While investigating firecracker-microvm/firecracker-containerd#253, we have found a small gap regarding virtio block device support on Firecracker.

Currently Firecracker is using std::fs::metadata() to check the size of a virtio block device, which ultimately calls stat() on Unix.

However firecracker-containerd is using a loopback device as the backing file for a virtio block device. On a lookback device, the size from stat() is always zero.

Can Firecracker support our usecase by inspecting the size of the file more deeply? You probably can use BLKGETSIZE64 ioctl.

Thanks,