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Home > Cloud & Networking > Servers > Troubleshooting CentOS
Troubleshooting CentOS
Troubleshooting CentOS

Troubleshooting CentOS: A practical guide to troubleshooting the CentOS 7 community-based enterprise server

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Profile Icon Jonathan Hobson
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Troubleshooting CentOS

Chapter 2. Troubleshooting Active Processes

A deeper understanding of the underlying active processes in CentOS 7 is an essential skill for any troubleshooter. From high load averages to slow response times, system overloads to dead and dying processes, there comes a time when every server may start to feel sluggish, act impoverished, or fail to respond, and as a consequence, it will require your immediate attention.

In this chapter, we will:

  • Learn about memory management, swap, swappiness, and thrashing
  • Learn how to analyze active processes using the vmstat, top, and ps commands
  • Learn how to monitor the server with iotop, iostat, and lsof
  • Learn about system load and systemd
  • Learn how to find process IDs, identify parent process IDs and orphaned processes, and initiate the various forms of the kill signal

Tuning server performance with memory management and swap

Regardless of how you look at it, the question of memory usage remains critical to the life cycle of a system, and whether you are maintaining system health or troubleshooting a particular service or application, you will always need to remember that the use of memory is a critical resource to your system. For this reason, we will begin by calling the free command in the following way:

# free -m

The main elements of the preceding command will look similar to this:

         Total    used    free    shared    buffers    cached
Mem:      1837     274    1563         8          0       108
-/+ buffers/cache: 164    1673
Swap:     2063       0    2063

In the example shown, I have used the -m option to ensure that the output is formatted in megabytes. This makes it easier to read, but for the sake of troubleshooting, rather than trying to understand every numeric value shown, let's reduce the scope of the original output to highlight...

Managing memory with vmstat

A different aspect of memory management can be achieved by using the vmstat command. Considered to be a summary reporting feature associated with memory, processes, and paging, vmstat can be seen in action by typing:

# vmstat -a

Having used the -a option to call on all active and inactive memory, the most endearing columns shown under vmstat's output are best described as follows:

  • si: This column shows the value swapped in from disk
  • so: This column shows the value swapped out to disk
  • bi: This column shows the value sent to block devices
  • bo: This column shows the value received from block devices
  • us: This column shows the user time
  • sy: This column shows the system time
  • id: This column shows the idle time

The display does look quite confusing to begin with, but for our purposes, we want to concentrate on the following columns contained under the swap column:

free           si   so
1645452          0    0

Where free shows the current allocation of free memory, si shows...

Checking the system load with the top command

The top command can be called at any time by typing:

# top

The top command is the standard command for checking system load (RAM/MEM and CPU). It contains a lot of information related to tasks associated with the kernel; the display is updated in real-time and the highest load factors are expressed as a percentage of CPU or MEM. However, it is important to realize that top may take these values above the expected percentile range. This is because all individual cores are expressed as a percentage and multiple instances of these cores are totaled. For example, a dual core system may have the first core at 70 percent and the second core at 60 percent, and in this instance, top may show a combined result of 130 percent, but you will not know the individual values.

You can use the M key to sort top by memory, but as you will see, rather than simply showing the amount of free memory (as seen with the free command), top will provide the swap details...

Monitoring disk I/O with iotop

Every administrator knows that a system can begin to slow down as a result of heavy disk I/O activities. However, in the role of a troubleshooter you will probably want to know which processes or (in the case of multi-user systems) which users are the culprits that and it is for this reason, you will want to turn to iotop—a tool that shows a list of the most I/O intensive processes in real time in a top-like interface.

To begin with, you will need to install iotop by typing:

# yum install iotop

The download is only small, and to start a discovery session, simply use the following command:

# iotop

Running iotop without any arguments will result in a list of all existing processes regardless of their disk I/O activities, so if you want iotop to only report on processes that are committed to disk I/O activity, you should use the following instead:

# iotop –o

The output is verbose as it works in a way similar to the top command, so familiarity should...

