Xen: How to Convert An Image-Based Guest To An LVM-Based Guest
Version 1.0
Author: Falko Timme
This short article explains how you can move/convert a Xen guest that uses disk images to LVM volumes. Virtual machines that use disk images are very slow and heavy on disk IO, therefore it's often better to use LVM. Also, LVM-based guests are easier to back up (using LVM snapshots).
I do not issue any guarantee that this will work for you!
1 Preliminary Note
To use LVM-based guests, you need a volume group that has some free space that is not allocated to any logical volume. In this example, I use the volume group /dev/vg0 with a size of approx. 465GB...
vgdisplay
server1:~# vgdisplay
--- Volume group ---
VG Name vg0
System ID
Format lvm2
Metadata Areas 1
Metadata Sequence No 3
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 2
Open LV 2
Max PV 0
Cur PV 1
Act PV 1
VG Size 465.28 GB
PE Size 4.00 MB
Total PE 119112
Alloc PE / Size 59842 / 233.76 GB
Free PE / Size 59270 / 231.52 GB
VG UUID gnUCYV-mYXj-qxpM-PEat-tdXS-wumf-6FK3rA
server1:~#
... that contains the logical volume /dev/vg0/root with a size of approx. 232GB and the logical volume /dev/vg0/swap_1 (about 1GB) - the rest is not allocated and can be used for Xen guests:
lvdisplay
server1:~# lvdisplay
--- Logical volume ---
LV Name /dev/vg0/root
VG Name vg0
LV UUID kMYrHg-d0ox-yc6y-1eNR-lB2R-yMIn-WFgzSZ
LV Write Access read/write
LV Status available
# open 1
LV Size 232.83 GB
Current LE 59604
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 254:0
--- Logical volume ---
LV Name /dev/vg0/swap_1
VG Name vg0
LV UUID SUI0uq-iTsy-7EnZ-INNz-gjvu-tqLD-rGSegE
LV Write Access read/write
LV Status available
# open 2
LV Size 952.00 MB
Current LE 238
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 254:1
server1:~#
I have an image-based Xen guest called xen1.example.com that I created using the following command:
xen-create-image --hostname=xen1.example.com --size=4Gb --swap=256Mb --ip=192.168.0.101 --memory=128Mb --arch=amd64 --role=udev
This is its Xen configuration file:
vi /etc/xen/xen1.example.com.cfg
#
# Configuration file for the Xen instance xen1.example.com, created
# by xen-tools 3.9 on Mon Mar 9 19:22:40 2009.
#
#
# Kernel + memory size
#
kernel = '/boot/vmlinuz-2.6.26-1-xen-amd64'
ramdisk = '/boot/initrd.img-2.6.26-1-xen-amd64'
memory = '128'
#
# Disk device(s).
#
root = '/dev/xvda2 ro'
disk = [
'file:/home/xen/domains/xen1.example.com/swap.img,xvda1,w',
'file:/home/xen/domains/xen1.example.com/disk.img,xvda2,w',
]
#
# Hostname
#
name = 'xen1.example.com'
#
# Networking
#
vif = [ 'ip=192.168.0.101,mac=00:16:3E:F2:DC:FA' ]
#
# Behaviour
#
on_poweroff = 'destroy'
on_reboot = 'restart'
on_crash = 'restart'
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As you see, the guest is using two disk images, /home/xen/domains/xen1.example.com/disk.img (4GB) and /home/xen/domains/xen1.example.com/swap.img (256MB).
We need the exact image sizes so that we can create logical volumes of the same size. If you don't remember the exact disk and swap sizes anymore, you can go to the directory where the images are stored...
cd /home/xen/domains/xen1.example.com
... and run the following command - it will show the image sizes in human-readable format:
ls -lh
2 Converting The Images To LVM
Before we convert the images, we must shut down the guest:
xm shutdown xen1.example.com
Then we create logical volumes of the same size as the disk images, e.g. as follows:
lvcreate -L4G -n xen1_root vg0
lvcreate -L256M -n xen1_swap vg0
This creates the logical volumes /dev/vg0/xen1_root (4GB) and /dev/vg0/xen1_swap (256MB):
lvdisplay
server1:~# lvdisplay
--- Logical volume ---
LV Name /dev/vg0/root
VG Name vg0
LV UUID kMYrHg-d0ox-yc6y-1eNR-lB2R-yMIn-WFgzSZ
LV Write Access read/write
LV Status available
# open 1
LV Size 232.83 GB
Current LE 59604
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 254:0
--- Logical volume ---
LV Name /dev/vg0/swap_1
VG Name vg0
LV UUID SUI0uq-iTsy-7EnZ-INNz-gjvu-tqLD-rGSegE
LV Write Access read/write
LV Status available
# open 1
LV Size 952.00 MB
Current LE 238
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 254:1
--- Logical volume ---
LV Name /dev/vg0/xen1_root
VG Name vg0
LV UUID MQzhrS-OpOt-2IbY-BozD-l5vN-3doB-GRtyMc
LV Write Access read/write
LV Status available
# open 0
LV Size 4.00 GB
Current LE 1024
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 254:2
--- Logical volume ---
LV Name /dev/vg0/xen1_swap
VG Name vg0
LV UUID GHwsIT-a0sj-M72J-OVof-Ydju-Sexf-Ex824b
LV Write Access read/write
LV Status available
# open 0
LV Size 256.00 MB
Current LE 64
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 254:3
server1:~#
Now we can convert the images as follows:
dd if=/home/xen/domains/xen1.example.com/disk.img of=/dev/vg0/xen1_root
dd if=/home/xen/domains/xen1.example.com/swap.img of=/dev/vg0/xen1_swap
(This can take a lot of time, depending on how big the images are.)
Afterwards, we must open /etc/xen/xen1.example.com.cfg...
vi /etc/xen/xen1.example.com.cfg
... and change...
[...]
disk = [
'file:/home/xen/domains/xen1.example.com/swap.img,xvda1,w',
'file:/home/xen/domains/xen1.example.com/disk.img,xvda2,w',
]
[...]
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... to ...
[...]
disk = [
'phy:/dev/vg0/xen1_swap,xvda1,w',
'phy:/dev/vg0/xen1_root,xvda2,w',
]
[...]
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You can now start the guest again:
xm create /etc/xen/xen1.example.com.cfg
If everything goes well, you can delete the disk images:
rm -f /home/xen/domains/xen1.example.com/disk.img
rm -f /home/xen/domains/xen1.example.com/swap.img
3 Links
- Xen: http://www.xen.org/
