Just to give a brief recap of why I'm writing this post, I've to describe a brief encounter I had with systemd which rendered my system unbootable. I first felt its systemd's problem but later figured out it was my mistake. So below is brief story divided into 2 sections Problem and Solution. Here Problem describes issue I faced with systemd and Solution discusses the .mount and .automount suffix files used by systemd.
I have several local bin folders and I don't want to modify PATH environment variable adding every other folder path. I first started using aufs as alternative to symlinks or modifying PATH variable. Recently I learnt about much more light weight union fs which is in kernel, overlayfs. And Google led me to Arch wiki article which showed me a fstab entry like below
overlay /usr/local/bin overlay noauto,x-systemd.automount,lowerdir=/home/vasudev/Documents/utilities/bin,upperdir=/home/vasudev/Documents/utilities/jonas-bin,workdir=/home/vasudev/Documents/utilities/bin-overlay 0 0
And after adding this entry, on next reboot systemd is not able to mount my LVM home and swap partition. It does however mount root partition. It gives me emergency target but login never returns. So without any alternative I had to reinstall the system. But funnily enough I started encountering the same issue (yeah after I had added above entry into fstab). Yeah that time I never thought that its culprit. My friend Ritesh finally got my system booting after the weird x-systemd.automount option. I never investigated further that time on why this problem occurred.
After re-encounter with similar problem and some other project I read the manual on systemd.mount, systemd-fstab-generator and systemd.automount and I had some understanding of what really went wrong in my case above.
So now let us see what really is happening. All the above happened because, systemd translates the /etc/fstab into .mount units at run time using systemd-fstab-generator. Every entry in fstab translates into a file named after the mount point. The / in the path of the mount point is replaced by a -. So / mount point is named as -.mount and /home is named as home.mount and /boot becomes boot.mount. All these files can be seen in directory /run/systemd/generator. And all these mount points are needed by local-fs.target , if any of these mount points fail, local-fs.target fails. And if local-fs.target fails it will invoke emergency.target.
systemd-fstab-generator manual suggests the ordering information in /etc/fstab is discarded. That means if you have union mounts, bind mounts or fuse mounts in the fstab which is normally at end of fstab and uses path from /home or some other device mount points it will fail to get mounted. This happens because systemd-fstab-generator didn't consider the ordering in fstab. This is what happened in my case my overlay mount point which is usr-local-bin.mount was some how happened to be tried before home.mount because there is no explicit dependency like Requires= or After= was declared. When systemd tried to mount it the path required under /home were not really present hence failed which in return invoked emergency.target as usr-local-bin.mount is Requires= dependency for local-fs.target.
Now what I didn't understand is why emregency.target never gave me the root shell after entering login information. I feel that this part is unrelated to the problem I described above and is some other bug.
To over come this what we can do is provided systemd-fstab-generator some information on dependency of each mount point. systemd.mount manual page suggests several such options. One which I used in my case is x-systemd.requires which should be placed in options column of fstab and specify the mount point which is needed before it has to be mounted. So my overlay fs entry translates to something like below
overlay /usr/local/bin overlay noauto,x-systemd.requires=/home,x-systemd.automount,lowerdir=/home/vasudev/Documents/utilities/bin,upperdir=/home/vasudev/Documents/utilities/jonas-bin,workdir=/home/vasudev/Documents/utilities/bin-overlay 0 0
There is another special option called x-systemd.automount this will make systemd-fstab-generator to create a .automount file for this mount point. What does systemd.automount do? It achieves on-demand file mounting and parallelized file system mounting. Something similar to systemd.socket the file system will be mounted when you access the mount point for first time.
So now if you try to see dependency of usr-local-bin.mount you will see following.
systemctl show -p After usr-local-bin.mount After=-.mount systemd-journald.socket local-fs-pre.target system.slice usr-local-bin.automount
This means now usr-local-bin.mount depends on usr-local-bin.automount. And let us see what usr-local-bin.automount needs.
systemctl show -p Requires usr-local-bin.automount Requires=-.mount home.mount systemctl show -p After usr-local-bin.automount After=-.mount home.mount
So clearly usr-local-bin.automount is activated after -.mount and home.mount is activated. Similar can be done for any bind mounts or fuse mounts which require other mount points before it can be mounted. Also note that x-systemd.autmount is not mandatory option for declaring dependencies, I used it just to make sure /usr/local/bin is mounted only when its really needed.
A lot of traditional way has been changed by systemd. I never understood why my system failed to boot in first place this happened because I was not really aware of how systemd works and was trying to debug the problem with traditional approach. So there is really a learning curve involved for every sysadmin out there who is going to use systemd. Most of them will read documentation before hand but others like me will learn after having a situation like above. :-).
So systemd interfering into /etc/fstab is good?. I don't know but since systemd parallelizes the boot procedure something like this is really needed. Is there a way to make systemd not touch /etc/fstab?. Yes there is you need to pass fstab=0 option in kernel command line and systemd-fstab-generator doesn't create any .mount or .swap files from your /etc/fstab.
!NB Also it looks like x-systemd.requires option was introduced recently and is not available in systemd <= 215 which is default in Jessie. So how do you declare dependencies in Jessie system?. I don't have any answer!. I did read that x-systemd.automount which is available in those versions of systemd can be used, but I'm yet to experiment this. If it succeeds I will write a post on it.