Moving a root partition to LVM
When I installed CentOS onto my laptop, I wasn’t comfortable with LVM at the time. I split out a few partitions onto LVM, but neglected to create my root partition on it. Of course, it filled up a few days ago and I had two options: 1) try to figure out how to move root to LVM or 2) re-install and re-partition to suit my needs. Well, I didn’t want to re-install because that takes way too much time especially when I have my laptop setup perfectly the way I want it.
It took a bit of playing around to figure out how to do this, but once I did it worked like a charm on a test box, so I thought I’d better document it so I don’t forget. This guide assumes LVM2 is in use since there are differences between LVM1 and LVM2.
I have a 40G drive installed, and had about 6G extra sitting on LVM unused. My previous root partition was 4G, and generally full file systems are a bad thing.
Previous layout
My drive looked like:
[root@laptop ~]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/vg0-root 4.0G 3.2G 689M 88% /
/dev/hda1 99M 9.4M 85M 11% /boot
none 506M 0 506M 0% /dev/shm
/dev/mapper/vg0-home 20G 8.4G 11G 45% /home
/dev/mapper/vg0-opt 2.0G 542M 1.4G 29% /opt
/dev/mapper/vg0-tmp 496M 16M 455M 4% /tmp
/dev/mapper/vg0-usr_local
1008M 179M 779M 19% /usr/local
/dev/mapper/vg0-var 2.0G 819M 1.1G 43% /var
Some previous LVM settings:
[root@laptop ~]# vgs VG #PV #LV #SN Attr VSize VFree vg0 1 5 0 wz–n- 29.62G 6.12G [root@laptop ~]# pvs PV VG Fmt Attr PSize PFree /dev/hda5 vg0 lvm2 a- 31.66G 160.00M
Create a new logical volume under LVM
Make sure you boot into single mode before you attempt to do anything at all. To do this, when Grub comes up hit ‘e’ to edit, and ‘a’ to add ‘1′ to the end of the kernel line.
The first thing to do is create a new logical volume which will eventually be your new root file system.
[root@laptop ~]# lvcreate -L6G -n root vg0
Obviously substitute your volume group name and what you want to call your partition with your configurations.
Next, make the filesystem an ext3 filesystem.
[root@laptop ~]# mk32fs -j /dev/vg0/root
Copy the original partition to the new one
The next thing you need to do is to actually move the data from the original root partition over to the new one created in the step above. You’ll need to create a mount point, mount the new LVM partiton, and then dump the data over to it.
[root@laptop ~]# mkdir /mnt/newroot [root@laptop ~]# mount /dev/vg0/root /mnt/newroot [root@laptop ~]# cd /mnt/newroot [root@laptop ~]# dump 0af - / | restore xfo -
The dump command is explained as follows: 0 means a full backup will be moved over and not an incremental, a means auto-size, f is writing the backup to a normal file.
You then pipe this to restore which pushes it onto your new parition. Restore is explained as follows: x means the backup is read from the given media, f means from file.
Change fstab & create a new boot image
In order for your new root partition to automatically be mounted, you’ll need to change the mount directory in /etc/fstab and create a new boot image for your system to boot from.
[root@laptop ~]# vim /mnt/newroot/etc/fstab # Change this line: LABEL=/ / ext3 defaults 1 1 # To read: /dev/vg0/root / ext3 defaults 1 1
Next, you’ll have to create the new boot image with mkinitrd. Make sure you point it at the fstab file under the mounted new partition, and not the old one!
[root@laptop ~]# mkinitrd -v –fstab=/mnt/newroot/etc/fstab /boot/initrd-lvm.img 2.6.9-42.0.10.EL
Again, substitute your kernel and the name of the actual image with your parameters.
Lastly, you’ll have to edit grub.conf to point to the new root partition and to the new boot image. Otherwise, your system won’t know how to start.
[root@laptop ~]# vim /boot/grub/grub.conf
default=0
timeout=5
splashimage=(hd0,0)/grub/robin.xpm.gz
hiddenmenu
title CentOS (2.6.9-42.0.10.EL)
root (hd0,0)
kernel /vmlinuz-2.6.9-42.0.10.EL ro root=/dev/vg0/root rhgb quiet
initrd /initrd-lvm.img
Reboot, but make sure you reboot into single mode again to complete the second half of finishing this job up. Everything should boot back up normal and you should be booted onto your new LVM partition.
[root@laptop ~]# mount /dev/mapper/vg0-root on / type ext3 (rw)
That’s it for getting things going. In case you’re worried about the new boot image, don’t be. When you upgrade the system will take that into account and nothing should be affected.
Create a new physical volume and extend into volume group
Next, you’ll need to turn the old /dev/hda2 partition into a physical volume that LVM can understand.
[root@laptop ~]# pvcreate /dev/hda2
This tells LVM that /dev/hda2 is a physical volume group container. Next, extend that physical volume into the current volume group vg0. This adds the pv into the vg.
[root@laptop ~]# vgextend vg0 /dev/hda2
That’s it as far as LVM cares about. I just took the 4G partition and added that 4G into my volume group which I can then, at a later time, push out to whichever logical volume group I need.
Change partition label
The very last step that needs completed is to turn the old partiton label into the new LVM status. Before, /dev/hda2 was just a regular Linux file system.
You’ll need to change that into a Linux LVM label like the other one shown.
[root@laptop ~]# fdisk /dev/hda Command (m for help): p Disk /dev/hda: 40.0 GB, 40007761920 bytes 255 heads, 63 sectors/track, 4864 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Device Boot Start End Blocks Id System /dev/hda1 * 1 13 104391 83 Linux /dev/hda2 14 535 4192965 83 Linux /dev/hda3 536 731 1574370 82 Linux swap /dev/hda4 732 4864 33198322+ 5 Extended /dev/hda5 732 4864 33198291 8e Linux LVM Command (m for help): t Partition number (1-5): 2 Hex code (type L to list codes): 8e Command (m for help): w
Just to make sure it changed, run fdisk -l and that’s all you need to do. When you reboot, the new partition table will be read and voila.
What you’ve accomplished is take a full 4G root partition, created a 6G LVM partition out of the 6G f free space left in the volume group, and moved the 4G of data over to the 6G LVM partition. Then you’ve taken the 4G static partition, created a physical volume, and added that into the LVM volume group vg0. Now you have 4G free on /dev/hda2 and 160M free on /dev/hda5, totalling the space left on vg0 to 4.12G.
[root@laptop ~]# vgs VG #PV #LV #SN Attr VSize VFree vg0 2 6 0 wz–n- 35.62G 4.12G [root@laptop ~]# pvs PV VG Fmt Attr PSize PFree /dev/hda2 vg0 lvm2 a- 3.97G 3.97G /dev/hda5 vg0 lvm2 a- 31.66G 160.00M
I don’t know about what you think, but this is much easier than re-installing Linux and spending the hours tinkering getting everything set back exactly on your desktop. Cheers!
