LVM-Thin Storage: Propagating Guest Trim to Host Thin Pool

Storage Reclamation in Proxmox VE 7.2

Proxmox VE 7.2 allows LVM-Thin storage to reclaim space when guest operating systems issue trim commands. This behavior depends on the interaction between the VM hardware configuration and the underlying block device support.

To achieve storage efficiency, the storage must be defined as lvmthin and the VM hardware settings must include the discard=unmap option. Without this configuration, thin volumes may grow silently as the host cannot identify which blocks are no longer in use by the guest, potentially leading to unexpected capacity exhaustion.

There is uncertainty regarding the most reliable method to verify that trim commands are successfully propagating from the guest OS through the hypervisor to the physical thin pool.

• What is the definitive way to confirm that fstrim in the guest is reducing the used percentage reported by pvesm thin list?
• Are there specific constraints on the underlying physical disk that prevent discard=unmap from functioning despite correct Proxmox configuration?