CentOS virt - Dec 2011 - Confusion over steps to add new logical volume to guest VM

Greetings -

I am hoping someone can confirm for me the steps that I am using to add an 
LV to an existing Guest in KVM, and what I am seeing as I do some of these 
steps.  My host (earth) is CentOS 6 and so is my guest (disect).  The guest 
(disect) is my testing server.  The main objective of what I am trying to do 
is add a logical volume for /var into my guest.  The LV (lv_disectvar) is 
part of an existing VG (vg_mei) on the host. When I installed my guest OS, I 
could only select one LV from my host for the installation, so the entire OS 
was installed into lv_disectroot.  I have added lv_disectvar into the 
storage pool available for VMs through the VirtManager GUI, and successfully 
added the lv_disectvar to the guest VM also through the VirtManager GUI. 
And now I want to move my /var over to the new LV.

My notes show that I created the LV about two weeks ago, and added it to the 
guest VM last week.  I don't see any other notes indicating that I did 
anything else with this LV.  Today I finally got around to the task of 
moving /var from lv_disectroot over to lv_disectvar and editing fstab so 
that it would automount at boot.  So here are the steps that I was going to 
do, and what I saw that made me pause.

1.  Verify that the device is identified by the guest, and get the UUID for 
fstab.
[jeffb at disect ~]$ ls -l /dev/disk/by-uuid
total 0
lrwxrwxrwx. 1 root root 10 Dec 12 14:37 
738923a4-c345-40bf-a850-4454caa152b6 -> ../../vda2
lrwxrwxrwx. 1 root root  9 Dec 12 14:37 
db1230fb-a061-4ea4-9229-c5953d29aef8 -> ../../vdb         
<----lv_disectvar
lrwxrwxrwx. 1 root root 10 Dec 12 14:37 
db70a6db-3640-4392-b9ba-49839d6e4b45 -> ../../vda1

2.  Create a mount point for /dev/vdb
[root at disect jeffb]# mkdir /mnt/var

3.  Mount the device
[root at disect jeffb]# mount -t ext4 /dev/vdb /mnt/var

4.  At this point I was going to move /var to /mnt/var but decided to check 
the mount first
[root at disect mnt]# cd /mnt/var
[root at disect var]# ls
cache  lib  lock  log  lost+found  run

5.  When I saw the directories listed I was initially confused and thought I 
had possibly mounted the wrong device.  But after double checking everything 
it appears I did not do anything wrong.  As I mentioned above my notes do 
not show that I did anything with this LV other than create it and attach it 
to the guest VM.  I am wondering about the source of these directories and 
all the subsequent files under them.  My assumption is that these were 
created during the creation of the LVM and are part of its file system.  Is 
that a correct assumption?  How could I confirm this assumption?

6.  Can I just add a /var directory to this and move my /var from /vda2 over 
to /vdb?

7.  Then I would add the following line to my /etc/fstab in the guest disect
UUID=db1230fb-a061-4ea4-9229-c5953d29aef8 /var ext4 defaults 1 2

8.  I initially tried just adding the above line to my fstab, but realized 
after the guest VM hung on rebooting that /var is already mounted under /, 
and that of course caused a problem when booting.

Thanks for any advice you can provide.


Jeff Boyce
Meridian Environmental
www.meridianenv.com

On Mon, December 19, 2011 18:04, Jeff Boyce wrote:
> Greetings -
>
> I am hoping someone can confirm for me the steps that
> I am using to add an LV to an existing Guest in KVM,
> and what I am seeing as I do some of these steps.
I think that you will find it easier to create guest
storage volumes entirely from within virt-manager or virsh
and not try and manipulate them directly on the host.  I
have done so in the past but it adds a layer of complexity
to the process that yields no discernible benefits.

Here is what I have hit upon in my own explorations of kvm:

1. Create a virtual storage pool and add it to the host. 
I use an lv on the host.

2. Create initial guest instance and allocate a new volume
from the storage pool using virt-manager -> details ->
storage window through the guest storage browser.  Name
the new storage volume to something related to the vm
guest name.

3. Complete creating the vm guest.

4. To add additional storage to an existing vm guest first
open the guest's -> details -> hardware menu tab and then
select Add Storage.

5. In the guest hardware storage window select VirtIO
type, raw format, and press the browse button.

6. In the host storage window select the storage pool to
allocate storage from.

7. Select add a New Volume.

8. Assign a storage volume name (some variant of the base
storage volume such that all volumes assigned to a single
guest appear together in the host storage volume window
works best for me) and set the new volume size.  Refresh
the host storage display, select the new volume name, and
return to the guest storage window.

9. Push the Finish button.  Restart the guest.

10. Now open the guest console, find the newly added
device (fdisk -l ), say /dev/vdb for example, and
partition it using fdisk or parted. I always make one
partition for the entire device.  Refresh the devices
using parted.

11. Now add the newly partitioned device to the guest's
own vg using the normal lvm tools.

12. Now create new or expand existing lvs on the guest
using lvm.

The only trouble I had, well towards the end the only
trouble that I had left, was discovering that a VirtIO
storage volume is not automatically partitioned when
created.  Until it had a partition I could not add it to
the guest's vg even though I could see the device.

HTH.

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