Tuning Systems System administrators can optimize the performance of a system by adjusting various device settings based on a variety of use case workloads. The tuned daemon applies tuning adjustments both statically and dynamically, using tuning profiles that reflect particular workload requirements. Configuring Static Tuning The tuned daemon applies system settings when the service starts or upon the selection of a new tuning profile. Static tuning configures predefined kernel parameters … [Read more...] about Beginners Guide to Tuning Profiles in CentOS/RHEL
Linux
Beginners Guide to Managing Package Module Streams in CentOS/RHEL 8
Introduction to Application Streams CentOS/RHEL 8.0 introduces the concept of Application Streams. Multiple versions of user-space components shipped with the distribution are now delivered at the same time. They may be updated more frequently than the core operating system packages. This provides you with greater flexibility to customize Red Hat Enterprise Linux without impacting the underlying stability of the platform or specific deployments. Traditionally, managing alternate versions of an … [Read more...] about Beginners Guide to Managing Package Module Streams in CentOS/RHEL 8
Common Error Messages from Command xfs_repair in Linux
The post explains some of the common error messages from command xfs_repair and the repairs that it performs. disconnected inode 242002, moving to lost+found xfs_repair found an inode that is in use but is not connected to the filesystem. The inode is moved to the filesystem's lost+found directory. Its name is its inode number (in this example, 242002). If the disconnected inode is a directory, the directory's subtree is preserved--all of its child inodes are automatically moved with it. So, … [Read more...] about Common Error Messages from Command xfs_repair in Linux
How to Permanently set the ethtool settings in CentOS/RHEL 6
Modifications made with the ethtool command are by default disappeared after a reboot. For permanent settings, the interface configuration file has to be edited. For each network device, there will be a file in the /etc/sysconfig/network-scripts/ For example, for device eth0 there will be /etc/sysconfig/network-scripts/ifcfg-eth0 containing the properties of the device. A line can be added to the file to automatically implement the settings allowed by the "ethtool -s" command. For example: # … [Read more...] about How to Permanently set the ethtool settings in CentOS/RHEL 6
How to Check if any of the RPM files were tempered with
Question: In some scenarios which it is suspected that the files in the system from an rpm installation, have been altered, changed, or tampered. For example, the system was hacked and a binary file of ssh tampered. How to check if changes were made on it comparing since installation? To verify and check if the files installed on a system with rpm or yum altered since installation using the following command: # rpm -V [PACKAGE_NAME] If you have any of the files altered in the rpm since … [Read more...] about How to Check if any of the RPM files were tempered with
How to extract RAR files in CentOS/RHEL 7 and 8
There is no default utility in CentOS/RHEL 7 systems to extract a .rar file. This post outlines the steps to install the unar package and use of unar command to extract files with .rar extension. 1. Log in as a root user (or use sudo) and configure the latest EPEL repositories to be able to install the unar package: # yum install epel-release Note: I recommend to disable all the existing repositories and just enable the EPEL repository if you do not want to face any conflict. 2, Now … [Read more...] about How to extract RAR files in CentOS/RHEL 7 and 8
What is the purpose of utmp, wtmp and btmp files in Linux
In a Linux system, everything is logged in a log file under the directory called /var/log. This directory contains logs related to different services and applications. In this directory we have some files such as utmp, wtmp and btmp. Unlike the system log files and the authentication log files, all of these files are binary files. So, we can't use our normal text tools, such as less or grep, to read them or extract information from them. Instead, we'll use some special tools that can read these … [Read more...] about What is the purpose of utmp, wtmp and btmp files in Linux
How to (Correctly) Change the UID and GID of a user/group in Linux
Changing the UID and GID of a user might seem a trivial task to most of the system admins. But it's not so trivial and it involves a lot more changes in the backend. In this post, we have outlined the exact steps to change the UID and GID of a user "user01". Username: user01 Group: group01 Existing UID: 800 Existing GID: 700 New UID: 900 New GID: 600 Pre-requisites 1. Make sure the user for which UID and GID is to be changed is currently not having any active process running in the … [Read more...] about How to (Correctly) Change the UID and GID of a user/group in Linux
cp: omitting directory – error while copying a directory in Linux
The Problem When we are trying to copy a directory to other location, we get below error: $ cp /data01 /data02 cp: omitting directory '/data01' $ The Solution The error above is a common mistake done by Linux newbies while copying a directory to other locations without using the recursive copy option in the 'cp' command. In order to avoid this error use the "-r" or the "-a" option in the copy command. From the man page of 'cp' command: # man cp -R, -r, --recursive copy … [Read more...] about cp: omitting directory – error while copying a directory in Linux
How to enable additional scsi logging in CentOS/RHEL
Question: How to turn on scsi extended debug messages? What are the field/flag definitions of scsi_logging_level? Additional scsi logging messages can be enabled by writing to /proc/sys/dev/scsi/logging_level either via use of the echo or preferably the sysctl command. The kernel parameter consists of ten packed fields, each 3 bits in length. Each field can have a value of 0 to 7. The higher the field's value, the more verbose the logging of messages associated with that field type. NOTE: … [Read more...] about How to enable additional scsi logging in CentOS/RHEL