Logical Volume Manager is a device mapper in LINUX which provides Logical Volume management.

With LVM, we can create logical partitions that can span across one or more physical hard drives.

LVM virtualizes the memory and the biggest advantage of LVM is that it allows disks to be added, replaced, reduced, resized without any downtime or service disruption.

LVM allows file systems to be easily resized later as per our need.

Creating and managing LVM is very simple. Just follow 3 steps process:

1. First the hard drives are divided into physical volumes(PV).
2. Physical volumes are combined together to create volume group(VG).
3. Create logical volumes(LV) from the volume group.

So, we need and un-partitioned  disk from which we shall select physical storage devices for LVM. Next is we shall group them OR club them together to create vloume groups. 

Volume groups(VG) are nothing but a circular birthday cake and when we cut a piece of cake out of it, it becomes the logical volumes(LV).

LVM is a part of lvm2 package. If this package is not installed in your system, install it using below command:

sudo apt-get install lvm2

Lets start creating LVM.

To check all available partitions , we can use below command or cat /proc/partitions:

fdisk -l

We can use cfdisk to create partitions:

cfdisk /dev/sdb

Now partitions are ready to be used. Next is we have to create physical volumes.

#pvcreate /dev/sdb1

Physical volume '/dev/sdb1' created.

#pvcreate /dev/sdb2

Physical volume '/dev/sdb2' created.

To display all created physical volumes, use pvdisplay command.

Or to display information about a particular PV, use pvdisplay /dev/sdb1

Create Volume Group

Now we shall create volume groups using these physical volumes. It will act as a container for our physical volumes

Lets create a volume group with name myVolGroup which will include /dev/sdb1 and /dev/sdb2.

#vgcreate myVolGroup /dev/sdb1 /dev/sdb2

Volume group created successfully

 To display volume groups in your system,use vgdisplay command.

Here, you can add new partition into you existing volume group using vgextend.

#vgextend myVolGroup /dev.sdb3

Volume group extended successfully.

Create Logical Volume

Now we have VG ready, we shall cut pieces(LV) out of the cake (VG) .

From the Big cake i.e. VG, we can cut as many pieces i.e. logical volumes as we want  which will be treated as logical partitions for our system.

So, lets create a logical volume with name lvol1 with size 400 MB from the VG myVolGroup.

#lvcreate -L 400 -n lvol1 myVolGroup

Logical volume lvol1 created.

To display logical volumes in your system, use lvdisplay command.

Similarly, to create logical volume of size 1000 mb ith name lvol2 from the same VG:

#lvcreate -L 1000 -n lvol2 myVolGroup

or

lvcreate -L 1G -n lvol2 myVolGroup.

Now, we have created two LVs from our volume group, we can check available size in Volume group for any more possible LVs. 

Lets make our LV usable

To use the newly created LV, we need to create a file system.

#mkfs-ext3 -m 0 /dev/myVolGroup/lvol1

Here -m 0 means that the percentage of space reserved for superuser is 0. If we don't give this option, the space reserved for superuser will be 5% by default.

Mount Logical volumes

Before we mount this LV, we need to created a mount point that is a location where this filesystem will be mounted.

#mkdir /home/shanky1

#mount -a

or 

#mount -t ext3 /dev/myVolGroup/lvol1 /home/shanky1

Add this file system entry into your /etc/fstab file to make it permanent.

/dev/myVolGroup/lvol1 /home/shanky1 ext3 defaults 0 0

Now navigate to your new mount point.

#cd /home/shanky1

You can check all your file systems and their mounted locations using df -hP command.

Biggest advantage of LVM is that we can extend the LVM anytime if we are running out of space.

#lvextend -L +800 /dev/myVolGroup/lvol1

The above command will increase the size of logical volume by another 800 MB.

But this will not actually increase the physical size of the volume. To actually resize the volume, we have to use resize2fs command.

#resize2fs /dev/myVolGroup/lvol1

Can't resize a mounted filesystem.

So, we need to unmount the file system before we resize.

#unmount /home/shanky1 

 Now lets try that resize command again.

#resize2fs /dev/myVolGroup/lvol1

Please run e2fsck -f /dev/myVolGroup/lvol1 first!

So we execute e2fsck -f /dev/myVolGroup/lvol1 and then we execute the resize command again.

 Finally we mount the file system to make it usable.

#mount -a

Note that mount -a command will mount all file systems mentioned in /etc/fstab file as indicated except for those whose line contains noauto keyword.

Reduce logical volume

#lvreduce  -L -10G -n /dev/myVolGroup/lvol1

The above command will reduce LV size by 10 GB.

To remove a logical volume

#lvremove /dev/myVolGroup/lvol1