SSD + HDD storage setup in a Linux desktop with bind mount

Hi everyone! Today I’m going to share with you how I have set up my storage in my desktop PC using a 250 GB SSD and a 2 TB HDD.

Background

Back in august 2020, I built my own desktop PC with the help of a friend, and the storage I decided to use was:

• a Samsung MZ-V7S250BW 250 GB NVMe M.2 SSD
• and a Seagate ST2000DM008 Barracuda 2TB HDD

And my choice for the main OS was Linux (specifically, Arch Linux).

First idea: EFI and root in SSD, home in HDD

Before building my PC, I had been thinking about the partition layout I was going to use. After doing some research on the internet, my initial idea was to use the SSD for the EFI System Partition, mounted in /boot, and the root partition, mounted in /, and the HDD for a single partition mounted in /home.

The idea was to store the system files, which usually need a fast access, in the fastest drive (and also the smaller and the most expensive), the SSD, and the user files, which are mostly less frequently accessed and which access speed is not critical, in the slowest drive (and also the biggest and the cheapest), the HDD.

The filesystem I decided to use for the / and /home partitions was ext4, since it is simple and rock-solid. I’m curious about other filesystems like btrfs, which have really interesting features, but at the moment I’m sticking to ext4.

So, when my friend and I finished building the PC, I did a first Arch Linux install with the layout I just described. However, when I booted up my machine after installing the graphical enviroment, I noticed the configuration didn’t load correctly. I assumed that was because the configuration files need a fast access just like the system files, and the user config files are stored in $HOME/.config, which was a subdirectory of the mountpoint of the HDD (/home).

Second idea: EFI and root in SSD, HDD single partition mounted in home and symlink dirs

Then, I did a second installation from scratch, using a different partition layout: just using the SSD like the previous time and no separate partition for /home. I would figure out later what to do with the HDD.

The idea I came up with was mounting the HDD in a subdirectory under my $HOME directory called .hdd, and inside making directories for every standard XDG user directory I wanted to be stored in the HDD (Documents, Music, Pictures and Videos). The rest of the directories (Downloads, hidden directories such as .config, .cache, etc.) would be stored in the SSD. Then, I would symlink those directories in .hdd to $HOME/Documents, $HOME/Music, etc.

At first it seemed like it was a good setup and worked as intended, but later I realized it wasn’t. Why?

• Mounting a device that could be used at a system level in a hidden user directory sounds dirty.
• Using the symlinked directories was a problem for certain things.

Final idea: EFI and root in SSD, HDD single partition mounted in root and bind mount dirs

Bind mount

I wasn’t satisfied with my previous setup, so I did a bit more research and heard about bind mount, which is basically mounting a subdirectory from a partition that is already mounted in another directory. This, unlike symlinks, doesn’t work at a filesystem level, so we’re not actually changing anything in our filesystems. Instead, we are showing the same directory in two different points.

This means that we can mount a partition somewhere in our root filesystem and then selectively remount directories in it in another part of our root filesystem or even in another filesystem so they are available in both locations.

For example, let’s say we have a directory under /home/myuser/Pictures/MyDownloadedPics/Wallpapers that we want to be available at /home/otheruser/MyWallpapers as well. We can bind mount the first directory in the latter… and that’s it! We can access the same directory using two different locations, and any modification on one directory is immediately effective on the other one, as they are showing the same data.

Setup

So the final idea for my setup was mounting the HDD in /mnt/home, creating a directory for my user, and inside it, the XDG user directories I wanted in the HDD (Documents, Music, Pictures and Videos). Finally, I would bind mount those directories in $HOME/Documents, $HOME/Music, etc.

That way, I would have everything stored in the SDD but the user directories which are usually used to store large files (or a large number of files) that doesn’t require fast access (pictures, videos, music, etc.).

I started using this setup several months ago and I think it is just the best solution. Of course, this is totally subjective and there are probably other and better ways to do this, so feel free to let me know if that is the case.

Tutorial

So… how can we achieve this setup?

I’m going to explain it step by step, covering both the cases of a new and an existing Linux-based installation.

I’m assuming the device file for the SSD is /dev/nvme0n1 and the device file for the HDD is /dev/sda, which is my case and may be yours, but it also may not be, so replace them if necessary.

New installation

Create and format partitions

If we are in the case of a new installation, the first thing we have to do when it is required during the installation process is to create the partitions and the filesystems.

We are using the cfdisk tool with this purpose. If your distro uses a graphical installer, create the described partition layout using the appropriate tool.

First, we’re going to proceed with the SSD:

# cfdisk /dev/nvme0n1

Create a GPT partition table.

Create a new partition for the EFI System Partition. A size of 512M should be more than enough. Set the partition type to EFI System.

Create a new partition for the root filesystem. Use the remaining size.

Write changes and quit.

Format partitions accordingly (FAT32 for EFI System Partition as required, and ext4 for the root partition for simplicity):

# mkfs.ext4 /dev/nvme0n1p2
# mkfs.fat -F32 /dev/nvme0n1p1

Then, with the HDD:

# cfdisk /dev/sda

Create a GPT partition table.

Create a new partition for /mnt/home. Use the total size of the disk.

Write changes and quit.

Format the newly created partition:

# mkfs.ext4 /dev/sda1

Mount partitions

First, mount the root partition:

# mount /dev/nvme0n1p2 /mnt

Create the mountpoints for the other partitions:

# mkdir -p /mnt/boot /mnt/mnt/home

and mount them:

# mount /dev/nvme0n1p1 /mnt/boot
# mount /dev/sda1 /mnt/mnt/home

Prepare bind mountpoints

When the installation process is finished and we have successfully booted up our system and created and logged in as an unprivileged user, we can move on to the exciting part: setting up bind mount!

