Short Notes: Operating System - File Systems | Short Notes for Computer Science Engineering - Computer Science Engineering (CSE) PDF Download

 Page 1


Logically related data items on the secondary storage are usually organised into 
named collections called files. A file may contain a report, an executable program 
as a set of commands to the operating system. A file often appears to the users as 
a linear array of characters or record structures.
The file system consists of two parts:
1. A collection of files
2. A directory structure
The file management system can be implemented as one or more layers of the 
operating system.
The common responsibilities of the file management system includes the following
• Mapping of access requests from logical to physical file address space.
• Transmission of file elements between main and secondary storage.
• Management of the secondary storage such as keeping track of the status 
allocation and deallocation of space.
• Support for protection and sharing of files and the recovery and possible 
restoration of the files after system crashes.
File Attributes
Each file is referred to by its name. The file is named for the convenience of the 
users and when a file is named, it becomes independent of the user and the 
process. Below are file attributes
• Name
• Type
• Location
• Size
Page 2


Logically related data items on the secondary storage are usually organised into 
named collections called files. A file may contain a report, an executable program 
as a set of commands to the operating system. A file often appears to the users as 
a linear array of characters or record structures.
The file system consists of two parts:
1. A collection of files
2. A directory structure
The file management system can be implemented as one or more layers of the 
operating system.
The common responsibilities of the file management system includes the following
• Mapping of access requests from logical to physical file address space.
• Transmission of file elements between main and secondary storage.
• Management of the secondary storage such as keeping track of the status 
allocation and deallocation of space.
• Support for protection and sharing of files and the recovery and possible 
restoration of the files after system crashes.
File Attributes
Each file is referred to by its name. The file is named for the convenience of the 
users and when a file is named, it becomes independent of the user and the 
process. Below are file attributes
• Name
• Type
• Location
• Size
• Protection
• Time and date
Disk Scheduling
One of the responsibilities of the OS is to use the hardware efficiently. For the disk 
drives, meeting this responsibility entails having fast access time and large disk 
bandwidth.
Access time has two major components
• Seek time is the time for the disk arm to move the heads to the cylinder 
containing the desired sector.
• The rotational latency is the additional time for the disk to rotate the desired 
sector to the disk head. It is not fixed, so we can take average value.
„ . , , Onccomplete revolution time
Rotational latencv = ---------- -------------------------
Disk bandwidth is the total number of bytes transferred, divided by the total time 
between the first for service and the completion of last transfer.
FCFS Scheduling: This is also known as First In First Out (FIFO) simply queues 
processes in the order that they arrive in the ready queue.
The following features which FIFO scheduling have.
• First come first served scheduling.
• Processes request sequentially.
• Fair to all processes, but it generally does not provide the fastest service.
total head movement of 640 cylinders
queue = 98, 183, 37, 122, 14, 124, 65, 67 
head starts at 53
Shortest Seek Time First (SSTF) Scheduling: It 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. It is not an optimal 
algorithm but its improvement over FCFS.
Page 3


Logically related data items on the secondary storage are usually organised into 
named collections called files. A file may contain a report, an executable program 
as a set of commands to the operating system. A file often appears to the users as 
a linear array of characters or record structures.
The file system consists of two parts:
1. A collection of files
2. A directory structure
The file management system can be implemented as one or more layers of the 
operating system.
The common responsibilities of the file management system includes the following
• Mapping of access requests from logical to physical file address space.
• Transmission of file elements between main and secondary storage.
• Management of the secondary storage such as keeping track of the status 
allocation and deallocation of space.
• Support for protection and sharing of files and the recovery and possible 
restoration of the files after system crashes.
File Attributes
Each file is referred to by its name. The file is named for the convenience of the 
users and when a file is named, it becomes independent of the user and the 
process. Below are file attributes
• Name
• Type
• Location
• Size
• Protection
• Time and date
Disk Scheduling
One of the responsibilities of the OS is to use the hardware efficiently. For the disk 
drives, meeting this responsibility entails having fast access time and large disk 
bandwidth.
Access time has two major components
• Seek time is the time for the disk arm to move the heads to the cylinder 
containing the desired sector.
• The rotational latency is the additional time for the disk to rotate the desired 
sector to the disk head. It is not fixed, so we can take average value.
„ . , , Onccomplete revolution time
Rotational latencv = ---------- -------------------------
Disk bandwidth is the total number of bytes transferred, divided by the total time 
between the first for service and the completion of last transfer.
FCFS Scheduling: This is also known as First In First Out (FIFO) simply queues 
processes in the order that they arrive in the ready queue.
The following features which FIFO scheduling have.
• First come first served scheduling.
• Processes request sequentially.
• Fair to all processes, but it generally does not provide the fastest service.
total head movement of 640 cylinders
queue = 98, 183, 37, 122, 14, 124, 65, 67 
head starts at 53
Shortest Seek Time First (SSTF) Scheduling: It 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. It is not an optimal 
algorithm but its improvement over FCFS.
queue = 98, 183. 37, 122. 14,124. 65. 67 
head starts at 53
0 14 37 536567 98 122124 18319S
|_I ___ I_ _ L _ U ____ I ___ U _________ I_|
SCAN Scheduling: In the SCAN algorithm, the disk arm starts at one end of the disk 
and moves toward the other end, servicing requests as it reaches each cylinder 
until it gets to the other end of the disk. At the other end, the direction of head 
movement is reversed and servicing continues. The head continuously scans back 
and forth across the disk. The SCAN algorithm is sometimes called the elevator 
algorithm, since the disk arm behaves just like an elevator in a building, first 
servicing all the request going up and then reversing to service requests the other 
way.
queue = 98, 183, 37, 122, 14, 124, 65, 67 
head starts at 53
0 14 37 536567 98 122124 183199
C-SCAN Scheduling: Circular SCAN is a variant of SCAN, which is designed to 
provide a more uniform wait time. Like SCAN, C-SCAN moves the head from one 
end of the disk to the other, servicing requests along the way. When the head 
reaches the other end, however it immediately returns to the beginning of the disk 
without servicing any requests on the return trip. The C-SCAN scheduling algorithm 
essentially treats the cylinders as a circular list that wraps around from the final 
cylinder to the first one.
Page 4


