Use guestfish, virt tools with remote disks

New in libguestfs ≥ 1.21.30 is the ability to use guestfish and some of the virt tools with remote disks.

Currently you can use remote disks over NBD, GlusterFS, Ceph, Sheepdog and (recently upstream) SSH.

For this example I’ll use SSH because it needs no setup, although this requires absolutely the latest qemu and libguestfs (both from git).

Since we don’t have libvirt support for ssh yet, so this only works with the direct backend:

$ export LIBGUESTFS_BACKEND=direct

I can use a ssh:// URI to add disks with guestfish, guestmount and most of the virt tools. For example:

$ virt-rescue -a ssh://localhost/tmp/f17x64.img
[... lots of boot messages ...]
Welcome to virt-rescue, the libguestfs rescue shell.

Note: The contents of / are the rescue appliance.
You have to mount the guest's partitions under /sysroot
before you can examine them.

><rescue> mount /dev/vg_f17x64/lv_root /sysroot
><rescue> cat /sysroot/etc/redhat-release
Fedora release 17 (Beefy Miracle)

Apart from being a tiny bit slower, it just works as if the disk was local:

$ virt-df -a ssh://localhost/tmp/f17x64.img
Filesystem                           1K-blocks       Used  Available  Use%
f17x64.img:/dev/sda1                    487652      63738     398314   14%
f17x64.img:/dev/vg_f17x64/lv_root     28316680    4285576   22586036   16%
$ guestmount -a ssh://localhost/tmp/f17x64.img -i /tmp/mnt
$ ls /tmp/mnt
bin   dev  home  lib64       media  opt   root  sbin  sys  usr
boot  etc  lib   lost+found  mnt    proc  run   srv   tmp  var
$ cat /tmp/mnt/etc/redhat-release
Fedora release 17 (Beefy Miracle)
$ guestunmount /tmp/mnt
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