I noticed the dump of my virtual machine with 80 GB used disk was 148 GB (Proxmox, dump with Proxmox backup, discard on in proxmox disk settings). I experimented with fstrim, afterwards dump was 60 GB smaller. Reading discussions it seems fstrim should run about once a week even for rotating media. My virtual machine has Ext4 file system, the host has SSD disks where the image is stored. The fstrim only took 30 seconds so it is small load, and may go faster now that FS is almost trimmed. Any experiences with fstrim? Are you running it from cron and how often?