LVM in Linux step by step

LVM stands for Logical Volume Manager.

With LVM, we can create logical partitions that can span across one or more physical hard drives. First, the hard drives are divided into physical volumes, then those physical volumes are combined together to create the volume group and finally the logical volumes are created from volume group.

The LVM commands listed in this article are used under Ubuntu Distribution. But, it is the same for other Linux distributions.

Before we start, install the lvm2 package as shown below.

$ sudo apt-get intall lvm2

To create a LVM, we need to run through the following steps.

Select the physical storage devices for LVM
Create the Volume Group from Physical Volumes
Create Logical Volumes from Volume Group

Select the Physical Storage Devices for LVM – Use pvcreate, pvscan, pvdisplay Commands

In this step, we need to choose the physical volumes that will be used to create the LVM. We can create the physical volumes using pvcreate command as shown below.

$ sudo pvcreate /dev/sda6 /dev/sda7 
Physical volume "/dev/sda6" successfully created                                                 
Physical volume "/dev/sda7" successfully created

As shown above two physical volumes are created – /dev/sda6 and /dev/sda7.

If the physical volumes are already created, you can view them using the pvscan command as shown below.

$ sudo pvscan                                                                   
  PV /dev/sda6                      lvm2 [1.86 GB]                                                 
  PV /dev/sda7                      lvm2 [1.86 GB]                                                 
  Total: 2 [3.72 GB] / in use: 0 [0   ] / in no VG: 2 [3.72 GB]

You can view the list of physical volumes with attributes like size, physical extent size, total physical extent size, the free space, etc., using pvdisplay command as shown below.

$ sudo pvdisplay 
--- Physical volume --- 
  PV Name             /dev/sda6 
  VG Name             
  PV Size               1.86 GB / not usable 2.12 MB 
  Allocatable           yes 
  PE Size (KByte)    4096 
  Total PE              476 
  Free PE               456 
  Allocated PE          20 
  PV UUID               m67TXf-EY6w-6LuX-NNB6-kU4L-wnk8-NjjZfv 
 
  --- Physical volume --- 
  PV Name             /dev/sda7 
  VG Name             
  PV Size               1.86 GB / not usable 2.12 MB 
  Allocatable           yes 
  PE Size (KByte)    4096 
  Total PE              476 
  Free PE               476 
  Allocated PE          0 
  PV UUID               b031x0-6rej-BcBu-bE2C-eCXG-jObu-0Boo0x

Note : PE – Physical Extents are nothing but equal-sized chunks. The default size of extent is 4MB.

Create the Volume Group – Use vgcreate, vgdisplay Commands

Volume groups are nothing but a pool of storage that consists of one or more physical volumes. Once you create the physical volume, you can create the volume group (VG) from these physical volumes (PV).

In this example, the volume group vol_grp1 is created from the two physical volumes as shown below.

$ sudo vgcreate vol_grp1 /dev/sda6 /dev/sda7                                  
  Volume  group "vol_grp1" successfully created

LVM processes the storage in terms of extents. We can also change the extent size (from the default size 4MB) using -s flag.

vgdisplay command lists the created volume groups.

$ sudo vgdisplay 
  --- Volume group ---              
  VG Name                     vol_grp1  
  System ID                         
  Format                        lvm2        
  Metadata Areas            2           
  Metadata Sequence No  1           
  VG Access                   read/write  
  VG Status                    resizable   
  MAX LV                       0           
  Cur LV                        0           
  Open LV                      0           
  Max PV                       0           
  Cur PV                        2           
  Act PV                       2           
  VG Size                      3.72 GB     
  PE Size                      4.00 MB     
  Total PE                     952         
  Alloc PE / Size             0 / 0       
  Free  PE / Size            952 / 3.72 GB 
  VG UUID                     Kk1ufB-rT15-bSWe-5270-KDfZ-shUX-FUYBvR

LVM Create: Create Logical Volumes – Use lvcreate, lvdisplay command

Now, everything is ready to create the logical volumes from the volume groups. lvcreate command creates the logical volume with the size of 80MB.

$ sudo lvcreate -l 20 -n logical_vol1 vol_grp1 
  Logical volume "logical_vol1" created

Use lvdisplay command as shown below, to view the available logical volumes with its attributes.

 
$ sudo lvdisplay                                  
  --- Logical volume ---                                             
  LV Name                /dev/vol_grp1/logical_vol1              
  VG Name                vol_grp1                                  
  LV UUID                 ap8sZ2-WqE1-6401-Kupm-DbnO-2P7g-x1HwtQ      
  LV Write Access      read/write                                  
  LV Status              available                                   
  # open                  0                                           
  LV Size                  80.00 MB                                    
  Current LE              20                                          
  Segments               1                                           
  Allocation               inherit                                     
  Read ahead sectors  auto                                        
  - currently set to     256                                         
  Block device            252:0

After creating the appropriate filesystem on the logical volumes, it becomes ready to use for the storage purpose.

$ sudo  mkfs.ext3 /dev/vol_grp1/logical_vol1

LVM resize: Change the size of the logical volumes – Use lvextend Command

We can extend the size of the logical volumes after creating it by using lvextend utility as shown below. The changes the size of the logical volume from 80MB to 100MB.

$ sudo lvextend -L100 /dev/vol_grp1/logical_vol1 
  Extending logical volume logical_vol1 to 100.00 MB 
  Logical volume logical_vol1 successfully resized

We can also add additional size to a specific logical volume as shown below.

$ sudo lvextend -L+100 /dev/vol_grp1/logical_vol1 
  Extending logical volume logical_vol1 to 200.00 MB 
  Logical volume logical_vol1 successfully resized

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