guestfish -i, that’s i for “inspector” not “interactive”

I wrote this FAQ entry today …

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• What’s the deal with guestfish -i?
• Why does virt-cat only work on a real VM image, but virt-df works on any disk image?
• What does “no root device found in this operating system image” mean?

These questions are all related at a fundamental level which may not be immediately obvious.

At the libguestfs API and guestfish level, a “disk image” is just a pile of partitions and filesystems.

In contrast, when the virtual machine boots, it mounts those filesystems into a consistent hierarchy such as:

 /          (/dev/sda2)
 |
 +-- /boot  (/dev/sda1)
 |
 +-- /home  (/dev/vg_external/Homes)
 |
 +-- /usr   (/dev/vg_os/lv_usr)
 |
 +-- /var   (/dev/vg_os/lv_var)

(or drive letters on Windows).

The libguestfs API however knows nothing about how filesystems are mounted. In general, a disk image that we just made up might not contain an operating system or “mountpoints” at all.

Nevertheless, users expect some tools (like virt-cat) to work with VM paths:

virt-cat fedora.img /var/log/messages

How does virt-cat know that /var is a separate partition?

The trick is a large external program called virt-inspector. Virt-inspector contains all sorts of tests and heuristics, so it can take a disk image about which nothing is known beforehand and work out both what it is, and how filesystems would be mounted.

Some tools, such as virt-cat, virt-edit, virt-ls and virt-tar use the same virt-inspector code to map VM paths. Other tools, such as virt-df and virt-list-filesystems operate entirely at the raw “big pile of filesystems” level of the libguestfs API, and don’t use virt-inspector code.

guestfish is in an interesting middle ground. If you use the -a and -m command line options, then you have to tell guestfish exactly how to add disk images and where to mount partitions. This is the raw API level.

However there is also a guestfish -i (for “inspector”) mode. In this mode, guestfish runs virt-inspector, and then (if successful) virt-inspector runs guestfish a second time with the correct -a and -m options. This is much slower, but usually simpler for the user. They can just do this and it all works:

guestfish -i SomeLibvirtGuest

The error “no root device found in this operating system image” is related to this. It means virt-inspector was unable to locate an operating system within the disk image you gave it. You might see this from programs like virt-cat if you try to run them on something which is just a disk image, not a virtual machine.

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