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

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-write head :
cylinder c
arm
platter
rotation
continued

4. Mapping to the hardware:
The Logical View:
one-dimensional array of logical blocks
a block is the smallest transfer unit (typically 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: High disk bandwidth
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. Example
Fig. 13.1, p.433
Queue: 98, 183, 37, 122, 14, 124, 65, 67
Head starts at cylinder 53. 14 37 53 65 67 98
122
124
183
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. Example
Fig. 13.2, p.434
Queue: 98, 183, 37, 122, 14, 124, 65, 67
Head starts at cylinder 53. 14 37 53 65 67 98
122
124
183
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. Example
Fig. 13.3, p.435
Queue: 98, 183, 37, 122, 14, 124, 65, 67
Head starts at cylinder 53, and moves left. 14 37 53 65 67 98
122
124
183
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. Example
Fig. 13.4, p.436
Queue: 98, 183, 37, 122, 14, 124, 65, 67
Head starts at cylinder 53, and moves right. 14 37 53 65 67 98
122
124
183

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. 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. 14 37 53 65 67 98
122
124
183

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. data blocks (subdirectories)
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
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