Proxmox freezes during backup when CIFS storage is full – how to prevent?

[thiagomespb]

I saw a report in a Proxmox user group from a colleague who encountered a critical issue:


During a backup job targeting a CIFS-mounted storage on Proxmox, the remote storage reached 100% usage. As a result, Proxmox froze, the backup failed, and even some VMs became unstable or froze due to the full mount point.


According to him, the job is configured with a retention policy (Keep Last: 3), but it seems Proxmox tries to write the new backup before removing the old ones — which causes the failure if the storage is already at its limit.


My questions are:

• Is there a native way for Proxmox to check available space on the CIFS target before starting the backup?
• Is it possible to configure it to delete old backups before writing new ones?
• Any best practices for dealing with this kind of scenario when space is limited?

Any guidance to help prevent this kind of situation would be greatly appreciated.

 

[BobhWasatch]

1. Is hard because of compression. It is difficult to know up-front how big the final archive will be. Making assumptions will lead to wrong answers and user grief (in both directions).
2. Is unsafe because if the new backup fails you are down by one.
3. Best practice is for the administrator to make sure the backup volume has sufficient space. And by "sufficient", I mean "more than you think you need" rather than "barely enough, maybe".

 

[thiagomespb]

Thank you for the explanation.


I understand the difficulty due to compression unpredictability and the risk of deleting old backups before securing a new one.


However, maybe there could be a middle ground — for example, the backup job itself could fail gracefully if there's insufficient space, without impacting the entire system or causing VMs to freeze. That way, only the backup would fail, not the whole environment.


Would it be feasible for future versions to include at least a pre-check for free space (even if approximate) and abort the backup job before mounting or writing data?