Checking processes with the ps command

For most troubleshooters who want a more complete picture of the processes running on their system, we can employ the ps command in the following way:

# ps aux | less

Alternatively, the information can be displayed in a user-friendly, tree-view mode like this:

# ps axjf | less

If you prefer a little less detail, try:

# ps auxf | less

Of course, there are always a lot more options that we can use with ps. For example, the command can be piped and applied with grep or tail, and you can use explicit statements such as ps -e (to show every process on the system). Alternatively, you can target a specific process by typing the following command:

# ps aux | grep <process_name>

Moreover, you can even extend its usage to show every process (except those running as root) with the following variation:

# ps -U root -u root -N

For a specific user, you can use:

# ps -u <username> u

Finally, you can then obtain additional security information and output the...

Tuning server performance with memory management and swap

Regardless of how you look at it, the question of memory usage remains critical to the life cycle of a system, and whether you are maintaining system health or troubleshooting a particular service or application, you will always need to remember that the use of memory is a critical resource to your system. For this reason, we will begin by calling the free command in the following way:

# free -m

The main elements of the preceding command will look similar to this:

         Total    used    free    shared    buffers    cached
Mem:      1837     274    1563         8          0       108
-/+ buffers/cache: 164    1673
Swap:     2063       0    2063

In the example shown, I have used the -m option to ensure that the output is formatted in megabytes. This makes it easier to read, but for the sake of troubleshooting, rather than trying to understand every numeric value shown, let's reduce the scope of the original output to highlight the...

Managing memory with vmstat

A different aspect of memory management can be achieved by using the vmstat command. Considered to be a summary reporting feature associated with memory, processes, and paging, vmstat can be seen in action by typing:

# vmstat -a

Having used the -a option to call on all active and inactive memory, the most endearing columns shown under vmstat's output are best described as follows:

  • si: This column shows the value swapped in from disk

  • so: This column shows the value swapped out to disk

  • bi: This column shows the value sent to block devices

  • bo: This column shows the value received from block devices

  • us: This column shows the user time

  • sy: This column shows the system time

  • id: This column shows the idle time

The display does look quite confusing to begin with, but for our purposes, we want to concentrate on the following columns contained under the swap column:

free           si   so
1645452          0    0

Where free shows the current allocation of free memory, si shows page...

Checking the system load with the top command

The top command can be called at any time by typing:

# top

The top command is the standard command for checking system load (RAM/MEM and CPU). It contains a lot of information related to tasks associated with the kernel; the display is updated in real-time and the highest load factors are expressed as a percentage of CPU or MEM. However, it is important to realize that top may take these values above the expected percentile range. This is because all individual cores are expressed as a percentage and multiple instances of these cores are totaled. For example, a dual core system may have the first core at 70 percent and the second core at 60 percent, and in this instance, top may show a combined result of 130 percent, but you will not know the individual values.

You can use the M key to sort top by memory, but as you will see, rather than simply showing the amount of free memory (as seen with the free command), top will provide the swap details...

Monitoring disk I/O with iotop

Every administrator knows that a system can begin to slow down as a result of heavy disk I/O activities. However, in the role of a troubleshooter you will probably want to know which processes or (in the case of multi-user systems) which users are the culprits that and it is for this reason, you will want to turn to iotop—a tool that shows a list of the most I/O intensive processes in real time in a top-like interface.

To begin with, you will need to install iotop by typing:

# yum install iotop

The download is only small, and to start a discovery session, simply use the following command:

# iotop

Running iotop without any arguments will result in a list of all existing processes regardless of their disk I/O activities, so if you want iotop to only report on processes that are committed to disk I/O activity, you should use the following instead:

# iotop –o

The output is verbose as it works in a way similar to the top command, so familiarity should make you feel...

