Logical volume management in GNU/Linux

Logical volume management or LVM is the answer to the problem of dynamically allocating more space to a logical volume without having to bother about moving data as the logical volume is grown. Many popular GNU/Linux distributions now provide LVM as an option to partition the disk during installation itself, though logical volumes can be setup any time after it.

Now let’s understand how logical volumes work. The basic disk area which a LVM uses is a physical volume (PV) which is nothing more than a partition on the disk or the complete disk itself. The partition type needs to of physical volume for LVM for it to be considered by LVM as a PV. Once a few PVs have been created you can allocate them to volume group (VG). Volume groups are a repository of PVs and can be extended or reduced dynamically by adding or removing PVs from it respectively. Many logical volumes (LV) can then be carved out of a VG. Size of a LV cannot be greater than the size of the VG it resides in.

Let’s say you have 3 PVs, PV₁, PV2 and PV₃ of size 20GB, 10GB and 5GB respectively. Now you created a VG, VG₁ using PV₁ and PV₂, so now you have VG₁ of size 30GB. You can now create a LV of size 30GB or less to begin with which can be dynamically grown. Let’s say you created a LV, LV₁ of size 30GB and started using it to fill up data. After consuming 90% of the space in LV₁ you started to feel the need to add more space to the LV. You can do so by first allocating PV₃, which was lying unallocated to VG₁. Once you have done so you can increase LV₁ to the size of 35GB which makes 5GB of extra free space available to LV₁ in a transparent manner without affecting any data already residing on LV₁. You also need to resize the filesystem of the LV for the operating system make use of the extra space allocated to the LV.

The steps to achieve the above in GNU/Linux is as follows. You need to have lvm2 package installed in your system for the below commands to be available.

Creating a LV from scratch

Increasing LV by extending VG

Increasing LV to make use of free space in VG

Important note regarding choice of filesystem for LV

If it is important for you to both grow and shrink your LV then you should pay prior attention to the choice of filesystem which you plan to use on the LV. Not all filesystems in the GNU/Linux system can be grown or shrunk. Also some filesystems allow growing or shrinking the filesystem while mounted, others do not. Make use of the below table for choosing the filesystem as per your needs.

^* ReiserFS is not actively maintained in the Linux kernel now, so consider it carefully before choosing it as your LVM filesystem.