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Chapter 13. Getting started with kernel logging

Log files provide messages about the system, including the kernel, services, and applications running on it. The syslog service provides native support for logging in Red Hat Enterprise Linux. Various utilities use this system to record events and organize them into log files. These files are useful when auditing the operating system or troubleshooting problems.

13.1. What is the kernel ring buffer
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Capture early boot and kernel messages by using the kernel ring buffer, preventing data loss during startup. This cyclic buffer stores the printk() output, which you can view with the dmesg command or save to permanent logs by using the syslog service.

The ring buffer is a cyclic data structure that has a fixed size, and is hard-coded into the kernel. Users can display data stored in the kernel ring buffer through the dmesg command or the /var/log/boot.log file. When the ring buffer is full, the new data overwrites the old.

13.2. Role of printk on log-levels and kernel logging
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The kernel assigns a log-level to every message to indicate its importance. While the ring buffer collects all messages, the kernel.printk parameter determines which messages appear on the console, which effectively filters output based on severity.

The log-level values break down in this order:

0
Kernel emergency. The system is unusable.
1
Kernel alert. Action must be taken immediately.
2
Condition of the kernel is considered critical.
3
General kernel error condition.
4
General kernel warning condition.
5
Kernel notice of a normal but significant condition.
6
Kernel informational message.
7
Kernel debug-level messages.

By default, kernel.printk in RHEL 8 has the following values: 

# sysctl kernel.printk
kernel.printk = 7	4	1	7

The four values define the following, in order:

  1. Console log-level, defines the lowest priority of messages printed to the console.
  2. Default log-level for messages without an explicit log-level attached to them.
  3. Sets the lowest possible log-level configuration for the console log-level.

  4. Sets default value for the console log-level at boot time.

    Each of these values defines a different rule for handling error messages.

Important

The default 7 4 1 7 printk value allows for better debugging of kernel activity. However, when coupled with a serial console, this printk setting might cause intense I/O bursts that might lead to a RHEL system becoming temporarily unresponsive. To avoid these situations, setting a printk value of 4 4 1 7 typically works, but at the expense of losing the extra debugging information.

Also note that certain kernel command line parameters, such as quiet or debug, change the default kernel.printk values.

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