Debian Wheezy: Split and resize Xen disk image

Background

One of my Xen virtual servers needed to have its 400G disk split and resized over the weekend, and I wanted to keep downtime to a minimal. The disk had 240G used, 235G of which was a /backup directory, and the remaining 5G dedicated to a regular Debian Linux installation. I wanted to split the disk into a two disks - a 20GB disk containing the Linux system, and a 300G disk to mount as /backup. Then I could apply my DRBD setup to replicate that 300G backup partition to a separate 2nd backup machine.

I didn't want to end up in a situation of having to copy a load of data on the physical host, and I certainly didn't want the virtual machine down while I did it.

Set up images

The virtual machine has its disk image located in /vm/domains/b1/disk.img and an accompanying swap.img that we don't need to worry about.

Issue:

losetup -a

To locate all used loop devices. In my case, /dev/loop20 was the first free device. Next, I'll set up a loopback device for the image, and then mount it:

mkdir -p /mnt/loop-copy-from /mnt/loop-copy-to

losetup /dev/loop20 /vm/domains/b1/disk.img

mount /dev/loop20 /mnt/loop-copy-from

Create a new empty sparse image

Now, we're not going to move /backup, instead, we're going to move everything else to the new 20G image. We'll create a sparse image file of 20G with:

dd if=/dev/zero of=/vm/domains/b1/disk-root.img bs=1k count=1 seek=$(((20*1024*1024)-1))

Next, we'll need to set up a new loop device, create a filesystem and finally mount this new image

losetup /dev/loop21 /vm/domains/b1/disk-root.img

mkfs.ext3 /dev/loop21

mount /dev/loop21 /mnt/loop-copy-to

Copy root filesystem

Now, I'll copy everything from /mnt/loop-copy-from to /mnt/loop-copy-to aside from the 235G backup directory:

rsync -arplogu --exclude "/backup" --stats --progress /mnt/loop-copy-from/ /mnt/loop-copy-to

Clean up existing filesystem

I want to remove everything on /mnt/loop-copy-from except the backup directory:

cd /mnt/loop-copy-from; ls | grep -v 'backup' | xargs rm -rf

We'll then unmount both devices:

umount /mnt/loop-copy-from

umount /mnt/loop-copy-to

Shrink existing filesystem

I'll now need to shrink /dev/loop20 from 400G to 300G:

e2fsck -f /dev/loop20

resize2fs /dev/loop20 300G

Copy boot data

And then, copy the boot data before destroying the loopback devices:

dd if=/dev/loop20 of=/dev/loop21 bs=1 count=512

losetup -d /dev/loop20

losetup -d /dev/loop21

Xen configuration and VM boot

Now edit the Xen VM configuration file, in my case, /etc/xen/b1.cfg, changing:

disk = [
  'file:/vm/domains/b1/disk.img,xvda2,w',
  'file:/vm/domains/b1/swap.img,xvda1,w',
]

With:

disk = [
  'file:/vm/domains/b1/disk-root.img,xvda2,w',
  'file:/vm/domains/b1/disk.img,xvda3,w',
  'file:/vm/domains/b1/swap.img,xvda1,w',
]
And finally:

xm create b1.cfg

IO Digital Sec

Debian Wheezy XEN + Guest Setup

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