Most Proxmox tuning lines you find online are one blob of kernel flags. Half of them belong on the hypervisor, half belong inside the guest, two of the popular ones do nothing at all, and a few change meaning depending on which side of the boundary they land.
QEMU can emulate a real NVMe controller, so the guest uses its own in-box NVMe driver with no VirtIO media required. It is also unmigratable by design, invisible to Proxmox backups, and no faster than VirtIO SCSI. Here is how to add one and when it is worth it.
A virtual machine’s clock is built on an assumption virtualisation breaks: that the CPU keeps counting. Why guest time drifts on every hypervisor, what the skew actually breaks — Kerberos, TLS, Ceph, Windows — and how ptp_kvm fixes it properly on QEMU/KVM.
Why PCIe devices lose throughput when passed through to a VM via VFIO, and the practical tuning steps that claw most of it back.
On multi-socket systems, a VM with its vCPUs on one NUMA node and its passed-through device on another loses 20–30% throughput before you’ve even looked at anything else.
Active State Power Management saves a few watts on idle PCIe links. Under VFIO passthrough, it adds latency jitter that’s hard to diagnose and easy to fix.
QEMU’s virtual root complex defaults to 128-byte TLP payloads. Most devices support 256 or 512. One kernel parameter fixes it.
The two QEMU virtual chipsets are not interchangeable. Q35 provides a proper PCIe topology that passthrough, modern Windows, and the wider KVM ecosystem all depend on.