If you’re reading this than, you probably searched online and were looking into virtual filesystems and how to merge the contents from multiple hard drives, mount points, or folders into one virtual point.

Why use Unionfs?

In my case the dilemma I was facing was how to combine the contents of multiple remote backup shares and folders for Proxmox vps backups to show in one mount point(directory). This allows clients to see all 7 days of the weekly backups vs me having to attach 7 different ones to show all 7 backup files.

This saves me a lot of time and frustration.
Proxmox backup(
vzdump) creates one backup file per day and it’s labeled by the container ID and date like (vzdumpqemu-100-2019_08_25-00_05_02.vma.lzo). As the remote backup storage I’m using rsync.net does daily snapshots and rotations via ZFS. I dump the backup file to their server every day and then remove the previous version on the live read/write mounted share. So only one backup per VPS per day exists in the snapshot for that day. This saves me money as they manage rotation and there readonly so I’m not able to remove them and can’t lose them, if someone were to somehow compromise the backup login as its read only file system for snapshots.

My provider rsync.net it creates 7 daily snapshots by date accessible over the .zsf hidden directory.


As I have this mounted locally via autofs over sshfs the below hidden snapshot paths all needed to be unified into this folder “/sdd/rsyncnet-backups-link “

This is configured in Proxmox as a local directory storage for backups and also used in the WHMCS Proxmox module for the backup storage path.

So, all these paths are going to be merged so their contents are unified under “/sdd/rsyncnet-backups-link” The first one is the read and write rsync mount point setup via autofs for my rsync.net account. The second one is another server in the same rack I sometimes use for temp storage and monthly snapshots and figured, I’d merge while I’m at it.


If you feel it looks complex it definitely is. I needed to dynamically mount only the available snapshots and which rotate each day with new name.

It would be tedious to maintain and redo everyday after the snapshot is taken and rotated. The second obstacle in my case was scripting how to detect the current folders and being able to use that in a cron. My script is customized for my needs but, it should be reusable for most people with slight mods for your paths and mount points.

If we we’re going to do this once off via unionfs the command would look like this:

 unionfs-fuse -o cow,max_files=32768 \
              -o allow_other,use_ino,suid,dev,nonempty \
              /mnt/rsyncnet/coby-backups=RW:/mnt/morgan=RW:/mnt/rsyncnet/.zfs/snapshot/daily_2019-08-23/coby-backups=RO:/mnt/rsyncnet/.zfs/snapshot/daily_2019-08-24/coby-backups=RO:/mnt/rsyncnet/.zfs/snapshot/daily_2019-08-25/coby-backups=RO:/mnt/rsyncnet/.zfs/snapshot/daily_2019-08-26/coby-backups=RO:/mnt/rsyncnet/.zfs/snapshot/daily_2019-08-27/coby-backups=RO:/mnt/rsyncnet/.zfs/snapshot/daily_2019-08-28/coby-backups=RO:/mnt/rsyncnet/.zfs/snapshot/daily_2019-08-29/coby-backups=RO \

You‘re probably like man that is nuts. You’re absolutely correct, it is and the documentation is a bit vague online for it. I had to pull relevant pieces from different sites and documentation to get the command I wanted to work in Debian/Ubuntu unionfs.

The order is important as you do it in order/priority and also the structure is as follows /mnt/path=RW: would mount “/mnt/path” as read and write file system so files can be written to it.

For example new backups are going to write it to the RW accessible mount point in my case “/mnt/rsyncnet/coby-backups” followed by the second RW path “/mnt/morgan”. Then, these are followed by all the “Read Only” zfs snapshot folders for the last week. I recommend you first build the command you are going to use and then test that it works before you automate the mounting and remounting as paths update.

Installing unionfs-fuse for the system

root@coby:~# apt list --installed | grep unionfs
WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

unionfs-fuse/oldstable,now 1.0-1+b1 amd64 [installed]

So to install it we did the below as root/sudo:

 sudo apt-get -y install unionfs-fuse

Now we need to create the path to the mnt point. In my case I needed to create the dir “/sdd/rsyncnet-backups-link”

 mkdir /sdd/rsyncnet-backups-link

Now that it’s installed we can test your command from the above step. Please note the \ and line breaks were necessary so I edited the example I found in gedit but Notepad++ editor should work too if using Windows.

