I wanted to explore overhead with the new drive-backup command and I noticed if I set the target to something like '/dev/null' the guest VM starts having IO errors and loses write access to its root file system. Here is the qmp-shell command I'm using:
> drive-backup sync=none device=virtio0 target=/dev/null format=raw > mode=existing I have a guest running with a single virtio root disk (ext4, Ubuntu guest). After that command, the guest sees write errors to its root block device (virtio0). I didn't trace syscalls or dig deeper yet, but was wondering if you had an idea on why '/dev/null' as a target in a block job would cause the origin device to lockup/fail? My overall goal is to drop the extra write traffic as early as possible to measure overhead of the drive-backup command in a few different scenarios, thus I was hoping /dev/null would help here. -- Wolf
