Reduce Proxmox LXC Backup Size and Time

Proxmox backs up guests byte-for-byte in a compressed archive. Looking at LXC backups specifically, the file system is compressed into the target backup file with just a few exceptions – temp files aren’t included. You can also add your own exceptions by editing the vzdump.conf to exclude specific file patterns.

All that said, one of the biggest disk space wasters is the cache directory for apt which caches the installation packages for software you have installed. This can generally be safely removed on internet connected machines which will reduce your overall backup size.

For example, a newly created Debian LXC that’s been recently updated shows a total of 206MB of disk used.

du -hs /var/cache/apt/
206M    /var/cache/apt/

After clearing this with the command apt-get clean we can see the space has mostly been freed.

apt-get clean
du -hs /var/cache/apt/
28K    /var/cache/apt/

Considering this whole container is only consuming approximately 1GB of disk space, 200MB is quite significant.

vzdump hooks

Now we can see how much space we can save, we need to make Proxmox issue the apt-get clean command before it creates the backup of our container.

vzdump, the utility which Proxmox uses to perform backups has the ability to call a script for various stages of the backup process – these stages are:

• backup-start
• backup-end
• backup-abort
• log-end
• pre-stop
• pre-restart
• post-restart

We can use these hooks to run our own commands at any of these points of the backup. For the goal of this blog post, we want to run the apt-get clean command at the point of backup-start.

Create a script on your Proxmox host with the following content:

#!/bin/bash
 
if [ "$1" == "backup-start" ] && [ ${VMTYPE} == "lxc" ]; then
    echo "Running pre backup guest cleanup for $3"
    pct exec "$3" -- bash -c "apt-get clean"
fi

Now edit your vzdump.conf file and add the following line to point to your new script. Remember to change the location of where your script is – I’ve just saved mine in /root/.

script: /root/backup-hook.sh