Logically related data items on the secondary storage are usually organised into 
named collections called files. A file may contain a report, an executable program 
as a set of commands to the operating system. A file often appears to the users as 
a linear array of characters or record structures.
The file system consists of two parts:
1. A collection of files
2. A directory structure
The file management system can be implemented as one or more layers of the 
operating system.
The common responsibilities of the file management system includes the following
• Mapping of access requests from logical to physical file address space.
• Transmission of file elements between main and secondary storage.
• Management of the secondary storage such as keeping track of the status 
allocation and deallocation of space.
• Support for protection and sharing of files and the recovery and possible 
restoration of the files after system crashes.
File Attributes
Each file is referred to by its name. The file is named for the convenience of the 
users and when a file is named, it becomes independent of the user and the 
process. Below are file attributes
• Name
• Type
• Location
• Size
• Protection
• Time and date
Disk Scheduling
One of the responsibilities of the OS is to use the hardware efficiently. For the disk 
drives, meeting this responsibility entails having fast access time and large disk 
bandwidth.
Access time has two major components
• Seek time is the time for the disk arm to move the heads to the cylinder 
containing the desired sector.
• The rotational latency is the additional time for the disk to rotate the desired 
sector to the disk head. It is not fixed, so we can take average value.
„ . , , Onccomplete revolution time
Rotational latencv = ---------- -------------------------
Disk bandwidth is the total number of bytes transferred, divided by the total time 
between the first for service and the completion of last transfer.
FCFS Scheduling: This is also known as First In First Out (FIFO) simply queues 
processes in the order that they arrive in the ready queue.
The following features which FIFO scheduling have.
• First come first served scheduling.
• Processes request sequentially.
• Fair to all processes, but it generally does not provide the fastest service.
total head movement of 640 cylinders
queue = 98, 183, 37, 122, 14, 124, 65, 67 
head starts at 53
Shortest Seek Time First (SSTF) Scheduling: It 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. It is not an optimal 
algorithm but its improvement over FCFS.
queue = 98, 183. 37, 122. 14,124. 65. 67 
head starts at 53
0 14 37 536567 98 122124 18319S
|_I ___ I_ _ L _ U ____ I ___ U _________ I_|
SCAN Scheduling: In the SCAN algorithm, the disk arm starts at one end of the disk 
and moves toward the other end, servicing requests as it reaches each cylinder 
until it gets to the other end of the disk. At the other end, the direction of head 
movement is reversed and servicing continues. The head continuously scans back 
and forth across the disk. The SCAN algorithm is sometimes called the elevator 
algorithm, since the disk arm behaves just like an elevator in a building, first 
servicing all the request going up and then reversing to service requests the other 
way.
queue = 98, 183, 37, 122, 14, 124, 65, 67 
head starts at 53
0 14 37 536567 98 122124 183199
C-SCAN Scheduling: Circular SCAN is a variant of SCAN, which is designed to 
provide a more uniform wait time. Like SCAN, C-SCAN moves the head from one 
end of the disk to the other, servicing requests along the way. When the head 
reaches the other end, however it immediately returns to the beginning of the disk 
without servicing any requests on the return trip. The C-SCAN scheduling algorithm 
essentially treats the cylinders as a circular list that wraps around from the final 
cylinder to the first one.
queue = 98, 183, 37, 122, 14, 124, 65, 67 
head starts at 53
0 14 37 536567 98 122124 183199
1 I_______ I I II__________I_______ L I___________________ I I
\
C-LOOK Scheduling: This is just an enhanced version of C-SCAN. In this the 
scanning doesn't go past the last request in the direction that it is moving. It too 
jumps to the other end but not all the way to the end. Just to the furthest request.
queue 98, 183. 37. 122, 14, 124. 65. 67 
head starts at 53
0 14 37 536567 98 122124 183199