Random small break-fix or enlightening ideas

Ubuntu names the volume group according to the hostname of the computer, but sometimes I want to change it after the fact, and doing so is pretty easy.

Since writing this procedure, I did run into a reason why unique volume groups could be useful. I had a system crash, which required me to take the HDD and connect it into a working system. Both machines used the VG name of “system”, so this caused a conflict with mounting both VGs. In this case, I had to rename one of them to perform my data recovery.

I created a test system with the initial hostname of dev-xfce-01. Take a look at one of the device names.

[root]$ df -h
Filesystem                          Size  Used Avail Use% Mounted on
/dev/mapper/dev--xfce--01--vg-root  6.7G  1.1G  5.3G  17% /
udev                                494M  4.0K  494M   1% /dev
tmpfs                               201M  300K  200M   1% /run
none                                5.0M     0  5.0M   0% /run/lock
none                                501M     0  501M   0% /run/shm
/dev/sda1                           228M   24M  192M  12% /boot

That just looks messy. The swap partition, not shown here, looks just as bad. Why do I care? I want to have a consistent VG name across all my systems for easy monitoring.

To see the actual VG name, run this.

[root]$ vgs
  VG             #PV #LV #SN Attr   VSize VFree 
  dev-xfce-01-vg   1   2   0 wz--n- 7.76g 32.00m

I used this command to rename the system VG (volume group) to something more generic:

[root]$ vgrename dev-xfce-01-vg system

Rebooting the system at this point is a very bad idea. Take a look at what is now in /dev/mapper, and how that compares to /etc/fstab.

[root]$ ls -alh /dev/mapper
total 0
drwxr-xr-x  2 root root     100 Nov  7 19:45 .
drwxr-xr-x 15 root root    4.1K Nov  7 19:45 ..
crw-------  1 root root 10, 236 Nov  7 19:33 control
lrwxrwxrwx  1 root root       7 Nov  7 19:45 system-root -> ../dm-0
lrwxrwxrwx  1 root root       7 Nov  7 19:45 system-swap_1 -> ../dm-1

[root]$ cat /etc/fstab | grep /dev/mapper
/dev/mapper/dev--xfce--01--vg-root /               ext4    errors=remount-ro 0       1
/dev/mapper/dev--xfce--01--vg-swap_1 none            swap    sw              0       0

So although we changed the name, the mount points are not automatically updated. Let’s change that.

[root]$ vim /etc/fstab

Find any drives that referenced the old VG name, and update to the new /dev/mapper/ path. On a typical system, you will have at least the root and swap paths to change.

We want to be uber sure the changes are correct. Run these informational commands and check your work. The /dev/mapper/ paths must match the entries in the /etc/fstab file.

[root]$ ls -alh /dev/mapper
total 0
drwxr-xr-x  2 root root     100 Nov  7 19:45 .
drwxr-xr-x 15 root root    4.1K Nov  7 19:45 ..
crw-------  1 root root 10, 236 Nov  7 19:33 control
lrwxrwxrwx  1 root root       7 Nov  7 19:45 system-root -> ../dm-0
lrwxrwxrwx  1 root root       7 Nov  7 19:45 system-swap_1 -> ../dm-1

[root]$ cat /etc/fstab | grep /dev/mapper
/dev/mapper/system-root /               ext4    errors=remount-ro 0       1
/dev/mapper/system-swap_1 none            swap    sw              0       0

Much better. Finally, update grub.

[root]$ update-grub

Now, it should be safe to reboot.