Page 180 - DCAP403_Operating System

Unit 9: I/O & Secondary Storage Structure

system available memory has been used. The unused quarter of memory allows a buffer between Notes
the system and the swapper to give the system computational fl exibility.
When the number of processes created approaches capacity, the system might exhibit thrashing
and a decrease in system response time.

9.8.2 Physical Swap Space

There are two kinds of physical swap space: device swap and fi le-system swap.

Device Swap Space

Device swap space resides in its own reserved area (an entire disk or logical volume of an LVM

disk) and is faster than file-system swap because the system can write an entire request (256 KB)
to a device at once.
File-system Swap Space

File-system swap space is located on a mounted file system and can vary in size with the system’s
swapping activity. However, its throughput is slower than device swap, because free fi le-system
blocks may not always be contiguous; therefore, separate read/write requests must be made for
each fi le-system block.

9.8.3 Three Rules of Swap Space Allocation

1. Start at the lowest priority swap device or file system. The lower the number, the higher

priority; that is, space is taken from a system with a zero priority before it is taken from a
system with a one priority.
2. If multiple devices have the same priority, swap space is allocated from the devices in a
round-robin fashion. Thus, to interleave swap requests between a number of devices, the

devices should be assigned the same priority. Similarly, if multiple file systems have the
same priority, requests for swap are interleaved between the file systems. In the fi gure,

swap requests are initially interleaved between the two swap devices at priority 0.
3. If a device and a file system have the same swap priority, all the swap space from the

device is allocated before any file-system swap space. Thus, the device at priority 1 will be
filled before swap is allocated from the file system at priority 1.

Task List various types of physical swap space.

9.9 RAID Structure

RAID stands for Redundant Array of Independent (or Inexpensive) Disks. It involves the
configuration (setting up) of two or more drives in combination for fault tolerance and

performance. RAID disk drives are used frequently on servers and are increasingly being found
in home and office personal computers.

Disks have high failure rates and hence there is the risk of loss of data and lots of downtime for
restoring and disk replacement. To improve disk usage many techniques have been implemented.
One such technology is RAID (Redundant Array of Inexpensive Disks). Its organisation is based
on disk striping (or interleaving), which uses a group of disks as one storage unit. Disk striping

LOVELY PROFESSIONAL UNIVERSITY 173