Re: Is it safe to read-only mount a running bhyve image from the host?

On Sun, Mar 23, 2025, at 2:11 PM, Pat Maddox wrote:
> I'm running a bhyve VM (freebsd guest), and want to back up individual 
> files from the VM. I thought I would need to use SSH, or possibly stop 
> the VM and mount the image from the host.
>
> However, I tried mounting the image while the VM was running... and to 
> my surprise, it worked (I was expecting a kernel panic):
>
> 1. `mdconfig -a /vm/myvm/myvm.img`
> 2. `zpool import -f -o readonly=on -t -R /tmp/myvm-root -d /dev/md0p2 
> zroot myvm-root`
>
> I was then able to read the files from the host _as of the time I ran 
> `mdconfig`_. So if I add / edit / delete files in the VM, the host 
> doesn't see it until I unmount and remount.
>
> Is this a wonderful feature that lets me back up individual files from 
> a running VM? Or a giant footgun that will eventually corrupt my VM 
> image?
>
> Pat

I got a much better suggestion on discord: snapshot the filesystem on the host. Mount the image file from the snapshot, and proceed. -Pat