We had a power outage on Monday that took out the drive that holds our VMs. There are backups, but the backup drive copies had superblock errors and all sorts of issues. To recover our data, I learned all sorts of new things — firstly that you can mount a QCOW file and copy data out. First, you have to connect a network block device to the file. Once it is connected, you can use fdisk to list the partitions on the drive and mount those partitions. In this example, I had a partition called nbd0p1 that I mounted to /mnt/data_recovery

modprobe nbd max_part=2
qemu-nbd --connect=/dev/nbd0 /path/to/server_file.qcow2
fdisk /dev/nbd0 -l
mount /dev/nbd0p1 /mnt/data_recovery

Once you are done, unmount it and disconnect from the network block device.

umount /mnt/data_recovery
qemu-nbd --disconnect /dev/nbd0
rmmod nbd