Extracting filesystems from guest images, reconstructing guest images from filesystems, part 3

In part 1 I “exploded” a disk image into its constituent filesystems. In part 2 I “imploded” those filesystems back into a disk image without LVM. Now let’s see if we can get this thing to boot.

Since there is no boot sector nor grub, the disk image produced by virt-implode won’t boot normally. You could boot it using an external kernel and initrd, but let’s see if we can install grub first.

Using virt-rescue we can interactively run programs from the guest:

$ virt-rescue -a output.img
...
><rescue> mount /dev/sda2 /sysroot
><rescue> mount /dev/sda1 /sysroot/boot
><rescue> mount --bind /dev /sysroot/dev
><rescue> mount --bind /sys /sysroot/sys
><rescue> mount --bind /proc /sysroot/proc
><rescue> chroot /sysroot
sh: no job control in this shell
><rescue> cat /etc/redhat-release
Red Hat Enterprise Linux AS release 4 (Nahant Update 8)
><rescue> vi /etc/fstab

You may need to fix /etc/fstab in the guest so that it points to the new partitions. For guests using LABELs or UUIDs, this won’t be necessary.

At this point, it should be simply a matter of running grub-install. But here’s where I remember how much I hate grub, because it just throws up peculiar, non-actionable error messages for every conceivable variation in the command.

So I give up that, and decide to extract the kernel and initrd and use the external boot method after all:

$ guestfish --ro -a output.img -i

Welcome to guestfish, the libguestfs filesystem interactive shell for
editing virtual machine filesystems.

Type: 'help' for help on commands
      'man' to read the manual
      'quit' to quit the shell

Operating system: Red Hat Enterprise Linux AS release 4 (Nahant Update 8)
/dev/sda2 mounted on /
/dev/sda1 mounted on /boot

><fs> ll /boot
total 4397
drwxr-xr-x.  5 root root    1024 Nov 15 13:49 .
drwxr-xr-x. 24 root root    4096 Nov 15 13:50 ..
-rw-r--r--.  1 root root  909034 Apr 20  2009 System.map-2.6.9-89.EL
drwxr-xr-x   3 root root    1024 Nov 15 13:49 boot
-rw-r--r--.  1 root root   45145 Apr 20  2009 config-2.6.9-89.EL
drwxr-xr-x.  2 root root    1024 Nov 15 13:58 grub
-rw-r--r--.  1 root root 1546843 Oct 27  2010 initrd-2.6.9-89.EL.img
drwx------   2 root root   12288 Oct 27  2010 lost+found
-rw-r--r--.  1 root root   23108 Aug  3  2005 message
-rw-r--r--.  1 root root   21282 Aug  3  2005 message.ja
-rw-r--r--.  1 root root   67352 Apr 20  2009 symvers-2.6.9-89.EL.gz
-rw-r--r--.  1 root root 1829516 Apr 20  2009 vmlinuz-2.6.9-89.EL
><fs> download /boot/vmlinuz-2.6.9-89.EL /tmp/vmlinuz-2.6.9-89.EL
><fs> download /boot/initrd-2.6.9-89.EL.img /tmp/initrd-2.6.9-89.EL.img
$ qemu-kvm -m 512 \
  -kernel vmlinuz-2.6.9-89.EL \
  -initrd initrd-2.6.9-89.EL.img \
  -append 'ro root=/dev/hda2' \
  -hda output.img

(I should really use libvirt, but this quick test proves that the guest works fine)

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1 Comment

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One response to “Extracting filesystems from guest images, reconstructing guest images from filesystems, part 3

  1. jamesjustjames

    Nice writeup, and I agree that you should really use libvirt ;) Maybe someone should keep a bin/* of this type of “helper” script, wrapping explode, implode and grub-install all into one. All these could be distributed with guestfish.

    Thanks!

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