Chapter 12. Getting started with kernel logging

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.

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:

By default, kernel.printk in RHEL 9 has the following values:

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

The four values define the following, in order: