Tuesday, November 29, 2011

LVM - Logical Volume Manager

The Logical Volume Manager (LVM) enables you to resize your partitions without having to modify the partition tables on your hard disk. This is most useful when you find yourself running out of space on a filesystem and want to expand into a new disk partition versus migrating all or a part of the filesystem to a new disk.
  • Physical Volume: A physical volume (PV) is another name for a regular physical disk partition that is used or will be used by LVM.
  • Volume Group: Any number of physical volumes (PVs) on different disk drives can be lumped together into a volume group (VG). Under LVM, volume groups are analogous to a virtual disk drive.
  • Logical Volumes: Volume groups must then be subdivided into logical volumes. Each logical volume can be individually formatted as if it were a regular Linux partition. A logical volume is, therefore, like a virtual partition on your virtual disk drive.
  •  Physical Extent: Real disk partitions are divided into chunks of data called physical extents (PEs) when you add them to a logical volume. PEs are important as you usually have to specify the size of your volume group not in gigabytes, but as a number of physical extents.
 Logical Volume Manager

2. Create Partitions

For this Linux lvm example you need an unpartitioned hard disk /dev/sdb. First you need to create physical volumes. To do this you need partitions or a whole disk. It is possible to run pvcreate command on /dev/sdb, but I prefer to use partitions and from partitions I later create physical volumes.
Logical Volume Manager List Partitions with fdisk
Use your preferred partitioning tool to create partitions. In this example I have used cfdisk.
Create partitions with cfdiskLogical Volume Manager List Partitions with fdisk

Partitions are ready to use.

3. Create physical volumes

Use the pvcreate command to create physical volumes.
# pvcreate /dev/sdb1
# pvcreate /dev/sdb2 
The pvdisplay command displays all physical volumes on your system.
# pvdisplay 
Alternatively the following command should be used:
# pvdisplay /dev/sdb1 
Create physical volumes with pvcreate

4. Create Virtual Group

At this stage you need to create a virtual group which will serve as a container for your physical volumes. To create a virtual group with the name "mynew_vg" which will include /dev/sdb1 partition, you can issue the following command:
# vgcreate mynew_vg /dev/sdb1 
To include both partitions at once you can use this command:
# vgcreate mynew_vg /dev/sdb1 /dev/sdb2 
Create Virtual Group
Feel free to add new physical volumes to a virtual group by using the vgextend command.
# vgextend mynew_vg /dev/sdb2 
virtual group extend

5. Create Logical Volumes

From your big cake (virtual group) you can cut pieces (logical volumes) which will be treated as a partitions for your linux system. To create a logical volume, named "vol01", with a size of 400 MB from the virtual group "mynew_vg" use the following command:
  • create a logical volume of size 400 MB -L 400
  • create a logical volume of size 4 GB -L 4G
# lvcreate -L 400 -n vol01 mynew_vg
lvm lvcreate
With a following example you will create a logical volume with a size of 1GB and with the name vol02:
# lvcreate -L 1000 -n vol02 mynew_vg

Note the free size in virtual group.
lvcrete free space

6. Create File system on logical volumes

The logical volume is almost ready to use. All you need to do is to create a filesystem.:
# mkfs.ext3 -m 0 /dev/mynew_vg/vol01 
the -m option specifies the percentage reserved for the super-user, set this to 0 if you wish not to waste any space, the default is 5%. create file system on the logical volumes

7. Edit /etc/fstab

Add an entry for your newly created logical volume into /etc/fstab
edit /etc/fstab to include logical volumes

7.1. Mount logical volumes

Before you mount do not forget to create a mount point.
# mkdir /home/foobar 
mount logical volumes

8. Extend logical volume

The biggest advantage of logical volume manager is that you can extend your logical volumes any time you are running out of the space. To increase the size of a logical volume by another 800 MB you can run this command:
# lvextend -L +800 /dev/mynew_vg/vol01 
Extend logical volume
The command above does not actually increase the physical size of volume, to do that you need to:
# resize2fs  /dev/mynew_vg/vol01 
Look at the figure below to see what problems you may encounter when extending a volume:
Problem with extending a logical volume

9. Remove logical volume

The command lvremove can be used to remove logical volumes. Make sure that before you attempt to remove logical volumes your logical volume does not have any valuable data stored on it, moreover, make sure the volume is unmounted.
# lvdisplay 
display volume group
# lvremove /dev/mynew_vg/vol02 

Note that this document comes without warranty of any kind. But every effort has been made to provide the information as accurate as possible. I welcome emails from any readers with comments, suggestions, and corrections at webmaster_at admin@linuxhowto.in

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