Storing Big Directories on a Separate Disk

August 16, 2016

Don’t tell rms, but sometimes I play Steam games. Fitting a bunch of 10+ GiB games onto a Chromebook with a ~20 GiB disk, however, is practically impossible¹. Two solutions came to mind:

1. Buy another drive
2. Expand my storage with external media

Option 1 requires hundreds of dollars and, worst of all, effort, so I took the second route using an SD card.

Putting Directories on an SD Card

Overview

I tell systemd via /etc/fstab to mount an SD card at /mnt/somewhere as soon as I connect it. Then, the tricky part: I create systemd mount units for bind mounts to the directories I want on the SD card. And to get systemd to do the bind mounts automatically, I make the auto-generated mnt-somewhere.mount unit “want” the bind-mount units.

Step-by-Step

On systemd 215 on Debian Jessie, I did the following:

1. Formatted the SD card as f2fs because I’m brave and using a newer-ish kernel. Use whatever filesystem you want; this isn’t particularly important.

   # mkfs.f2fs -l mylabel /dev/sdX
   

2. Made a mountpoint:

   # mount /mnt/bonus
   

3. Added an /etc/fstab entry for the card:

   UUID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX	/mnt/bonus	f2fs	rw,nofail	0 0
   

   The systemd-specific nofail option tells systemd not to complain if the card isn’t connected at bootup.

4. Now, after doing a systemctl daemon-reload, the SD card should be mounted:

   NAME   FSTYPE LABEL   UUID                                 MOUNTPOINT
   ...
   sdb                                                        
   └─sdb1 f2fs   mylabel XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX /mnt/bonus
   ...
   

5. Next I created an ‘overlay’, a directory structure mimicking my root filesystem so that I can easily identify which directory goes where (e.g., I’d bind-mount /mnt/bonus/overlay/var/x to /var/x).

   # mkdir -p /mnt/bonus/overlay/home/austin/{.steam,Documents/bonus}
   # chown austin:austin /mnt/bonus/overlay/home/austin/{.steam,Documents/bonus}
   

   Creating mountpoints:

   $ mkdir ~/.steam ~/Documents/bonus
   

6. From /etc/fstab, systemd automatically created a mount unit named mnt-bonus.mount, which I used for dependency management. You can see it with systemctl cat:

   $ systemctl cat mnt-bonus.mount
   # /run/systemd/generator/mnt-bonus.mount
   # Automatically generated by systemd-fstab-generator
   
   [Unit]
   SourcePath=/etc/fstab
   Documentation=man:fstab(5) man:systemd-fstab-generator(8)
   
   [Mount]
   What=/dev/disk/by-uuid/e7c2e32a-4a7c-4a34-b72c-0274f81bba6f
   Where=/mnt/bonus
   Type=f2fs
   Options=rw,nofail
   

7. So, to tell systemd to make the bind-mounts automatically once it mounts the SD card at /mnt/bonus, I created a directory named /etc/systemd/system/mnt-bonus.mount.wants/ and added home-austin-Documents-bonus.mount and home-austin-.steam.mount as follows:

   # mkdir /etc/systemd/system/mnt-bonus.mount.wants
   # cat >/etc/systemd/system/mnt-bonus.mount.wants/home-austin-Documents-bonus.mount <<EOF
   [Unit]
   After=mnt-bonus.mount
   
   [Mount]
   What=/mnt/bonus/overlay/home/austin/Documents/bonus
   Where=/home/austin/Documents/bonus
   Type=none
   Options=bind,nofail
   EOF
   # cat >/etc/systemd/system/mnt-bonus.mount.wants/home-austin-.steam.mount <<EOF
   [Unit]
   After=mnt-bonus.mount
   
   [Mount]
   What=/mnt/bonus/overlay/home/austin/.steam
   Where=/home/austin/.steam
   Type=none
   Options=bind,nofail
   EOF
   

8. After running another systemctl daemon-reload, ~/Documents/bonus and ~/.steam should begin pointing to the SD card!

   $ mount
   ...
   /dev/sdb1 on /mnt/bonus type f2fs (rw,relatime,background_gc=on,user_xattr,acl,inline_data,extent_cache,active_logs=6)
   /dev/sdb1 on /home/austin/Documents/sr type f2fs (rw,relatime,background_gc=on,user_xattr,acl,inline_data,extent_cache,active_logs=6)
   /dev/sdb1 on /home/austin/.steam type f2fs (rw,relatime,background_gc=on,user_xattr,acl,inline_data,extent_cache,active_logs=6)
   

9. Later, when I want to unmount the SD card and respective bind-mounts, I can simply run:

   # eject /dev/sdX
   

Caveats

• From my brief testing, my SD card is way slower than my laptop’s SSD. We’re talking 50 MB/s versus 5 MB/s here — an order of magnitude or more. So games struggle much more than usual to load data from the disk.

• If I remove the SD card unexpectedly, any programs opened to bind-mounted files or running in bind-mounted directories get seriously disoriented and need restarting.

  Consequently, I have to close any applications using the bind-mounted directories before unmounting and removing the card.