Creating a LVM in Linux

I am sure anybody who have used windows (2000 and above) have come across the term dynamic disks. Linux/Unix also have its own dynamic disk management called LVM.

What is an LVM ?

LVM stands for Logical Disk Manager which is the fundamental way to manage UNIX/Linux storage systems in a scalable manner. An LVM abstracts disk devices into pools of storage space called Volume Groups. These volume groups are in turn subdivided into virtual disks called Logical Volumes. The logical volumes may be used just like regular disks with filesystem created on them and mounted in the Unix/Linux filesystem tree. The logical volumes can span multiple disks. Even though a lot of companies have implemented their own LVM's for *nixes, the one created by Open Software Foundation (OSF) was integrated into many Unix systems which serves as a base for the Linux implementation of LVM.

Note: Sun Solaris ships with LVM from Veritas which is substantially different from the OSF implementation.

Benefits of Logical Volume Management

• LVM created in conjunction with RAID can provide fault tolerance coupled with scalability and easy disk management.
• Create a logical volume and filesystem which spans multiple disks.
  
  By creating virtual pools of space, an administrator can create dozens of small filesystems for different projects and add space to them as needed without (much) disruption. When a project ends, he can remove the space and put it back into the pool of free space.
  

Note : Before you move to implement LVM's in linux, make sure your kernel is 2.4 and above. Or else you will have to recompile your kernel from source to include support for LVM.

LVM Creation
To create a LVM, we follow a three step process.

Step One : We need to select the physical storage resources that are going to be used for LVM. Typically, these are standard partitions but can also be Linux software RAID volumes that we've created. In LVM terminology, these storage resources are called "physical volumes" (eg: /dev/hda1, /dev/hda2 ... etc).

Our first step in setting up LVM involves properly initializing these partitions so that they can be recognized by the LVM system. This involves setting the correct partition type (usually using the fdisk command, and entering the type of partition as 'Linux LVM' - 0x8e ) if we're adding a physical partition; and then running the pvcreate command.

# pvcreate /dev/hda1 /dev/hda2 /dev/hda3
# pvscan

The above step creates a physical volume from 3 partitions which I want to initialize for inclusion in a volume group.

Step Two : Creating a volume group. You can think of a volume group as a pool of storage that consists of one or more physical volumes. While LVM is running, we can add physical volumes to the volume group or even remove them.

First initialize the /etc/lvmtab and /etc/lvmtab.d files by running the following command:

# vgscan

Now you can create a volume group and assign one or more physical volumes to the volume group.

# vgcreate my_vol_grp /dev/hda1 /dev/hda2

Behind the scenes, the LVM system allocates storage in equal-sized "chunks", called extents. We can specify the particular extent size to use at volume group creation time. The size of an extent defaults to 4Mb, which is perfect for most uses.You can use the -s flag to change the size of the extent. The extent affects the minimum size of changes which can be made to a logical volume in the volume group, and the maximum size of logical and physical volumes in the volume group. A logical volume can contain at most 65534 extents, so the default extent size (4 MB) limits the volume to about 256 GB; a size of 1 TB would require extents of atleast 16 MB. So to accomodate a 1 TB size, the above command can be rewriten as :

# vgcreate -s 16M my_vol_grp /dev/hda1 /dev/hda2

You can check the result of your work at this stage by entering the command:

# vgdisplay

This command displays the total physical extends in a volume group, size of each extent, the allocated size and so on.

Step Three : This step involves the creation of one or more "logical volumes" using our volume group storage pool. The logical volumes are created from volume groups, and may have arbitary names. The size of the new volume may be requested in either extents (-l switch) or in KB, MB, GB or TB ( -L switch) rounding up to whole extents.

# lvcreate -l 50 -n my_logical_vol my_vol_grp

The above command allocates 50 extents of space in my_vol_grp to the newly created my_logical_vol. The -n switch specifies the name of the logical volume we are creating.

Now you can check if you got the desired results by using the command :

# lvdisplay

which shows the information of your newly created logical volume.

Once a logical volume is created, we can go ahead and put a filesystem on it, mount it, and start using the volume to store our files. For creating a filesystem, we do the following:

# mke2fs -j /dev/my_vol_grp/my_logical_vol

The -j signifies journaling support for the ext3 filesystem we are creating.
Mount the newly created file system :

# mount /dev/my_vol_grp/my_logical_vol /data

Also do not forget to append the corresponding line in the /etc/fstab file:

#File: /etc/fstab
/dev/my_vol_grp/my_logical_vol /data ext3 defaults 0 0

Now you can start using the newly created logical volume accessable at /data mount point.
Next : Resizing Logical Volumes