This section describes
virt-rescue, which can be considered analogous to a rescue CD for virtual machines. It boots a guest virtual machine into a rescue shell so that maintenance can be performed to correct errors and the guest virtual machine can be repaired.
There is some overlap between virt-rescue and guestfish. It is important to distinguish their differing uses. virt-rescue is for making interactive, ad-hoc changes using ordinary Linux file system tools. It is particularly suited to rescuing a guest virtual machine that has failed . virt-rescue cannot be scripted.
In contrast, guestfish is particularly useful for making scripted, structured changes through a formal set of commands (the libguestfs API), although it can also be used interactively.
21.6.2. Running virt-rescue
Before you use
virt-rescue on a guest virtual machine, make sure the guest virtual machine is not running, otherwise disk corruption will occur. When you are sure the guest virtual machine is not live, enter:
virt-rescue -d GuestName
(where GuestName is the guest name as known to libvirt), or:
virt-rescue -a /path/to/disk/image
(where the path can be any file, any logical volume, LUN, or so on) containing a guest virtual machine disk.
You will first see output scroll past, as virt-rescue boots the rescue VM. In the end you will see:
Welcome to virt-rescue, the libguestfs rescue shell.
Note: The contents of / are the rescue appliance.
You have to mount the guest virtual machine's partitions under /sysroot
before you can examine them.
bash: cannot set terminal process group (-1): Inappropriate ioctl for device
bash: no job control in this shell
The shell prompt here is an ordinary bash shell, and a reduced set of ordinary Red Hat Enterprise Linux commands is available. For example, you can enter:
><rescue> fdisk -l /dev/vda
The previous command will list disk partitions. To mount a file system, it is suggested that you mount it under
/sysroot, which is an empty directory in the rescue machine for the user to mount anything you like. Note that the files under
/ are files from the rescue VM itself:
><rescue> mount /dev/vda1 /sysroot/
EXT4-fs (vda1): mounted filesystem with ordered data mode. Opts: (null)
><rescue> ls -l /sysroot/grub/
-rw-r--r--. 1 root root 63 Sep 16 18:14 device.map
-rw-r--r--. 1 root root 13200 Sep 16 18:14 e2fs_stage1_5
-rw-r--r--. 1 root root 12512 Sep 16 18:14 fat_stage1_5
-rw-r--r--. 1 root root 11744 Sep 16 18:14 ffs_stage1_5
-rw-------. 1 root root 1503 Oct 15 11:19 grub.conf
When you are finished rescuing the guest virtual machine, exit the shell by entering
virt-rescue has many command-line options. The options most often used are:
--ro: Operate in read-only mode on the guest virtual machine. No changes will be saved. You can use this to experiment with the guest virtual machine. As soon as you exit from the shell, all of your changes are discarded.
--network: Enable network access from the rescue shell. Use this for example if you need to download RPM or other files into the guest virtual machine.