LVM on Linux: Create, Resize, and Manage Logical Volumes

LVM is useful for managing disks intelligently through volumes. Learn about this technology used in many distributions

LVM is a disk manager for the Linux Kernel. It allows disks to be swapped without service interruption (hotswap), resize volumes, create backup images of volumes, create a single volume from several # mount /dev/meuvolume/logico1 /dadosdiscos (similar to RAID 0) or create mirrored volumes on more than one disk (similar to RAID 1).

LVM makes it possible to expand the filesystem that traditionally is seen as a set of physical disks and partitions. Its goal is to allow great flexibility for the administrator in managing the disks.

Imagine that the user has the following partition scheme without LVM:

/boot   /dev/hda1 50 Megabytes

swap    /dev/hda2 256 Megabytes

/            /dev/hda3 2 Gigabytes

/home                    /dev/hda4 6 Gigabytes

In this example, if the user wants to increase the root partition, they would have to reformat their disk, or even move part of the data to another disk and mount the new partition as a directory of root.

Whereas, if the user uses LVM, they could simply shrink the size of /home and increase root, or even add another disk and increase root, without needing to backup the data, format the partition and copy the data back.

Observe the same example using volumes:

/boot    /dev/hda1  50Mb

swap              /dev/vg00/swap          256Mb

/             /dev/vg00/root             2 Gigabytes

/home            /dev/vg00/home          6 Gigabytes

In large systems with many disks, it is practically unfeasible to manage the disks without using LVM.

It is important for you to know that there are two versions of LVM: 1 and 2. Version 2 is supported by Kernel 2.6 and 3.x, and by Kernel 2.4 with application of some patches.

The only Linux partition that cannot be used with the volume manager is /boot.

LVM Terminology

• Physical Volume: It is a disk or some hardware that behaves like a disk (such as a storage that uses RAID);
• Volume Group: It is an abstraction of LVM that brings together logical volumes and physical volumes in the same administrative unit;
• Logical Volume: It is the equivalent to a partition in a non-LVM system.

Step by step to work with LVM

To work with LVM you need to do the following steps:

1. If you are going to use only one partition of a disk, you need to create it with fdisk and change its type;
2. Initialize the physical volumes (partitions or disks) with the pvcreate command;
3. Create a volume group with the vgcreate command;
4. Activate a volume group with the vgchange command;
5. Create a logical volume with the lvcreate command;
6. Format the logical volume with the desired filesystem.

Prepare the partitions for LVM with fdisk

If you do not want to use the entire disk as LVM, you can use fdisk to create one or more LVM type partitions.

To create an LVM partition with fdisk, you create the partition normally and change its type to 8e.

For example, let's imagine that the system was installed on disk sda. And two disks will be used for LVM:  sdb and sdc which are not partitioned.

Before adding a disk or partition as a physical volume of LVM you need to initialize it with the pvcreate command.

Initialize physical volumes with pvcreate

To initialize physical volumes of entire disks the command is pvcreate and the full path of the partition or disk:

# pvcreate /dev/sdb

Physical volume "/dev/sdb" successfully created

# pvcreate /dev/sdc

Physical volume "/dev/sdc" successfully created

Remember to first change the partition types with fdisk to 8e.

Creating a volume group with vgcreate

After initializing, you need to create a volume group with the disks with the vgcreate command:

# vgcreate meuvolume /dev/sdb /dev/sdc

  Volume group "meuvolume" successfully created

Activating a volume group with vgchange

After creating the volume group, it is necessary to activate it with the vgchange command:

# vgchange -a y meuvolume

  0 logical volume(s) in volume group "meuvolume" now active

After rebooting the system it is necessary to activate the volume group again. Therefore, it is necessary to include this command in the system boot scripts.

Remove a volume group with vgremove

To remove a volume group there cannot be logical volumes inside the group.

The command to remove volumes is vgremove and the volume needs to be deactivated:

# vgchange -a n meuvolume

# vgremove meuvolume

Add new "physical volumes to the volume group with vgextend

New disks can be added to the volume group with the command:

# vgextend meuvolume   /dev/sdd

Remove "physical volumes from the volume group" with vgreduce

Disks can be removed from the volume group with the vgreduce command.

You need to make sure there is no logical volume on the physical volume, which can be migrated to another disk.

# vgreduce meuvolume /dev/sdd

Creating logical volumes with lvcreate

The lvcreate command creates logical volumes. In the example, a 1GB logical volume called logico1 will be created on the meuvolume volume:

# lvcreate -L 1000 -n logico1 meuvolume

Logical volume "logico1" created

Since sdb and sdc each have 2GB, it's possible to create up to 4 volumes of 1GB each, or 1 single logical volume of 4GB, as in the example below:

# lvcreate -L 4000 -n logico1 meuvolume

  Logical volume "logico1" created

Activating the logical volume with lvchange

The lvchange command activates / deactivates the logical volume for use:

ACTIVATE:

# lvchange -a y /dev/meuvolume/logico1

DEACTIVATE:

# lvchange -a n /dev/meuvolume/logico1

Formatting the logical volume with mkfs

Any file system can be used to format the logical volume:

# mkfs.ext4 /dev/meuvolume/logico1

mke2fs 1.41.14 (22-Dec-2010)

Filesystem label=

OS type: Linux

(…)

Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 25 mounts or

180 days, whichever comes first.  Use tune2fs -c or -i to override.

After formatting the logical volume, it needs to be mounted:

# mount /dev/meuvolume/logico1 /dados

After these steps the logical volume will be ready for use.

Increasing the volume size

First you need to create the physical volume:

# pvcreate /dev/hda8

Assign it to the group:

# vgextend meugrupo /dev/hda8

Unmount the logical volume:

# umount /dev/meuvolume/logico1

Extend the logical volume group:

# lvextend -L +13090M /dev/meuvolume/logico1

Check for errors and repair:

# e2fsck -f /dev/meuvolume/logico1

Finally, resize:

# resize2fs /dev/meuvolume/logico1

Now just mount:

# mount /dev/meuvolume/logico1 /dados

Decreasing the volume size

Unmount the file system:

# umount /dev/meuvolume/logico1

Check the logical volume for errors:

# e2fsck -f /dev/meuvolume/logico1

Reduce the partition:

# resize2fs /dev/meuvolume/logico1 524288

Reduce the logical volume:

# lvreduce -L -50M //dev/meuvolume/logico1

Remount the file system:

# mount /dev/meuvolume/logico1 /dados

Learn much more about Linux in our online course. You can enroll here. If you already have an account, or want to create one, just log in or create your user here.

Did you like it? Share