Monitoring Storage - Red Hat ENTERPRISE LINUX 3 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual

Chapter 2. Resource Monitoring
Page Ins/Page Outs
These statistics make it possible to gauge the flow of pages from system memory to attached
mass storage devices (usually disk drives). High rates for both of these statistics can mean that
the system is short of physical memory and is thrashing, or spending more system resources on
moving pages into and out of memory than on actually running applications.
Active/Inactive Pages
These statistics show how heavily memory-resident pages are used. A lack of inactive pages can
point toward a shortage of physical memory.
Free, Shared, Buffered, and Cached Pages
These statistics provide additional detail over the more simplistic active/inactive page statistics.
By using these statistics, it is possible to determine the overall mix of memory utilization.
Swap Ins/Swap Outs
These statistics show the system's overall swapping behavior. Excessive rates here can point to
physical memory shortages.
Successfully monitoring memory utilization requires a good understanding of how demand-paged
virtual memory operating systems work. While such a subject alone could take up an entire book, the
basic concepts are discussed in Chapter 4 Physical and Virtual Memory. This chapter, along with time
spent actually monitoring a system, gives you the the necessary building blocks to learn more about
this subject.

2.4.4. Monitoring Storage

Monitoring storage normally takes place at two different levels:
Monitoring for sufficient disk space
•
Monitoring for storage-related performance problems
•
The reason for this is that it is possible to have dire problems in one area and no problems whatsoever
in the other. For example, it is possible to cause a disk drive to run out of disk space without once
causing any kind of performance-related problems. Likewise, it is possible to have a disk drive that
has 99% free space, yet is being pushed past its limits in terms of performance.
However, it is more likely that the average system experiences varying degrees of resource shortages
in both areas. Because of this, it is also likely that — to some extent — problems in one area impact
the other. Most often this type of interaction takes the form of poorer and poorer I/O performance as
a disk drive nears 0% free space although, in cases of extreme I/O loads, it might be possible to slow
I/O throughput to such a level that applications no longer run properly.
In any case, the following statistics are useful for monitoring storage:
Free Space
Free space is probably the one resource all system administrators watch closely; it would be a
rare administrator that never checks on free space (or has some automated way of doing so).
File System-Related Statistics
These statistics (such as number of files/directories, average file size, etc.) provide additional
detail over a single free space percentage. As such, these statistics make it possible for system
administrators to configure the system to give the best performance, as the I/O load imposed by
a file system full of many small files is not the same as that imposed by a file system filled with
a single massive file.
17