seek time • move head to the right cylinder quickly • low rotational latency • rotate the disk under the head quickly so the required sector can be accessed • High disk bandwidth • total number of bytes transferred in the time between the initial request and its completion

  7. Controller Selection • Disk I/O requests are placed on a queue managed by the hard drive controller. • When the controller comes to do the next request, its scheduler chooses a request which: • improves access time • improves disk bandwidth

  8. Types of Disk Scheduling • First-Come First-Served (FCFS) • Shortest Seek-Time First (SSTF) • SCAN (and C-SCAN) • LOOK (and C-LOOK)

  9. 2.1. FCFS Scheduling • Fair • Not the fastest service • Large swings across the disk are possible. • The following example(s) will assume a disk queue of I/O requests to blocks on cylinders.

  10. 14 37 53 65 67 98 122 124 183 Example Fig. 13.1, p.433 • Queue: 98, 183, 37, 122, 14, 124, 65, 67 • Head starts at cylinder 53. Total head movement = 640 cylinders

  11. 2.2. SSTF Scheduling • The next request to be serviced is the one closest to the current head position • minimize seek time • Reduces overall head movement. • May lead to starvation for some requests.

  12. 14 37 53 65 67 98 122 124 183 Example Fig. 13.2, p.434 • Queue: 98, 183, 37, 122, 14, 124, 65, 67 • Head starts at cylinder 53. Total head movement = 236 cylinders

  13. Not Optimal • If the queue was serviced in a slightly different order: • 53, 37, 14, 65, 67, 98, 122, 124, 183 • then the total head movement would be less: 208 cylinders

  14. 2.3. SCAN Scheduling • Scan back and forth across the disk • sometimes called the elevator algorithm • Problem: if a request arrives just behind the head then it will have to wait until the head returns.

  15. 14 37 53 65 67 98 122 124 183 Example Fig. 13.3, p.435 • Queue: 98, 183, 37, 122, 14, 124, 65, 67 • Head starts at cylinder 53, and moves left. Total head movement = 236 cylinders

  16. C-SCAN Scheduling • ‘C’ for “circular list”. • When a scan reaches one end of the disk, it jumps to the other end • no point reversing since it is unlikely that there will be requests in recently serviced areas

  17. 14 37 53 65 67 98 122 124 183 Example Fig. 13.4, p.436 • Queue: 98, 183, 37, 122, 14, 124, 65, 67 • Head starts at cylinder 53, and moves right.

  18. 2.4. LOOK Scheduling • SCAN (C-SCAN) moves the head across the full width of the disk. • LOOK (C-LOOK) moves the head only as far as the last request in each direction, and then reverses (wraps around).

  19. 14 37 53 65 67 98 122 124 183 C-LOOK Example Fig. 13.5, p.437 • Queue: 98, 183, 37, 122, 14, 124, 65, 67 • Head starts at cylinder 53, and moving right.

  20. 2.5. Which Disk Scheduling Algorithm? • SSTF - commonly used • SCAN/C-SCAN • good for heavily loaded disks since they avoid starvation • Choice depends on number/type of requests • e.g. file I/O will generate sequences of requests to adjacent blocks continued

  21. Caching? • Comparisons can utilise seek distances (as here) and disk rotational latency. • Logical addresses hide useful physical information (e.g. sector and track positions), that an OS-level scheduler could use. continued

  22. There may be conflict between the OS and disk controller scheduling policies: • e.g. the OS may want disk writes to happen immediately and in a fixed order when a transaction is being carried out

  23. 3. Disk Formatting • The disk controller uses low-level formatting (physical formatting) • a data structure per each sector • consists of • header, data area (512 bytes), trailer • utilises an error-correcting code (ECC) continued

  24. The OS adds its own data structures: • partitions the disk into groups of cylinders • logical formatting (make the file system) • includes the directory/file structure,and lists of free and allocated pages

  25. 4. Boot Blocks • Most book blocks hold a simple bootstrap loader whose only job is to load and execute a fuller bootstrap program from disk • the program is located in a fixed place on disk (a boot partition) sector 0 book block sector 1 FAT root directory data blocks(subdirectories) MS-DOS

  26. Bad Blocks • A bad block is a defective sector. • The OS can mark bad blocks and then skip them, or maintain a list of bad blocks. • Sector sparing (forwarding) • substitute a (close) good block for the bad one • Sector slipping • move related blocks to be contiguous if they contain a bad block

  27. 5. Swap space Management • Swap space algorithms use virtual memory (VM) • disk space treated as RAM • slow • management aim is to speed-up VM throughput

  28. Swap Space Implementation • As a file: • standard interface • easy to implement • inefficient use of physical memory • As a special partition: • no need to impose directory/file structures • faster • hard to reconfigure

  29. 5.1. UNIX Swap Space (4.3 BSD) • Each process is assigned a swap space which holds: • text segments (for pages of the program) • data segments (for runtime data) • The kernel uses two swap maps to track swap space usage in a process.

  30. Swap Maps Figs. 13.7 and 13.8, p.444 text segment swap map 512K 512K 512K 32K data segment swap map 16K 32K 64K 128K

  31. 6. Disk Reliability • Common approach is to use multiple disks. • Disk stripping (interleaving) • store data spread across several disks • reduces chance of complete failure • I/O data transfers can use parallelism continued

  32. RAID (Redundant Array of Independent Disks) • Mirroring (shadowing) duplicates data across disks • costly • parallelism can increase speed • Block interleaved parity • if data on one machine is corrupted, it can be recalculated from the remaining data and parity information stored on the other machines

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