Checking processes with the ps command

For most troubleshooters who want a more complete picture of the processes running on their system, we can employ the ps command in the following way:

# ps aux | less

Alternatively, the information can be displayed in a user-friendly, tree-view mode like this:

# ps axjf | less

If you prefer a little less detail, try:

# ps auxf | less

Of course, there are always a lot more options that we can use with ps. For example, the command can be piped and applied with grep or tail, and you can use explicit statements such as ps -e (to show every process on the system). Alternatively, you can target a specific process by typing the following command:

# ps aux | grep <process_name>

Moreover, you can even extend its usage to show every process (except those running as root) with the following variation:

# ps -U root -u root -N

For a specific user, you can use:

# ps -u <username> u

Finally, you can then obtain additional security information and output...

Checking performance with iostat and lsof

Having already discovered how vmstat can be used to provide statistics related to memory management, when troubleshooting performance-related issues an overburdened CPU is yet another area of concern. For this purpose, we can use the iostat command like this:

# iostat

However, to display a more interactive CPU utilization report, you can use the –c option (and provide a numeric value measured in seconds, such as 5 seconds) like this:

# iostat –c 5

Most of the columns should be self-explanatory, but if the system is getting busy, you will see an increase in %iowait, which is used to report on an increase in waiting time for any I/O requests to be completed. Based on this, if the server is transferring or copying a large amount of files, you may also notice additional time being spent at the system level as files will be moved in and out of relevant disk partitions. A feature that is particularly useful when attempting to monitor storage devices in...

Calculating the system load

The system load is a measure of the amount of processing a computer system is currently performing. It is not the perfect way to measure computer performance, but it does provide the troubleshooter with the additional evidence they need to fix a system.

The expression most commonly associated with calculating load is:

Actual Load = Total Load (uptime) / Number of CPUs

As you probably know the number of CPUs, you can calculate the uptime by reviewing the results of the top command or by typing:

# uptime

The output of the preceding command may look like this:

09:59:41 up  2:36,  1 user,  load average: 0.01, 0.02, 0.05

The server load is expressed as a value based on 1 minute, 5 minute, and 15 minute read times. So, by looking at the final three values in the preceding output, we can see that, for this system, the average load was 0.01 (at 1 minute), 0.02 (at 5 minutes), and 0.05 (at 15 minutes).

At this current time, the example system shows no sign of fatigue, but...

Discovering process IDs with pgrep and systemctl

Rather than using ps, another way of discovering a specific process ID is to use the pgrep command like this:

# pgrep <servicename>

In most cases, the use of this command will reveal the process ID or PID. However, by using this approach, it is also possible that the output will provide more than one value. So remember, if an application (such as httpd or ssh) provides one or more process IDs, you can safely assume that the lowest number (which represents the first PID generated by the system) is the most important. This value is known as the PPID or parent process ID.

On the other hand, a more succinct method could be based on taking advantage of systemd by using the following command:

# systemctl status <service_name>.service

The output of the preceding command will look similar to the following sample, and as we can see, the main PID for Apache is 2413:

httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd...
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Description

It is assumed that you will already have a server up and running, you have a good working knowledge of CentOS, and you are comfortable with the concept of working with those services used by your server.

Who is this book for?

It is assumed that you will already have a server up and running, you have a good working knowledge of CentOS, and you are comfortable with the concept of working with those services used by your server.

What you will learn

  • Consider the need to understand, manipulate, and make use of the relevant system log files
  • Analyze, review, and make decisions regarding how and what to do with troublesome active processes on a CentOS server
  • Discover how to approach issues regarding the network environment
  • Approach issues regarding package management and learn how to make the necessary steps to diagnose and fix the problems found in relation to their YUM and RPMbased needs
  • Diagnose and troubleshoot issues related to Samba, NFS, and various external storage methods
  • Diagnose and troubleshoot issues related to iptables, SELinux, some common firewalls, shell access, and SSH
Estimated delivery fee Deliver to New Zealand

Standard delivery 10 - 13 business days

NZ$20.95

Premium delivery 5 - 8 business days

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(Includes tracking information)

Product Details

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Publication date, Length, Edition, Language, ISBN-13
Publication date : Jun 24, 2015
Length: 190 pages
Edition : 1st
Language : English
ISBN-13 : 9781785289828
Category :
Cloud & Networking
Concepts :
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Tools :
CentOS

