Unit-4: Operating system lecturenotes (Disk Scheduling and Distributed Systems) P.R.Engineering college

Unit-4: Operating system lecturenotes (Disk Scheduling and Distributed Systems) in Operating System uploaded at P.R.Engineering college

Unit-4: Operating system lecturenotes (Disk Scheduling and Distributed Systems) in Operating System at P.R.Engineering college P.R.Engineering college Staff uploaded in lecturenotes.net

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UNIT IV
Disk Structure
 Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the 
logical block is the smallest unit of transfer. 
 The 1-dimensional array of logical blocks is mapped into the sectors of the disk 
sequentially.
 Sector 0 is the first sector of the first track on the outermost cylinder.
 Mapping proceeds in order through that track, then the rest of the tracks in that 
cylinder, and then through the rest of the cylinders from outermost to innermost.
Disk Scheduling
 The operating system is responsible for using hardware efficiently — for the disk drives, 
this means having a fast access time and disk bandwidth.
 Access time has two major components
 Seek time is the time for the disk are to move the heads to the cylinder containing 
the desired sector.
 Rotational latency is the additional time waiting for the disk to rotate the desired 
sector to the disk head.
 Minimize seek time
 Seek time  seek distance
 Disk bandwidth is the total number of bytes transferred, divided by the total time between 
the first request for service and the completion of the last transfer. 
 Several algorithms exist to schedule the servicing of disk I/O requests. 
 We illustrate them with a request queue (0-199).
98, 183, 37, 122, 14, 124, 65, 67
Head pointer 53
FCFS Disk Scheduling and Distributed Systems UNIT VI
Operating Systems Page 2
SSTF
 Selects the request with the minimum seek time from the current head position.
 SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests.
 Illustration shows total head movement of 236 cylinders.
SCAN
 The disk arm starts at one end of the disk, and moves toward the other end, servicing 
requests until it gets to the other end of the disk, where the head movement is reversed 
and servicing continues.
 Sometimes called the elevator algorithm.
 Illustration shows total head movement of 208 cylinders. Disk Scheduling and Distributed Systems UNIT VI
Operating Systems Page 3
C-SCAN
 Provides a more uniform wait time than SCAN.
 The head moves from one end of the disk to the other. servicing requests as it goes. 
When it reaches the other end, however, it immediately returns to the beginning of the 
disk, without servicing any requests on the return trip.
 Treats the cylinders as a circular list that wraps around from the last cylinder to the first 
one.
C-LOOK
 Version of C-SCAN
 Arm only goes as far as the last request in each direction, then reverses direction 
immediately, without first going all the way to the end of the disk. 
Selecting a Disk-Scheduling Algorithm
 SSTF is common and has a natural appeal
 SCAN and C-SCAN perform better for systems that place a heavy load on the disk.
 Performance depends on the number and types of requests.
 Requests for disk service can be influenced by the file-allocation method. Disk Scheduling and Distributed Systems UNIT VI
Operating Systems Page 4
 The disk-scheduling algorithm should be written as a separate module of the operating 
system, allowing it to be replaced with a different algorithm if necessary.
 Either SSTF or LOOK is a reasonable choice for the default algorithm.
Disk Management
 Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk 
controller can read and write.
 To use a disk to hold files, the operating system still needs to record its own data 
structures on the disk.
 Partition the disk into one or more groups of cylinders.
 Logical formatting or “making a file system”.
 Boot block initializes system.
 The bootstrap is stored in ROM.
 Bootstrap loader program.
 Methods such as sector sparing used to handle bad blocks.
MS-DOS Disk Layout
Swap-Space Management
 Swap-space — Virtual memory uses disk space as an extension of main memory.
 Swap-space can be carved out of the normal file system,or, more commonly, it can be in 
a separate disk partition.
 Swap-space management
 4.3BSD allocates swap space when process starts; holds text segment (the 
program) and data segment.
 Kernel uses swap maps to track swap-space use.
 Solaris 2 allocates swap space only when a page is forced out of physical 
memory, not when the virtual memory page is first created. Disk Scheduling and Distributed Systems UNIT VI
Operating Systems Page 5
File-System Interface
File Concept
 Contiguous logical address space
 Types: 
 Data
 numeric
 character
 binary
 Program
File Structure
 None - sequence of words, bytes
 Simple record structure
 Lines 
 Fixed length
 Variable length
 Complex Structures
 Formatted document
 Relocatable load file
 Can simulate last two with first method by inserting appropriate control characters.
 Who decides:
 Operating system
 Program
File Attributes
 Name – only information kept in human-readable form.
 Type – needed for systems that support different types.
 Location – pointer to file location on device.
 Size – current file size.
 Protection – controls who can do reading, writing, executing.
 Time, date, and user identification – data for protection, security, and usage 
monitoring.
Information about files are kept in the directory structure, which is maintained on the disk
File Operations
 Create
 Write
 Read
 Reposition within file – file seek
 Delete
 Truncate
 Open(Fi) – search the directory structure on disk for entry Fi, and move the content of 
entry to memory.
 Close (Fi) – move the content of entry Fi in memory to directory structure on disk

Title:
Unit-4: Operating system lecturenotes (Disk Scheduling and Distributed Systems)

Subject:
Operating System

Institute:
P.R.Engineering college

Unit-4: Operating system lecturenotes (Disk Scheduling and Distributed Systems) P.R.Engineering college

Unit-4: Operating system lecturenotes (Disk Scheduling and Distributed Systems) in Operating System uploaded at P.R.Engineering college

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