VM becomes unresponsive during Veeam backup? It’s almost always I/O starvation. The agent hammers the disk so hard the VM can’t get a word in edgewise. On Windows, VSS freezes the filesystem too. Here’s how to fix it on Proxmox.
Why the VM freezes
Veeam Agent isn’t officially supported on Proxmox. It’s built for VMware and Hyper-V, so it assumes it can talk to the hypervisor and quiesce the VM. On Proxmox, it just runs inside the guest like any other app. That means it eats whatever I/O the VM has. If your storage is slow or the VM has a small disk queue, the backup can starve the OS. Linux VMs also suffer from the agent’s snapshot mechanism, which can stall I/O.
On Windows, the Volume Shadow Copy Service (VSS) is the usual culprit. Veeam triggers a VSS snapshot, and if the VSS writers hang, the whole VM hangs. You’ll see the disk light solid and the console frozen.
Fix I/O starvation
The real fix is to limit the backup’s I/O. Veeam Agent has a throttling setting. On Linux, you can use ionice and cgroups to cap its disk bandwidth. On Proxmox, you can also limit the VM’s disk I/O from the host, but that affects everything. Better to throttle inside the guest.
For Linux, create a cgroup with a block I/O limit. Something like this:
# Create a cgroup for Veeam
sudo cgcreate -g blkio:/veeam
# Limit read/write to 50 MB/s
sudo cgset -r blkio.throttle.read_bps_device="8:0 52428800" veeam
sudo cgset -r blkio.throttle.write_bps_device="8:0 52428800" veeam
# Run Veeam inside the cgroup
sudo cgexec -g blkio:veeam veeamYou’ll need to find your disk’s major:minor number with lsblk. Adjust the limit to what your storage can handle. This keeps the VM responsive.
Fix VSS hangs on Windows
If you’re backing up a Windows VM, disable VSS integration in the Veeam job settings. It’s under Advanced Settings > Application-Aware Processing. Turn off “Enable application-aware processing”. That skips the VSS snapshot and the freeze. You lose application consistency, but at least the VM doesn’t hang.
If you need VSS, make sure the VSS writers are healthy. Run vssadmin list writers in an admin prompt. Any writer in a failed state will cause hangs. Restart the Volume Shadow Copy service and the COM+ Event System service. That often clears it.
Proxmox-specific tweaks
On the Proxmox host, make sure the VM’s disk is on storage that can handle the backup load. If you’re using ZFS, the agent’s random reads can tank the pool. Consider moving the VM to an SSD or limiting the backup window. Also, check the VM’s CPU type. If it’s set to kvm64, switch to host for better performance. That can help the VM keep up during the backup.
One more thing: if you’re using QEMU guest agent, make sure it’s installed and running. It won’t prevent the freeze, but it helps Proxmox manage the VM during heavy I/O.
My take
Veeam on Proxmox is a pain. The agent isn’t designed for it, and you’ll always be fighting these issues. For a homelab, I’d just use Proxmox’s built-in backup. It’s simpler and it works. If you need Veeam for some reason, throttle the hell out of it and disable VSS. That’s the only way to keep your VM alive.
Related: Proxmox VM Freezes During NFS Backup: Diagnose the Snapshot Hang
