Adding Space to an LVM

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 AN!Wiki :: How To :: Adding Space to an LVM

Note: This is a snippet from another doc and still needs to be cleaned up. Please follow this with caution. It starts after a modification to a Xen domU machine called 'dev01'.


Adding New Disk in domU to it's PV

Once dom0's LV has been added to domU, you will need to make it available to the domU OS. In this case, we will add it to domU's existing PV.

Log into the VM; We will use dev01 and it's new /dev/xvdb disk for our example.

Preparing the Partition

Use fdisk -l and make sure you now see:

Disk /dev/xvdb: 53.6 GB, 53687091200 bytes
255 heads, 63 sectors/track, 6527 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk /dev/xvdb doesn't contain a valid partition table

Enter the following:

fdisk /dev/xvdb

In fdisk, enter the following:

n           # New partition
p           # Primary partition
1           # Partition number
<enter>     # First free block
<enter>     # Last free block
t           # Change the new partiton's label
8e          # To 'Linux LVM'
p           # Print how the new partition will look and confirm that it is right
w           # Write the changes to disk and exit.

This should look like:

# fdisk /dev/xvdb
Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
Building a new DOS disklabel. Changes will remain in memory only,
until you decide to write them. After that, of course, the previous
content won't be recoverable.
The number of cylinders for this disk is set to 6527.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
   (e.g., DOS FDISK, OS/2 FDISK)
Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)
Command (m for help): n   
Command action
   e   extended
   p   primary partition (1-4)
Partition number (1-4): 1
First cylinder (1-6527, default 1): 
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-6527, default 6527): 
Using default value 6527
Command (m for help): t
Selected partition 1
Hex code (type L to list codes): 8e
Changed system type of partition 1 to 8e (Linux LVM)
Command (m for help): p
Disk /dev/xvdb: 53.6 GB, 53687091200 bytes
255 heads, 63 sectors/track, 6527 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
    Device Boot      Start         End      Blocks   Id  System
/dev/xvdb1               1        6527    52428096   8e  Linux LVM
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.

Creating the new PV

We need to turn the new partition into a Physical Volume so that we can then add it to the existing Volume Group. Do this by typing:

pvcreate dev01_snap /dev/xvdb1

Wait a few seconds for background steps to complete and then check that it is now visible by LVM as a PV by typing:


If it worked, you should now see this as the output:

  --- Physical volume ---
  PV Name               /dev/xvda2
  VG Name               dev01_vg1
  PV Size               244.04 GB / not usable 5.90 MB
  Allocatable           yes (but full)
  PE Size (KByte)       32768
  Total PE              7809
  Free PE               0
  Allocated PE          7809
  PV UUID               2bm15Z-vRGI-yKvp-NvGy-w1eC-mnrM-XE0GQe
  "/dev/xvdb1" is a new physical volume of "50.00 GB"
  --- NEW Physical volume ---
  PV Name               /dev/xvdb1
  VG Name               
  PV Size               50.00 GB
  Allocatable           NO
  PE Size (KByte)       0
  Total PE              0
  Free PE               0
  Allocated PE          0
  PV UUID               kJaG6L-R31a-NTRZ-NXT4-1ldP-Egnd-0Z4f1E

Adding the new PV to the Existing VG

Now that we have the second PV, we need to add it to the existing VG. We will continue with this example using the dev01 VM.

To sort out what PV you want to add to which VG, run the following two commands (output shown):

  PV /dev/xvda2   VG dev01_vg1       lvm2 [244.03 GB / 0    free]
  PV /dev/xvdb1                      lvm2 [50.00 GB]
  Total: 2 [294.03 GB] / in use: 1 [244.03 GB] / in no VG: 1 [50.00 GB]

Above, we see the new PV, /dev/xvdb1 is not yet assigned to a VG so we know it's the one we want and that it is ready.

  Reading all physical volumes.  This may take a while...
  Found volume group "dev01_vg1" using metadata type lvm2

Above, we see that the existing VG is dev01_vg1. This is the VG that the new PV will be added to.

So then, run:

vgextend dev01_vg1 /dev/xvdb1
  Volume group "dev01_vg1" successfully extended

If you see the above output, then it looks like it worked. Rerun pvscan and it should now look like:

  PV /dev/xvda2   VG dev01_vg1   lvm2 [244.03 GB / 0    free]
  PV /dev/xvdb1   VG dev01_vg1   lvm2 [49.97 GB / 49.97 GB free]
  Total: 2 [294.00 GB] / in use: 2 [294.00 GB] / in no VG: 0 [0   ]

We can see that both PVs are in the same VG.

Now create the mount point that the new snapshot partition will be mounted:

mkdir /mnt/snapshot


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