Diagnosis and Performance Monitoring - Deploying and Managing a Cloud Infrastructure - page 144

move data heavily should be defragmented regularly. Depending on the disk usage, it
should be done once a month or even once a week.
Swap Disk Space
Swap disk space , or swap space , is largely a Linux term that refers to a preconfigured space
on the disk that stores data being frequently used by the main memory. On Windows this
is called a page file, and the process is called paging. Swapping or paging is a memory man-
agement scheme in which snapshots of memory called pages are transferred to a specific
location on disk. This is an important part of virtual memory implementation; it allows the
system to use disk storage for data that does not fit into the main memory (RAM). This can
be viewed as increasing RAM size because main memory can now accommodate more data
by dumping older data into disk, virtually increasing capacity slightly.
It is typically the OS that sets a specific size for the swap file and then just increases that
if there is need for more. The size is returned to the default value once this need has been
fulfilled. However, the user is able to set a fixed amount, which cannot be changed by the
OS even if it needs to.
I/O Tuning
Since we are on the topic of tuning disks, keep in mind that you cannot really alter how a
disk operates and increase its mechanical performance. The best thing you can do is employ
proper I/O tuning and some file system tuning.
As we have discussed many times, applications and many other processes are limited by
disk input/output operations. And it is often the case that CPU activity is suspended until
a specific I/O operation completes. If the application is often waiting for an I/O operation,
it is said to be I/O bound . I/O tuning can help enhance performance if the applications are
really working more on the disks and saving and modifying files, but it cannot do much for
CPU bound applications.
The I/O requirements should be analyzed during the initial design phase for a system.
You must find out what resources you require to get the desired performance. This is a top-
down approach to I/O tuning. But if you already have a system in place, you must approach
tuning from the bottom up. You can go do so by following recommended steps:
1. Determine the exact number of disks being utilized by the system.
2. Determine the specific number of disks being used by the application you are tuning for.
3. Determine the type of I/O operations being performed by your system.
4. Determine if those I/O operations are going through the file system or directly
into devices.