Understanding the ring buffer in Linux
After the initialization of the system has been completed by the boot procedures (BIOS or UEFI, MBR, and GRUB), the kernel is loaded into memory, the initial ramdisk (initrd or initramfs) is connected to the kernel, and systemd is launched.
The OS is handed over to the startup routines, which finish the setup. When a system is first booted, it may take a while for logging daemons such as syslogd and rsyslogd to become operational. The kernel features a ring buffer that it employs as a message cache to ensure that critical error messages and warnings from this phase of initialization are not lost.