Data is stored on disks that are logically divided into “partitions”.

Partitions can exist on a portion of a disk, on an entire disk, or it may even span on multiple disks.

Logical Volume Manager (LVM)

LVM provides an abstraction layer between the physical storage and the file system - providing the following functionalities for the filesystem:

  • Enables resizing.
  • Span across multiple disks.
  • Use arbitrary disk space.
  • etc…

lvm

LVM Components

As shown in the image above, LVM is comprised of the following components explained below.

Physical Volumes (PVs)

A PV is created when a block storage device such as a partition or an entire disk is initialised and brought under LVM control - for example:

pvcreate /dev/sdb1

A PV is chopped into equally sized chunks known as physical extents (PEs) with the default being 4MiB on most Linux distros.

A PV belongs to exactly one Volume Group.

Volume Groups (VGs)

A VG is a pool of storage made by combining one or more PVs - for example:

vgcreate vg0 /dev/sda2 /dev/sdb1

The size of the VG is essentially the sum of the sizes of its PVs (minus metadata)

Basically, multiple disks / partitions (PVs) equates to one “big virtual disk” (VG).

Logical Volumes (LVs)

An LV is essentially carved out of a VG - for example:

lvcreate -L 50G -n lv_data vg0

To the OS, an LV looks like a normal block device (e.g. /dev/vg0/lv_data) on which you can create a filesystem and mount it.

However, the added functionality is since the mapping is flexible.

Check Your Understanding

A few questions for you to check your understanding…

Question: If you had two 500 GB disks and wanted a single 800 GB /home plus 200 GB left over for future use, how would you design the PVs, VG, and LV layout?

Answer:

  1. Create Physical volumes:
pvcreate /dev/sda
pvcreate /dev/sdb

(1 PV per disk)

  1. Create One Volume Group:
vgcreate vg0 /dev/sda /dev/sdb

VG size ≈ 1 TB combined.

  1. Create One Logical Volume for /home:
lvcreate -L 800G -n lv_home vg0

Make filesystem and mount at /home.

This leaves ~200 GB free space inside vg0 for future LVs or extending lv_home later.

Note: The reason it’s an estimate of combined VG size and LV free space is because of the metadata applied to the VG and LV meaning you don’t get the entire 1TB or 200GB of free space.

Question: If you want to grow /home by 100GB with the free space left in the same VG how would you go about doing it? (Clue: Read the man pages of lvextend!)

Answer:

  • To increase it:
lvextend -L +100G -r /dev/vg0/lv_home

The -r flag tells it to also resize the filesystem. This is because without the flag, the lvextend command only changes the block device size that LVM presents. However, the filesystem metadata on top (ext4, xfs, etc.) still thinks it has the old size until you run a filesystem-specific grow option.

If you didn’t use the -r flag, then to grow a filesystem it would depend on the filesystem type:

  • For example for an ext4 filesystem, the ordering would be:
lvextend -L +100G /dev/vg0/lv_home
resize2fs /dev/vg0/lv_home