The sysctl utility allows a Linux admin to query and modify kernel parameters in runtime. For example, to change the swappiness of a linux system to 0 we can:
echo 0 > /proc/sys/vm/swappiness
Or we can use sysctl
sysctl -w vm.swappiness=0
To make the value persistent, Archwiki suggests to writevm.swappiness=0 to /etc/sysctl.d/99-swappiness.conf file.
For persistent silent boot, Archwiki suggests to write kernel.printk = 3 3 3 3 to /etc/sysctl.d/20-quiet-printk.conf
Similarly I have a 99-sysrq.conf on my system which works without the number as well.
Archwiki has a sysctl page which mentions the importance of the number:
Note: From version 207 and 21x, systemd only applies settings from /etc/sysctl.d/.conf and /usr/lib/sysctl.d/.conf. If you had customized /etc/sysctl.conf, you need to rename it as /etc/sysctl.d/99-sysctl.conf. If you had e.g. /etc/sysctl.d/foo, you need to rename it to /etc/sysctl.d/foo.conf.
What does the number in 99-swappiness.conf and 20-quiet-printk.conf denote here?
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Answer
It’s the order in which the files are read. By prefixing it with a two-digit number you can simply order in by the ascii-value of its characters. 20-quiet….conf comes before 99-swapiness.conf. (Try it with ls which sorts by default).
This follows the convention already used by SysV init system. Quoted from the Sequencing directories section of the very useful boot manpage:
To define the starting or stopping order within the same run-level, the name of a link contains an order-number. Also, for clarity, the name of a link usually ends with the name of the service to which it refers. For example, the link
/etc/rc2.d/S80sendmailstarts the sendmail service on runlevel 2. This happens after/etc/rc2.d/S12syslogis run but before/etc/rc2.d/S90xfsis run.