About Disk Scheduling
The I/O system has got the following layers:
- User processes: The functions
of this layer including making I/O calls, formatting the I/O and spooling.
- Device independent software: Functions are naming, blocking, protection, allocating and buffering.
- Device drivers: Functions
include setting up the device registers and checking their status.
- Interrupt handlers: These
perform the function of waking up the I/O drivers up on the completion of
the I/O.
- Hardware: Performing the I/O
operations.
- Disk
drives can be pictured as large 1 – D array consisting of logical blocks that
are smallest unit of transfer.
- These
blocks are mapped in to the disk sectors in a sequential manner.
- Mapping is
done in the same manner.
- The responsibility of using the hardware efficiently
is the duty of the operating system for the disk drives for increasing the
speed of access and bandwidth of the disk.
Algorithms for Scheduling Disk Requests
There are several algorithms
existing for the scheduling of the disk requests:
Ø SSTF:
- In this method the
request having the minimum seek time is selected from the present head position.
- This method is a modification of the SJF (shortest job first) scheduling and
therefore contains some possibility of process starvation.
Ø SCAN:
- From one end of the
disk, the disk arm starts and continues in the direction of the other end,
serving to the requests till the opposite end.
- At this end the head is reversed
and the process continues.
- This is sometimes called as the elevator algorithm.
Ø C – SCAN:
- A better
algorithm then the previous one.
- This one offers a more uniform waiting time
than the previous one.
- The movement of the head is from one end to another
while it services the requests encountered along the way.
- However, the
difference is that when it comes to the other it straightaway goes to the
beginning without heeding to any of the requests in the way and then again
starts.
- The cylinders are treated as the circular list wrapped around last and
the first cylinder.
Ø C – Look:
- This is a
modified version of the C – SCAN.
- Here the arm or the head travels only up to
the last request rather than going till the far end.
- Then immediately the
direction is reversed and the process continues.
- For
disk scheduling it is important that the method be selected as per the
requirements only.
- The first one is the most commonly used and appeals to the
needs naturally.
- For a system where often there is a heavy load on the disk,
the SCAN and C- SCAN methods can help.
- The number as well as the kind of
requests affects the performance in a number of ways.
- On the other hand, the
file – allocation method influences the requests for the disk services.
- These
algorithms have to be written as an individual module of the OS so that if
required it can be replaced with a different one easily.
- As a default
algorithm, the LOOK or the SSTF is the most reasonable choice.
Ways to attach to a disk
There are two
ways of attaching the disk:
Ø Network attached: This
attachment is made via a network. This is called the network attached storage. All
such connected storage devices together form the storage area network.
Ø Host attached: This
attachment is made via the I/O port.
All
these disk scheduling methods are for the optimization of the secondary storage
access and for making the whole system efficient.
