Extending an LVM volume: Physical volumes (partitions) -> Volume groups -> Logical volume -> Filesystem

Logical Volume Management (AKA LVM) is a powerful, robust mechanism for managing storage space.

In TurnKey 11,  instead of installing the root filesystem directly to a fixed size partition, we setup LVM by default, and install the root filesystem to a Logical Volume, which may later be expanded, even across multiple physical devices.

Unfortunately, as with anything powerful, to get the most out of LVM you first have to negotiate a learning curve. From the feedback we've been getting it seems that confusion regarding LVM is  common with new users, so here's a quick "crash course"...

How LVM works

In LVM, there are several layers, each builds on top of the other:

PV[s] (Physical Volumes) -> VG[s] (Volume Groups) -> LV[s] (Logical Volumes) -> Filesystems.

Logical Volumes are allocated/extended within the boundaries of their underlying storage pool which is called a Volume Group in LVM terminology.

For example, in TurnKey the filesystem is installed by default to the /dev/turnkey/root Logical Volume, which is allocated within the turnkey Volume Group:

--- Logical volume ---
  LV Name                /dev/turnkey/root
  VG Name                turnkey
  LV UUID                OzX3fe-aRQa-81XM-0vCV-8aJo-eUL4-6J90XJ
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                17.0 GiB
  Current LE             4502
  Segments               2
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           251:0

Out of the box the turnkey Volume Group doesn't have too much free space:

# vgdisplay
  --- Volume group ---
  VG Name               turnkey
  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               18.14 GiB
  PE Size               4.00 MiB
  Total PE              4645
  Alloc PE / Size       4480 / 17.50 GiB
  Free  PE / Size       165 / 660.00 MiB
  VG UUID               IwaFL0-QCi8-iIUE-TWjQ-R906-PYpH-gMPaH9

We can only extend a Logical Volume within the free space of the underlying Volume Group. How much free space we currently have within the Volume Group can be seen in this part of the output:

Free  PE / Size       165 / 660.00 MiB

In the above example we only have 660 MB to allocate to LVMs within the turnkey Volume Group. So if we want to extend the root LV we'll have to first extend the VG backs it up.

Volume Groups group together Physical Volumes. That's why they're called Volume Groups. This command will show us which Physical Volumes have been registered into LVM, and to which volume groups they have been assigned:

# pvdisplay
  --- Physical volume ---
  PV Name               /dev/sda2
  VG Name               turnkey
  PV Size               18.15 GiB / not usable 4.00 MiB
  Allocatable           yes
  PE Size               4.00 MiB
  Total PE              4645
  Free PE               165
  Allocated PE          4480
  PV UUID               H1Prpv-0VXR-7moE-zsbt-eyVt-m0fQ-GkAT6w

In this example we only have one Physical Volume (the /dev/sda2 partition) in the turnkey Volume Group.

Extending a Logical Volume

Bottom line: if the underlying Volume Group doesn't have enough free space, to extend the Logical Volume you'll first have to extend the underlying Volume Group by adding another Physical Volume to it.

In VMWare you could either create a new virtual hard disk device to add to the volume group, or extend an existing virtual hard disk device, create a new partition with cfdisk, and add the new partition to the Volume Group:

# example #1: you've added to VMWare a new virtual hard disk called /dev/sdb
pvcreate /dev/sdb
vgextend turnkey /dev/sdb

# example #2: you've expanded the existing sda hard disk
cfdisk /dev/sda  # creating /dev/sda3 (you may need to reboot before you can see this)
pvcreate /dev/sda3
vgextend turnkey /dev/sda3

After you've extended the Volume Group, you are free to extend the underlying Logical Volume:

# lvextend -L+10G /dev/turnkey/root
Extending logical volume root to 27.0 GiB
Logical volume root successfully resized

Finally, you'll have to resize the filesystem within /dev/turnkey/root so it can see that the underlying block device just got 10G bigger:

# resize2fs /dev/turnkey/root
resize2fs 1.41.11 (14-Mar-2010)
Filesystem at /dev/turnkey/root is mounted on /; on-line resizing required
old desc_blocks = 2, new_desc_blocks = 2
Performing an on-line resize of /dev/turnkey/root to  7077888 (4k) blocks.
The filesystem on /dev/turnkey/root is now 7077888 blocks long.

Have you tried using LVM? Still confused? Post a comment to discuss.