systemd Drive Auto-Mounting

For the longest time, I’ve used external drives for many things at once, and the classical tool for this is the partition. Partitions, however have a fatal flaw - if you want to change the balance of size between them, you have to physically copy all the data around. This can be a slow process even on an SSD! In addition, manually mounting my drives got annoying.

Here’s how I setup something much nicer.

btrfs and subvolumes

The general idea is to, instead of using partitions, use subvolumes. These are a feature of btrfs and are dynamically sized. They essentially operate like folders that can be mounted and snapshotted as if individual filesystems.

They’re relatively easy to set up with the btrfs command line tools.

I set two of these up on my drive, corresponding to my use cases (osu-portable and a vaguely named usb-ssd).

From partitions to the subvolumes

Next, you have to copy all your data. No btrfs magic can make this better for you, they’re two separate partitions as it stands, so you have to copy all your data from the second partition into the subvolume on the first.

This takes a while, and if you don’t have enough free space in the first partition to contain the files from the second, you may have to do this in multiple stages with slow resizes in between. Ugh.

Once all your data is on one partition in subvolumes, you can delete the second partition and fill the disk with your one big partition.


So, I had my old mount points set up in fstab like so:

# <file system>   <dir>         <type> <options> <dump> <pass>
LABEL=osuportable /osu-portable btrfs  noauto    0      0
LABEL=usbssd      /ssd          btrfs  noauto    0      0

I’d need to change this, so run a subvolume list on the drive and note the IDs:

sudo btrfs subvolume list /mnt
ID 256 gen 5571 top level 5 path osu-portable
ID 257 gen 6403 top level 5 path usb-ssd

Now, I change the fstab to look like this.

# <file system> <dir>         <type> <options>                                                                                                      <dump> <pass>
LABEL=usbssd    /osu-portable btrfs  noauto,nofail,x-systemd.automount,x-systemd.idle-timeout=2,x-systemd.device-timeout=0.1,subvolid=256,nodiscard 0      0
LABEL=usbssd    /ssd          btrfs  noauto,nofail,x-systemd.automount,x-systemd.idle-timeout=2,x-systemd.device-timeout=0.1,subvolid=257,nodiscard 0      0
  • x-systemd.automount tells systemd to automatically attempt to mount the drive if a program tries to access it.
  • x-systemd.idle-timeout is how long the drive should be idle before it unmounts it automatically.
  • x-systemd.device-timeout is how long to wait for the drive to be plugged in before erroring on access, I set this low because Steam tries to access everything that looks mounted many times on start, and if the drive is missing then having this at a high value makes it very slow to start.

Then run

sudo systemctl daemon-reload
sudo systemctl restart


One complication here is that this is an SSD, so periodic TRIM commands are generally a good idea to keep it running ideally.

With btrfs specifically, the kernel will automatically TRIM by default as you use the drive. This is nice, but is a problem here because ‘Continuous Trimming’ causes a constant low level of drive activity, which means the drive will never be idle, and never unmounted by systemd automatically!

The better solution here is to use fstrim.timer, included with systemd. It will run TRIM on all connected drives once per week, which can replace continuous trimming.

The nodiscard fstab option disables continuous trim, and periodic trim can be enabled with

sudo systemctl enable fstrim.timer

Waybar Indicator

Given that my drive may or may not be actually mounted at any point, I wanted a way to know! So I threw together a custom module for my status bar, waybar.

I wrote this script file:


if lsblk | grep $1 > /dev/null; then
    echo $2
    echo $3

and this waybar config:

"custom/ssdmnt": {
    "format": "SSD mount: {} ",
    "exec": "$HOME/.config/waybar/ /ssd y n 2> /dev/null",
    "interval": 4, // should be >2, see fstab
    "tooltip": false
"custom/osumnt": {
    "format": "osu: {} ",
    "exec": "$HOME/.config/waybar/ /osu-portable y n 2> /dev/null",
    "interval": 4, // should be >2, see fstab
    "tooltip": false

Now I have an indicator on my status bar that reads SSD mount: n osu: n, and will say y if those subvolumes mount.

Testing time!

Trying to ls /ssd printed "/ssd": No such device (os error 19), and when I plugged my SSD in, not much happened - but then running the ls succeeded, and caused the SSD mount indicator to flip to y for a couple of seconds!


Current issues with this setup

My file manager of choice, pcmanfm, does not hold active locks on the open folder, and so if you open a folder on that drive, it’ll mount, but then unmount a couple seconds later and boot you back to ~. A possible fix for this is to increase the unmount timeout to something like 30 seconds, or to try a different file manager.

Having the drive unplugged makes Steam take longer to load as it will try to read from the mount points that appear to be there but aren’t multiple times before loading. The best fix for this is just a low device-timeout. The default is 90 seconds which is crazy, but at 100ms, it doesn’t take too long to blow through the amount of retries Steam will do. Steam doesn’t just listen to mount state changes, it also listens for USB events, so when you actually plug the drive in it will automatically update and find all the games, which is a nice touch.

Overall, I’m very happy with this setup, and will be using it as is for the forseeable future.