Once entered it should not provide an error. The unionfs mount will then show when doing a “df-h”. Like below very bottom entry:

root@coby:~# df -h
 Filesystem                                       Size  Used Avail Use% Mounted on
 udev                                              63G     0   63G   0% /dev
 tmpfs                                             13G  1.3G   12G  11% /run
 rpool/ROOT/pve-1                                 424G  276G  149G  65% /
 tmpfs                                             63G   37M   63G   1% /dev/shm
 tmpfs                                            5.0M     0  5.0M   0% /run/lock
 tmpfs                                             63G     0   63G   0% /sys/fs/cgroup
 rpool                                            149G     0  149G   0% /rpool
 rpool/ROOT                                       149G     0  149G   0% /rpool/ROOT
 rpool/data                                       149G     0  149G   0% /rpool/data
 tmpfs                                             13G     0   13G   0% /run/user/0
 /dev/fuse                                         30M   72K   30M   1% /etc/pve
 /dev/sdd                                         916G   80M  870G   1% /sdd
 user@hostname:/remote/path  708G  131G  577G  19% /mnt/morgan   <<<<<<<<<<<< remote sshfs mounts that are also mounted under the unionfs
 user@hostname:                    1.1T  538G  563G  49% /mnt/rsyncnet  <<<<<remote sshfs mounts that are also mounted under the unionfs
 unionfs-fuse                                     1.1T  539G  565G  49% /sdd/rsyncnet-backups-link  <<<< here is our unionfs mount of the 9 directories from above.

You can test this now by cd into the path and it should show the combined contents of all folders, their subfolders and the contents unified. If it worked you’re all set. This can be automated by using autofs. See our tutorial on that here.

Additional Source References

The next part is for those of you that have dynamically changing paths you need to mount via unionfs-fuse.

See the below script for reference and modify it to suit your needs. I recommend commenting out the mount till you test it by running the script with only the echo commands uncommented to see what paths would be output to ensure your formatting is proper.

# run via "/bin/bash snapshot_folder_detection.sh" as cron after snapshot rotation to remount with the current daily snapshots into one unified folder for Proxmox backups access.

#define base path to check for folders 

#Define unified mount point

#Define Snapshot/or dynamically updated folder's base location. In this example we want the paths of all the subdfolders under this path "/mnt/rsyncnet/.zfs/snapshot/" formatting is important.

#For testing output generated uncomment the below lines to see it detecting your folders. In my example i know its always going to be an array of 7 directories so I have only setup that many. Stuff with dynamic varying amounts would need some custom scripting and loop over the array.
echo "${dirs[0]}"
echo "${dirs[1]}"
echo "${dirs[2]}"
echo "${dirs[3]}"
echo "${dirs[4]}"
echo "${dirs[5]}"
echo "${dirs[6]}"

#next your base command would need to be put here and then copy it once below it and then modify it with the array variables so it merges it like you want it to.

#My base command commented out for example
#unionfs-fuse -o cow,max_files=32768 \
#             -o allow_other,use_ino,suid,dev,nonempty \
#             /mnt/rsyncnet/coby-backups=RW:/mnt/morgan=RW:/mnt/rsyncnet/.zfs/snapshot/daily_2019-08-23/coby-backups=RO:/mnt/rsyncnet/.zfs/snapshot/daily_2019-08-24/coby-#backups=RO:/mnt/rsyncnet/.zfs/snapshot/daily_2019-08-25/coby-backups=RO:/mnt/rsyncnet/.zfs/snapshot/daily_2019-08-26/coby-backups=RO:/mnt/rsyncnet/.zfs/snapshot/#daily_2019-08-27/coby-backups=RO:/mnt/rsyncnet/.zfs/snapshot/daily_2019-08-28/coby-backups=RO:/mnt/rsyncnet/.zfs/snapshot/daily_2019-08-29/coby-backups=RO \
#             $UnionMountMergePath

#My command edited with the array variables spliced in and commented out.
#unionfs-fuse -o cow,max_files=32768 \
#             -o allow_other,use_ino,suid,dev,nonempty \
#             /mnt/rsyncnet/coby-backups=RW:/mnt/morgan=RW:"${dirs[0]}"coby-backups=RO:"${dirs[1]}"coby-backups=RO:"${dirs[2]}"coby-backups=RO:"${dirs[3]}"coby-#backups=RO:"${dirs[4]}"coby-backups=RO:"${dirs[5]}"coby-backups=RO:"${dirs[6]}"coby-backups=RO \
#             $UnionMountMergePath


root@coby:~# bash snapshot_folder_detection.sh 

Next, you will want to adjust it if you’re not getting the desired effect. Once, that is corrected you can than uncomment or add in your working command and then set it up as a cron to remount the detected paths automatically on your desired schedule.


You can see an example of a merged directory in the below images.

The first image shows what’s on the live backup server read/write under “/mnt/rsyncnet/coby-backups/dump” when selected in Proxmox backups for this VPS. Which is only the last two days.

Now, when we select the unionfs-fuse mount setup notice how there are contents from all folders merged in nicely. I only recently started the daily backups or it would have had more. This gives you the idea about the functionality this provides and how cool it is when combined with something like rsync.net or a remote backup server setup like this.

In the client portal due to the unionfs magic the client sees all 4 backups from this unionfs mount spread across 3 different locations vs only the two they would have normally seen on the live file system.

If you would like to hire us for a project feel free to shoot us a message via our contact us page for a quote. We would love to assist you with your project and save you time.


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