Title: Persistency management of memory based filesystem on OpenBSD

	Title: Persistency management of memory based filesystem on OpenBSD
	Author: Solène
	Date: 15 December 2021
	Tags: openbsd performance nocloud
	Description: This explains how to automatically backup and restore mfs
	based filesystem on OpenBSD upon boot and shutdown.

	# Introduction

	For saving my SSD and also speeding up my system, I store some cache
	files into memory using the mfs filesystem on OpenBSD. But that would
	be nice to save the content upon shutdown and restore it at start,
	wouldn't it?

	I found that storing the web browser cache in a memory filesystem
	drastically improve its responsiveness, but it's hard to make
	measurements of it.

	Let's do that with a simple rc.d script.

	# Configuration

	First, I use a mfs filesystem for my Firefox cache, here is the line in
	/etc/fstab

	```fstab entry
	/dev/sd3b /home/solene/.cache/mozilla mfs rw,-s400M,noatime,nosuid,no…
	```

	This mean I have a 400 MB partition using system memory, it's super
	fast but limited. tmpfs is disabled in the default kernel because it
	may have issues and is not well enough maintained, so I stick with mfs
	which is available out of the box. (tmpfs is faster and only use memory
	when storing file, while mfs reserves the memory chunk at first).

	# The script

	We will write /etc/rc.d/persistency with the following content, this is
	a simple script that will store as a tgz file under /var/persistency
	every mfs mountpoint found in /etc/fstab when it receives the "stop"
	command. It will also restore the files at the right place when
	receiving the "start" command.


	```shell script
	#!/bin/ksh

	STORAGE=/var/persistency/

	if [[ "$1" == "start" ]]
	then
	install -d -m 700 $STORAGE
	for mountpoint in $(awk '/ mfs / { print $2 }' /etc/fstab)
	do
	tar_name="$(echo ${mountpoint#/} \| sed 's,/,_,g').tgz"
	tar_path="${STORAGE}/${tar_name}"
	test -f ${tar_path}
	if [ $? -eq 0 ]
	then
	cd $mountpoint
	if [ $? -eq 0 ]
	then
	tar xzfp ${tar_path} && rm ${tar_path}
	fi
	fi
	done
	fi

	if [[ "$1" == "stop" ]]
	then
	install -d -m 700 $STORAGE
	for mountpoint in $(awk '/ mfs / { print $2 }' /etc/fstab)
	do
	tar_name="$(echo ${mountpoint#/} \| sed 's,/,_,g').tgz"
	cd $mountpoint
	if [ $? -eq 0 ]
	then
	tar czf ${STORAGE}/${tar_name} .
	fi
	done
	fi
	```

	All we need to do now is to use "rcctl enable persistency" so it will
	be run with start/stop at boot/shutdown times.

	# Conclusion

	Now I'll be able to carry my Firefox cache across reboots while keeping
	it in mfs.

	* Beware! A situation like using a mfs for a cache can lead to getting
	a full filesystem because it's never emptied, I think I'll run into the
	mfs filesystem full after a week or two.
	* Beware 2! If the system has a crash, mfs data will be lost. The
	script remove the archives at boot after using it, you could change the
	script to remove them before creating the newer archive upon stop, so
	at least you could recover "latest known version", but it's absolutely
	not a backup. mfs data are volatile and I just want to save it softly
	for performance purpose.