Restore mount local disks after reinstall PBS Server

[LSX3]

Hi everyone,
I’m using Proxmox Backup Server (in my case as a VM, but this question should apply to any PBS installation type).

What I did (GUI workflow)​

I attached a dedicated backup disk to PBS and then in the PBS web UI I created a mount and datastore like this:

• Administration → Disks / Storage → Directory
• Created a new directory on the attached disk (formatted ext4)
• Let PBS mount it and then also use it as a datastore

So far everything works fine.

My concern (disaster recovery / reinstall scenario)​

If the PBS installation itself becomes unusable (VM broken, OS corruption, etc.), I want to be able to reinstall PBS and then reattach the existing disk with the backups.

However, when I try to re-run the assistant / GUI workflow after reinstall, it doesn’t really fit the recovery case because:

• the disk already contains data and/or is already partitioned/formatted,
• and the GUI flow seems oriented around “creating” a new setup rather than “reusing an existing datastore”.

What I observed while trying to understand PBS’ behavior​

I tried to trace what PBS actually changes in the system when going through the steps above:

• I don’t see an entry in /etc/fstab for that mount.
• It looks like PBS creates some kind of systemd mount unit somewhere under /etc/systemd/system/ (or similar), but I haven’t pinned down exactly where and which unit name is used.

---

Questions​

1. What exactly does PBS do under the hood when you create a directory on a disk via the GUI and “mount it” (ext4, directory storage)?
   • Does it create a systemd .mount unit?
   • Where is that configuration stored?
   • How is it named / derived (UUID, device path, mountpoint)?
2. What is the correct recovery procedure after reinstalling PBS if I want to reattach the local disk and reuse the existing backups?
   • What are the steps to mount the disk properly again?
   • How do I re-register the existing path as a PBS datastore without reinitializing/overwriting anything?
   • Is there an official doc / best-practice guide for this scenario?

I’ve searched for documentation or a step-by-step guide for “PBS reinstall + reattach existing local datastore disk” but couldn’t find something that clearly matches this use case.

Thanks a lot in advance for any pointers (especially regarding where PBS stores the mount configuration and the recommended reattach workflow).

 

[Chris]

Questions​

1. What exactly does PBS do under the hoodwhen you create a directory on a disk via the GUI and “mount it” (ext4, directory storage)?
   • Does it create a systemd .mount unit?

Yes, PBS does not use an fstab entry for mounting datastore disk but systemd mount unit are created.

1. • Where is that configuration stored?

They are written into /etc/systemd/system, e.g. /etc/systemd/system/mnt-datastore-test.mount

1. • How is it named / derived (UUID, device path, mountpoint)?

The mount path is used for the file name, following best practice. The mount unit will look something like, e.g. for a datastore test:

[Install]
WantedBy=multi-user.target

[Unit]
Description=Mount datatstore 'test' under '/mnt/datastore/test'

[Mount]
Options=defaults
Type=ext4
What=/dev/disk/by-uuid/<UUID>
Where=/mnt/datastore/test

1. What is the correct recovery procedureafter reinstalling PBS if I want to reattach the local disk and reuse the existing backups?
   • What are the steps to mount the disk properly again?

Best practice would be to restore the mount unit from a backup or recreate it yourself.

1. • How do I re-register the existing path as a PBS datastore without reinitializing/overwriting anything?
   • Is there an official doc / best-practice guide for this scenario?

You can simply use the reuse existing datastore flag in the advanced options of the datastore create window. Just make sure to provide the correct path and that the filesystem is already mounted there.