First, let’s create the standard XDG user directories (you may will the xdg-user-dirs package):

$ xdg-user-dirs-update

Then, create the directories we’re going to bind mount to our $HOME in our HDD. My personal choice is Documents, Music, Pictures and Videos, but feel free to make yours as needed. Think of which directories are you going to store large files (or a large number of files) that don’t need fast access in.

First, create a directory for our user in /mnt/home and change the ownership so the directory belongs to it:

$ cd /mnt/home
$ sudo mkdir -p myuser
$ sudo chown -R myuser:myuser myuser

and after that create the directories themselves:

$ cd myuser
$ mkdir -p Documents Music Pictures Videos

Define bind mountpoints

Finally, edit the /etc/fstab file, which describes how filesystems have to be mounted. Add the following lines at the end (modify them as needed if you’re using different directories):

# /mnt/home/myuser/Documents
/mnt/home/myuser/Documents  /home/myuser/Documents  none    rw,bind 0 0

# /mnt/home/myuser/Music
/mnt/home/myuser/Music      /home/myuser/Music      none    rw,bind 0 0

# /mnt/home/myuser/Pictures
/mnt/home/myuser/Pictures   /home/myuser/Pictures   none    rw,bind 0 0

# /mnt/home/myuser/Videos
/mnt/home/myuser/Videos     /home/myuser/Videos     none    rw,bind 0 0

Those lines are telling our system to bind mount /mnt/home/myuser/Documents to /home/myuser/Documents, /mnt/home/myuser/Music to /home/myuser/Music and so on.

Test the setup:

$ sudo mount -a

and try creating a file in one of the bind mounted directories. For example, if we create a file called test in /home/myuser/Documents, it should appear in /mnt/home/myuser/Documents as well.

If it works fine, we’re done! On the next reboots, the bind mount will happen automatically.

And that’s it! We will have everything stored in our SSD, excepting the files saved to our Documents, Music, Pictures and Videos user directories.

Existing installation

But what if we want to use this setup in an existing Linux installation? The steps to follow depend on the situation: if you are already using your drive for your home partition, it will be a little more tricky.

Extra drive unused or not mounted in the home directory

Let’s start with the easiest case: you just installed the new drive or you just were not using it, or it was mounted in your filesystem, but not in /home.

I’m assuming the drive is already partitioned and formatted with a single ext4 partition which device file is /dev/sda1.

First of all, if the partition is mounted, unmount it:

$ sudo umount /dev/sda1

Create the mountpoint for the partition:

$ sudo mkdir -p /mnt/home

and append a couple of lines to /etc/fstab file to define the mount:

$ sudo bash -c 'echo "# /dev/sda1" >> /etc/fstab'
$ sudo bash -c 'echo -e "UUID=$(blkid -s UUID -o value /dev/sda1)\t/mnt/home\text4\t\trw,relatime\t0 2" >> /etc/fstab'
$ sudo bash -c 'echo >> /etc/fstab'

If the partition used to be mounted in a different path, remove those lines, too.

Check /etc/fstab file manually to verify everything is correct and well-formatted and test the configuration:

$ sudo mount -a

If it works properly, now follow the guide for a new installation starting from here but, instead of creating directories in /mnt/home/myuser, move them from /home/myuser and create empty directories in the latter:

skip

$ cd myuser
$ mkdir -p Documents Music Pictures Videos

and instead do

$ mv /home/myuser/{Documents,Music,Pictures,Videos} /mnt/home/myuser
$ mkdir -p /home/myuser/{Documents,Music,Pictures,Videos}

Extra drive used for home partition

Another case would be that you are already using that extra drive and having it mounted in /home. How can we do the migration then?

First of all, log out as your user and log in as root in a tty (if you are using a desktop manager, hit Alt + F2 or Ctrl + Alt + F2 to switch to tty2). We’re doing this because regular users have their home directory under /home, which we are about to remount in a different path, while root user has its home under /root, so that won’t cause any problem.

I’m assuming the corresponding device for the former home partition is /dev/sda1.

Unmount the home partition:

# umount /dev/sda1

Create the mountpoint for the partition:

# mkdir -p /mnt/home

and edit /etc/fstab accordingly, replacing /home with /mnt/home, which is the new mountpoint for our partition:

# sed -i 's/home/mnt\/home/g' /etc/fstab

Check /etc/fstab file manually to verify everything is correct and test the configuration:

# mount -a

If it works properly, now recreate the home directory for your user in /home (and for every other regular user in your system):

# mkdir -p /home/myuser
# chown myuser:myuser /home/myuser

Then, we are going to move everything there except the directories you want to keep in your extra drive. First, enable a couple of shell options for glob matching:

# shopt -s extglob
# shopt -s dotglob

Those will let us match everything except certain files or directories, and match hidden files too when using * wildcard.

Then move everything inside our home directory except, for example, Documents, Music, Pictures and Videos, if those are the directories we want to keep in the extra drive:

# mv /mnt/home/myuser/!(Documents|Music|Pictures|Videos) /home/myuser

Make empty directories under our home directory for those that are going to stay in the extra drive so we can bind mount them and make them belong to your user:

# mkdir -p /home/myuser/{Documents,Music,Pictures,Videos}
# chown myuser:myuser /home/myuser/{Documents,Music,Pictures,Videos}

Now, reboot your system and log in as your regular user and follow the guide for a new installation starting from here.

Conclusion

And that’s all! I hope you find this setup useful and, as always, reach me out at albertomost@gmail.com if you have any questions, suggestions, etc.