In this tutorial, we are going to learn about Linux Volume Management. We will see what is LVM in Linux, advantages of LVM, how to create Volume group and Logical volumes in Linux with examples.
Table of Contents
What is Logical Volume Manager (LVM)?
Logical Volume Manager (LVM) is used to manage block storage in Linux. LVM handles storage management differently as well as efficiently when compared with standard partitions.
LVM accumulates spaces taken from the partitions or entire disks (known as Physical Volumes) to form a logical container (known as Volume Group). The Volume Group is further divided into logical partitions called Logical Volumes.
To put this simply, LVM groups all your storage space into the pool and allows you to create volumes (Logical Volumes) from that pool.
The advantage of using LVM over standard partition is, LVM offers you more flexibility and features. It allows online resizing of Logical Groups and Logical Volumes. So If any of your logical partitions run out of space, you can easily increase the partition size by using the space available in the storage pool.
You can also export and import partitions. LVM also supports mirroring and snapshotting of logical volumes.
We will go through all the features separately in the upcoming articles. I suggest you also look at the following guides to understand how standard partitions are created and mounted.
- Create And Manage Disk Partitions With Parted In Linux - OSTechNix
- Create Linux Disk Partitions With Fdisk - OSTechNix
Layers Of Abstraction In LVM
LVM provides an abstraction of layer between the physical storage and the file system, enabling the file system to be resized, span across multiple physical disks, and use arbitrary disk space.
LVM uses three layers of abstraction to create partitions.
- Physical Volume,
- Volume Group,
- Logical Volume.
PHYSICAL VOLUME
It all starts with a physical disk. Physical volume is the first layer of abstraction LVM uses to identify the disk tagged for LVM operations. To put it in simple terms, if you wish to work with LVM, your disk should be initialized as the physical volume. It can be an entire disk or standard partitions created from that disk.
VOLUME GROUP
Volume group is the combination of all the physical volume. Let's say you have five separate disks with a size of 1 TB each. You will first initialize the five disks as physical volume and then add them to the volume group.
Volume group will hold 5TB of space which is the space available from all the physical volume. From the volume group, you can create logical partitions.
LOGICAL VOLUMES
From the pool of space (Volume Group) you can create logical volumes. Think of this as equivalent to a standard disk partition.
Enough with the basics. Let us get started with Linux LVM commands.
I am using the Ubuntu server running in virtualbox for demonstration. If you are learning LVM for the first time, do your testing on any virtual machine.
Heads Up: LVM commands require root privilege. Either run all the commands as root user or with sudo privilege.
Step 1 - Initialize Physical Volume
I have added three disks with different sizes and it totals 10G.
$ lsblk /dev/sd[b-e]
Sample output:
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sdb 8:16 0 2G 0 disk sdc 8:32 0 3G 0 disk sdd 8:48 0 5G 0 disk
To initialize any disk as physical volume, use the pvcreate command, with the device name as the argument.
$ sudo pvcreate /dev/sdb /dev/sdc /dev/sdd
Sample output:
Physical volume "/dev/sdb" successfully created.
Physical volume "/dev/sdc" successfully created.
Physical volume "/dev/sdd" successfully created.
To check the list of physical volumes, you can run any of the following commands. Each command will give you a different output.
PVDISPLAY - The pvdisplay command will give you detailed information about each physical volume, under which volume group it is, the unique ID, and the size available.
$ sudo pvdisplay
PVS & PVSCAN - These two commands will give you information like physical volume, volume group, allocated, and free size.
$ sudo pvs
$ sudo pvscan
Step 2 - Creating Volume Group
Now I have three disks initialized as a physical volume with a total size of 10GB. These physical volumes should be added to a storage pool known as a volume group.
Run vgcreate command to create a volume group. You have to pass a name for the volume group. Here I am using "ostechnix_files" as the volume group name.
$ sudo vgcreate ostechnix_files /dev/sd[b-d]
Volume group "ostechnix_files" successfully created
Run any of the following commands to check the volume group details.
$ sudo vgdisplay
Or,
$ sudo vgdisplay <volume group name>
Step 3 - Display Volume Group Details
The vgs and vgscan commands will give you information related to all available volume groups, number of physical volumes, and number of logical volumes, allocated and free sizes from the volume group.
$ sudo vgs
$ sudo vgscan
Step 4 - Creating Logical Volumes
As I mentioned earlier, logical volume is similar to disk partitions. Now we have close to 10GB of free space in the "ostechnix_files" pool (volume group). On top of this volume group, we will create logical volumes, format the volume with the ext4 file system, mount and use the volume.
To create the logical volume, you can use the lvcreate command. The general syntax of lvcreate command is given below.
$ sudo lvcreate -L <volume-size> -n <logical-volume-name> <volume-group>
Here,
-L <volume-size>=> size in KB, MB, GB-n <logical-volume-name>=> Name for your volume<volume-group>=> Which volume group to be used
Now I am creating a logical volume with 3GB size. I named the logical volume "guides".
$ sudo lvcreate -L 3GB -n guides ostechnix_files Logical volume "guides" created.
Step 5 - Display Logical Volumes Information
You can use any one of following commands for viewing logical volume information.
The lvdisplay command gives you detailed information about the logical volume, the associated volume group, volume size, logical volume path, etc.
$ lvdisplay
Or, mention the logical volume's name explicitly:
$ lvdisplay guides
The lvscan and lvs commands will also provide some basic information about the logical volumes.
$ lvscan
$ lvs
Step 6 - Formatting and Mounting Logical Volumes
You need to format the logical volume with a file system and mount the volume. Here I am formatting the volume with the ext4 file system and mounting it under the /mnt/ directory.
You should be seeing the device file for logical volume under /dev/volume-group/logical-volume. In my case, the device file will be /dev/ostechinix_files/guides.
$ sudo mkfs.ext4 /dev/ostechnix_files/guides
Sample output:
mke2fs 1.45.5 (07-Jan-2020)
Creating filesystem with 786432 4k blocks and 196608 inodes
Filesystem UUID: a477d1b6-e806-451f-ab34-4be9978c1328
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912
Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: doneTo mount the directory run the following command. You can mount the directory to any location as you wish.
$ sudo mount /dev/ostechnix_files/guides /mnt/
To view the mounted volumes, run:
$ mount | grep -i guides
/dev/mapper/ostechnix_files-guides on /mnt type ext4 (rw,relatime)
You can also run the df command to check info about the mounted file system. You can see the file system is named with volume name. It will be quite easy for you to understand the underlying volume and its group with this naming convention.
$ df -h /mnt/
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/ostechnix_files-guides 2.9G 9.0M 2.8G 1% /mnt
To make the mount persistent across reboots, you should add an entry to the fstab. If you have no idea about fstab, I suggest you look at our comprehensive article on fstab.
Conclusion
In this introductory article, we have seen LVM abstraction and how to create volume group and logical volumes using LVM in Linux. In the next article, we will take a look at how to expand and shrink volume space.
