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,nodev 1 0 ``` 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.