| Symptom | Likely Cause | Fix |
| --- | --- | --- |
| VM freezes under disk load | Missing VirtIO drivers | Reinstall virtio-win, switch to virtio-blk. |
| qcow2 file grows forever | Windows 7 deleted files but no TRIM | Enable "Unmap" in virtio-scsi and run Optimize-Volume -DriveLetter C -ReTrim -Verbose in PowerShell. |
| High host CPU (~50% idle guest) | qcow2 encryption + old host CPU | Disable encryption, use LUKS on host instead. |
| Snapshot revert takes minutes | Deep snapshot chain | Commit snapshots, then create fresh qcow2 via qemu-img convert. |
| Windows 7 shows "Disk is busy 100%" | Antivirus real-time scan | Exclude .qcow2 files and VM process from host AV; inside guest, exclude C:\Windows\CSC. |
qemu-img create -f qcow2 -o cluster_size=2M,backing_file=win7-base.qcow2,backing_fmt=qcow2 win7-overlay.qcow2
Lazy refcounts delay metadata updates, drastically improving write performance on HDDs and SSDs: windows 7 qcow2 top
qemu-img amend -f qcow2 -o lazy_refcounts=on win7-overlay.qcow2