Backup Stategy

I run Windows 11 Pro as virtual machine in Proxmox host. Because the Windows OS has many important files, I need a backup solution. Especially after I experience the BitLocker locks down my VM.

Strategy

#

I need a backup strategy that will auto-backup my VM only when I shut down it. And I need to keep multiple backup copies while not to causing my local disk full.

Strategy:

• Auto-backup my VM after shutdown.
• Keep multiple copies at daily 2 and weekly 2 as Prune settings.

My challenges:

• Existing local disk is 67% full and limited to 100GB.
• Once it reaches 100%, my Proxmox host will crash.
• Keep multiple backup copies and each is at estiamted 41GB (after compression).

Luckily, my local-lvm partition is at 60% usage (700GB free).

Plan

#

I’ll showoff the “magic” of LVM-Thin without reboot the host.

Here’s the plan to expand my local storage:

1. Create a “Dummy Drive” for backup using local-lvm partition.

2. Expose the “Dummy Drive” to Proxmox for future backup.

3. Configure the backup retention at Datacenter level.

Preparation Steps

#

First, create the volume at the “thin” volume named backup-disk.

% lvcreate -V 200G --thinpool pve/data --name backup-disk

Format it to EXT4. Create the Mount Point and mount it.

% mkfs.ext4 /dev/pve/backup-disk
% mkdir -p /mnt/pve-backups
% mount /dev/pve/backup-disk /mnt/pve-backups
% echo '/dev/pve/backup-disk /mnt/pve-backups ext4 defaults 0 2' >> /etc/fstab

Next, expose it to Proxmox.

From Proxmox Web UI:

1. Datacenter > Storage > Add > Directory

2. ID: Backups

3. Directory: /mnt/pve-backups

4. Content: Backup

5. Nodes: proxmox

6. Click Add.

Last, I move existing backups to the new location.

% mkdir -p /mnt/pve-backups/dump
% sudo mv /var/lib/vz/dump/* /mnt/pve-backups/dump
% ln -lh /mnt/pve-backups/dump

Update Backup Retention

#

1. Goto Datacenter > Backup

2. Disable any existing Backup Jobs

3. Goto Datacenter > Storage

4. Select Backups ID (created earlier)

5. Click Backup Retention tab

6. Keep Daily: 2

7. Keep Weekly: 2

8. Click OK.

The “Shutdown-Triggered” Backup (Hook Script)

#

My goal is to tell Proxmox to run a script whenever a VM changes state (to shutdown). And I have gone some lessons learn before reaching this final version.

This “Backup on Shutdown” hook script will tells Proxmox: Whenever this VM stops, run a backup to the new Backups storage.

The strategy we use in our script here is called “Defer and Detach”.

First, intall with sudo apt install at.

Then, at the Proxmox host, create a file /var/lib/vz/snippets/backup-on-shutdown.sh.

#!/bin/bash

VMID=$1
PHASE=$2
FLAG_FILE="/tmp/backup_running_$VMID"

if [ "$PHASE" == "post-stop" ]; then
    # If flag exists, this stop was caused by the backup finishing
    if [ -f "$FLAG_FILE" ]; then
        rm "$FLAG_FILE"
        exit 0
    fi

    # Create flag and schedule the backup 1 minute out
    touch "$FLAG_FILE"
    echo "qm unlock $VMID && vzdump $VMID --storage Backups --mode snapshot" | at now + 1 minute
    
    exit 0
fi
exit 0

Proxmox locks the VM configuration during shutdown. The solution is to use the at command to schedule the backup one minute into the future, allowing the host to finish its housekeeping first.

Before we attach it to VM, we need to make it executable: chmod +x /var/lib/vz/snippets/backup-on-shutdown.sh

Last, we will attach it to VM (ID:100) with:

% qm set 100 --hookscript local:snippets/backup-on-shutdown.sh

🎓 Lessons Learned: The Road to Success

#

1. Race Conditions: Proxmox holds a system lock during shutdown. Attempting to backup immediately causes a “Lock Timeout.”

2. The Infinite Loop: The backup process sends a “Stop” signal when it finishes. Without a Flag File, the script would trigger a recursive loop of endless backups.

3. Process Detachment: Using the at command is essential. It completely detaches the backup from the Proxmox process tree, letting the host “clean the room” before the backup starts.

Retention Tip: Use Daily 2 / Weekly 2 prune settings to provide a 3-week safety net while keeping your backup partition lean.