On 20.12.2017 15:37, Paolo Bonzini wrote: > Passing through a port is not possible, but it's possible to hot-unplug > the entire virtio-blk disk and add it back, like: > > -drive if=none,id=bupdisk,file=/dev/sda,format=raw,cache=none > -device virtio-blk-pci,drive=bupdisk > > and then, to change the drive: > > in the guest: sync > in the QEMU monitor: device_del bupdisk > on the host: echo 1 > /sys/block/sda/device/delete > in the QEMU monitor: > drive_add 0 if=none,id=bupdisk,file=/dev/sda,format=raw,cache=none > device_add virtio-blk-pci,drive=bupdisk > > This uses PCI hotplug. An alternative is to use virtio-scsi; then you > can keep always the same controller and do the hotplug at the SCSI level: > > -device virtio-scsi-pci,id=scsi > -drive if=none,id=bupdisk,file=/dev/sda,format=raw,cache=none > -device scsi-hd,drive=bupdisk,bus=scsi.0 > > and then, to change the drive: > > in the guest: sync > in the QEMU monitor: device_del bupdisk > on the host: echo 1 > /sys/block/sda/device/delete > in the QEMU monitor: > drive_add 0 if=none,id=bupdisk,file=/dev/sda,format=raw,cache=none > device_add scsi-hd,drive=bupdisk,bus=scsi.0 At first, thank you very much for that invaluable answer. Of course, I had studied the man page of qemu-system-x86_x64, but that sort of expert knowledge isn't in there :-) I will try both solutions you proposed. The problem I currently can see is that the guest does not provide a sync command (not even Windows server versions do so). Instead, if Windows has identified a removable storage device, it shows a symbol in the tray which can be used to "eject" (safely remove) that device. Thus, at the first place (but see below), if your solution will work or not depends on whether the guest treats the backup disk as removable drive after I have changed the command line as advised. If it does not see it as removable, the tray symbol will be missing and there will be no way to eject the drive. Furthermore, as far as I remember, Windows could possibly crash or corrupt data if you would forcefully remove drives, even if a sync command would have been issued before. At least, from the times I have used SCSI devices, I haven't been able to disconnect SCSI peripherals from a running Windows without negative consequences. But maybe my SCSI hardware just has not been hot-pluggable. Therefore, I have an additional question: Are PCI hotplug (your first solution) and SCSI hotplug (your second solution) supported by Windows Server 2008 R2 and (probably more important) by the VirtIO drivers? Do I need the most recent version of the drivers? The following link seems to solve the problem of the missing sync command: https://docs.microsoft.com/en-us/sysinternals/downloads/sync I haven't tried it yet, but since it is from Mark Russinovich, I'll trust it. I also have thought of playing around with the mountvol command to mount or dismount the disk within the guest, but I think this is a bad idea. In any case, I'll begin to test now and report the results ... Many thanks to all for any further comment, Binarus