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Publication date : Jun 24, 2015
Length: 190 pages
Edition : 1st
Language : English
ISBN-13 : 9781785289828
Category :
Cloud & Networking
Concepts :
Servers
Tools :
CentOS

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Table of Contents

11 Chapters
1. Basics of Troubleshooting CentOS Chevron down icon Chevron up icon
1. Basics of Troubleshooting CentOS
Installing some basic tools
Gathering hardware information
Understanding dmesg
Understanding log files
Reading log files and affecting the output
Using tail to monitor log files
Using cat, less, and more
Using grep
Using diff
Using truncation
Summary
References
2. Troubleshooting Active Processes Chevron down icon Chevron up icon
2. Troubleshooting Active Processes
Tuning server performance with memory management and swap
Managing memory with vmstat
Checking the system load with the top command
Monitoring disk I/O with iotop
Checking processes with the ps command
Checking performance with iostat and lsof
Calculating the system load
Discovering process IDs with pgrep and systemctl
More about systemd
Issuing the kill signal
Dealing with an orphaned process
Summary
References
3. Troubleshooting the Network Environment Chevron down icon Chevron up icon
3. Troubleshooting the Network Environment
Using ping, dig, host, traceroute, and mtr
Monitoring network connections with the ss command
Packet analysis with tcpdump
Summary
References
4. Troubleshooting Package Management and System Upgrades Chevron down icon Chevron up icon
4. Troubleshooting Package Management and System Upgrades
Gathering software information
Using Yum plugins
Fixing Yum operations
Installing additional Yum repositories
Downloading an RPM package with Yum
Diagnosing a corrupt RPM database
Minor release upgrades
Summary
References
5. Troubleshooting Users, Directories, and Files Chevron down icon Chevron up icon
5. Troubleshooting Users, Directories, and Files
Users
Monitoring user activity with utmpdump
Resetting the root password and enhancing logging
Recovering lost or deleted files with Scalpel
Restoring file and directory permissions
Working with and extending the XFS filesystem
Running repairs on XFS
Investigating fragmentation on XFS
Auditing directories and files
Visualizing directories and files
Summary
References
6. Troubleshooting Shared Resources Chevron down icon Chevron up icon
6. Troubleshooting Shared Resources
Providing NFS shares on a CentOS 7 server
About NFS exports
Mounting NFS shares on a CentOS client
Mounting an external drive with CIFS
Using autofs to mount an external drive
Summary
References
7. Troubleshooting Security Issues Chevron down icon Chevron up icon
7. Troubleshooting Security Issues
Auditing SELinux with aureport and setroubleshoot
SSH banners
Tuning SSH
Intrusion detection with Tripwire
Firewalld – zone, service, and port management
Removing Firewalld and returning to iptables
Summary
References
8. Troubleshooting Database Services Chevron down icon Chevron up icon
8. Troubleshooting Database Services
Getting up-and-running with MariaDB
Resetting and recovering a root password with MariaDB
Tuning MariaDB and MySQL
Obtaining metrics from MariaDB and MySQL
Returning to MySQL
Installing and configuring PostgreSQL 9
Summary
References
9. Troubleshooting Web Services Chevron down icon Chevron up icon
9. Troubleshooting Web Services
Auditing the server with cURL
Debugging Akamai headers with cURL
Adding Varnish to Apache
Testing Varnish with cURL
Using cURL to access an FTP directory
Enabling mod_status in Apache
Summary
References
10. Troubleshooting DNS Services Chevron down icon Chevron up icon
10. Troubleshooting DNS Services
Changing the hostname and managing the FQDN
Performing system sanity checks with BIND
Monitoring bandwidth with iftop
Flushing the cache
Summary
References
Index Chevron down icon Chevron up icon
Index

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Customer reviews

Rating distribution
Full star icon Full star icon Full star icon Full star icon Empty star icon 4
(3 Ratings)
5 star 33.3%
4 star 33.3%
3 star 33.3%
2 star 0%
1 star 0%
Amazon Customer Mar 12, 2018
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Awesome book!
Amazon Verified review Amazon
Ed P Jul 10, 2015
Full star icon Full star icon Full star icon Full star icon Empty star icon 4
Jonathan Hobson's book is a handy intro to troubleshooting a variety of issues that CentOS system admins may encounter. In addition he covers some basic information on installing and configuring some of the more popular Linux services, like MySQL/MariaDB, file shares and DNS services. While not exhaustive in any one area, it does provide a place to start when trying to resolve problems on the operating system.
Amazon Verified review Amazon
Human Jan 29, 2017
Full star icon Full star icon Full star icon Empty star icon Empty star icon 3
This book mainly seems to be composed of various basic Linux commands with a few examples. I have a few months experience supporting small web servers (mainly VPS's) with no prior server experience and I don't feel that I learned a lot from this, though it probably would have been an OK way to get started when I was first trying to learn if this wasn't covered in the training I got.I need much more detail than this for work.
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Mastering Kubernetes
Mastering Kubernetes
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Jun 2023 746 pages
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The Ultimate Docker Container Book
The Ultimate Docker Container Book
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Aug 2023 626 pages
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Mastering Ubuntu Server
Mastering Ubuntu Server
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Sep 2022 584 pages
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About the author

Profile icon Jonathan Hobson
Jonathan Hobson
Jonathan is a web developer, systems engineer and applications programmer, who, for more than 20 years has been working behind the scenes to support companies, organizations and individuals around the world to realize their digital ambitions. With an honors degree in both English and History and as a respected practitioner of many computer languages Jonathan enjoys writing code, publishing articles, building computers, playing the video games and getting 'out and about' in the big outdoors. He has been using CentOS since its inception and over the years, it has not only earned his trust, but it has become his first choice server solution. CentOS is a first class community based enterprise class operating system, it is a pleasure to work with, and because of this Jonathan has written this book in order that his knowledge and experience can be passed on to others.
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Shipping Details

USA:

'

Economy: Delivery to most addresses in the US within 10-15 business days

Premium: Trackable Delivery to most addresses in the US within 3-8 business days

UK:

Economy: Delivery to most addresses in the U.K. within 7-9 business days.
Shipments are not trackable

Premium: Trackable delivery to most addresses in the U.K. within 3-4 business days!
Add one extra business day for deliveries to Northern Ireland and Scottish Highlands and islands

EU:

Premium: Trackable delivery to most EU destinations within 4-9 business days.

Australia:

Economy: Can deliver to P. O. Boxes and private residences.
Trackable service with delivery to addresses in Australia only.
Delivery time ranges from 7-9 business days for VIC and 8-10 business days for Interstate metro
Delivery time is up to 15 business days for remote areas of WA, NT & QLD.

Premium: Delivery to addresses in Australia only
Trackable delivery to most P. O. Boxes and private residences in Australia within 4-5 days based on the distance to a destination following dispatch.

India:

Premium: Delivery to most Indian addresses within 5-6 business days

Rest of the World:

Premium: Countries in the American continent: Trackable delivery to most countries within 4-7 business days

Asia:

Premium: Delivery to most Asian addresses within 5-9 business days

Disclaimer:
All orders received before 5 PM U.K time would start printing from the next business day. So the estimated delivery times start from the next day as well. Orders received after 5 PM U.K time (in our internal systems) on a business day or anytime on the weekend will begin printing the second to next business day. For example, an order placed at 11 AM today will begin printing tomorrow, whereas an order placed at 9 PM tonight will begin printing the day after tomorrow.


Unfortunately, due to several restrictions, we are unable to ship to the following countries:

  1. Afghanistan
  2. American Samoa
  3. Belarus
  4. Brunei Darussalam
  5. Central African Republic
  6. The Democratic Republic of Congo
  7. Eritrea
  8. Guinea-bissau
  9. Iran
  10. Lebanon
  11. Libiya Arab Jamahriya
  12. Somalia
  13. Sudan
  14. Russian Federation
  15. Syrian Arab Republic
  16. Ukraine
  17. Venezuela