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Setting up LVM in Linux

This is a Simple Quick HowTo on LVM setup.It's provides a quick overview of LVM setup (Logical Volume Manager) in Linux.Ready ? Let's go...
Here is my hard disk details.

[root@localhost ~]# fdisk -l /dev/sda

Disk /dev/sda: 80.0 GB, 80026361856 bytes
255 heads, 63 sectors/track, 9729 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1            4865        4995     1052257+  82  Linux swap / Solaris
/dev/sda3            4996        9468    35929372+  83  Linux
/dev/sda4               1        3163    25406766    5  Extended
/dev/sda5               1         903     7253284+  83  Linux
/dev/sda6   *         904        1947     8385898+  83  Linux
/dev/sda7            1948        2677     5863693+  83  Linux
/dev/sda8            2678        3163     3903763+  83  Linux

Partition table entries are not in disk order

We are going to create LVM using two partitions,

/dev/sda7 which has 5.5GB and

/dev/sda8 which has 3.7GB space.
In order to Create LVM , you need to three steps Physical volume,Volume group and Logical Volume.
Physical volume can be set of hard disk partitions and these partition combined together to form a Volume group. Logical volume is final resulting partition built from Volume group.
Step 1: Creating Physical Volumes.
The command pvcreate is used to initialize the partitions (/dev/sda7 and /dev/sda8) for use by LVM. So execute it on our partition like this,

[root@localhost ~]#  pvcreate /dev/sda7 /dev/sda8
 Physical volume "/dev/sda7" successfully created
  Physical volume "/dev/sda8" successfully created

Though above output shows Physical volume successfully created on our dis, We can recheck them using the command called pvscan which scans for list of physical volumes that can support LVM setup.

[root@localhost ~]# pvscan 
  PV /dev/sda7         lvm2 [5.59 GB]
  PV /dev/sda8         lvm2 [3.72 GB]
  Total: 2 [9.31 GB] / in use: 0 [0   ] / in no VG: 2 [9.31 GB]

Yeah,we are sure that two partitions are ready.
Step 2: Now,Create Volume Group over Physical volumes
The command to create new volume group is vgcreate - yes,you are right,it\'s very much similar to previous command pvcreate thus makes it easy to remember them. Following vgcreate command creates a new volume group called giis_vol_grp

using the block special device (/dev/sda7 and /dev/sda8) previously configured for LVM with pvcreate.

 
[root@localhost ~]# vgcreate giis_vol_grp /dev/sda7 /dev/sda8
  Volume group "giis_vol_grp" successfully created

We can display attributes of volume groups as shown below.Note the total size of giis_vol_grp volume group is sum of /dev/sda7 and /dev/sda8 ,that's 5.59GB + 3.72Gb = 9.3GB.(approx)

[root@localhost ~]# vgdisplay 
  --- Volume group ---
  VG Name               giis_vol_grp
  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               9.31 GB
  PE Size               4.00 MB
  Total PE              2384
  Alloc PE / Size       0 / 0   
  Free  PE / Size       2384 / 9.31 GB
  VG UUID               fC2zQj-jsNR-kH74-8JGx-mjk1-031x-2qJYKR

Step 3: Finally,Create Logical Volume On Volume Group.
By now you should know what lvcreate will do? right ?here it's ,lvcreate creates a new logical volume calledgiis_logical_vol

in a volume group. Here we can specify the size of logical volume based on our requirement.We have used physical extents ( above PE size is 4MB) to specify the size 50*4MB=200MB.You can also use --size option with lvcreate to specify the actual size.(see below simple example/demo)

[root@localhost ~]# lvcreate -l 50 -n giis_logical_vol giis_vol_grp
  Logical volume "giis_logical_vol" created

Now from the attributes of a logical volume,we can see it's size is set as 200MB.

[root@localhost ~]# lvdisplay 
  --- Logical volume ---
  LV Name                /dev/giis_vol_grp/giis_logical_vol
  VG Name                giis_vol_grp
  LV UUID                EHuylX-oJtd-J7OT-3tP9-feDb-T24z-ozOqC8
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                200.00 MB
  Current LE             50
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           253:0

That's it.Now we can create a file system on the logical volume.We create ext3fs for this example.
Step 4:Creating File system on Logical Volume
[root@localhost ~]# mkfs.ext3 /dev/giis_vol_grp/giis_logical_vol
now mount it under /mnt.
[root@localhost ~]# mount /dev/giis_vol_grp/giis_logical_vol /mnt/
Just check it's size.

[root@localhost ~]# df -h
Filesystem           			   Size  Used Avail Use% Mounted on


/dev/mapper/giis_vol_grp-giis_logical_vol   194M  5.6M  179M   4% /mnt

and unmount it.

[root@localhost ~]# umount /mnt/

LVM Usage -Simple Example/Demo

Say for example,Now I have created two logical volumes named "bkup_sys" with 5GB

and "main_sys" with 3GB

and formatted it as ext3fs and mounted them.

[root@localhost ~]# lvcreate --name bkup_sys --size 5G giis_vol_grp
  Logical volume "bkup_sys" created


[root@localhost ~]# mkfs.ext3 /dev/giis_vol_grp/bkup_sys 


[root@localhost ~]# df -h
Filesystem           			   Size  Used Avail Use% Mounted on
/dev/mapper/giis_vol_grp-bkup_sys
                      5.0G  139M  4.6G   3% /mnt



[root@localhost ~]# lvcreate --name main_sys --size 3G giis_vol_grp
  Logical volume "main_sys" created


 mkfs.ext3 /dev/giis_vol_grp/main_sys

Now both are mounted,using df command we get following output,

[root@localhost ~]# df -h


/dev/mapper/giis_vol_grp-main_sys
                      3.0G   69M  2.8G   3% /mnt/main_sys
/dev/mapper/giis_vol_grp-bkup_sys
                      5.0G  139M  4.6G   3% /mnt/bkup_sys

Few months later,we found, bkup_sys doesn't need 5GB and 4GB is enough. And main_sys needs extra 1GB.Following represents the currents status of LVM

[root@localhost ~]# lvscan
  ACTIVE            '/dev/giis_vol_grp/giis_logical_vol' [200.00 MB] inherit
  ACTIVE            '/dev/giis_vol_grp/bkup_sys' [5.00 GB] inherit
  ACTIVE            '/dev/giis_vol_grp/main_sys' [3.00 GB] inherit

Shrinking size
So first we reduce/shrink "bkup_sys" from 5GB to 4GB using lvreduce command.

[root@localhost ~]# umount /mnt/bkup_sys/
[root@localhost ~]# lvreduce -L 4G /dev/giis_vol_grp/bkup_sys
  WARNING: Reducing active and open logical volume to 4.00 GB
  THIS MAY DESTROY YOUR DATA (filesystem etc.)
Do you really want to reduce bkup_sys? [y/n]: y
  Reducing logical volume bkup_sys to 4.00 GB
  Logical volume bkup_sys successfully resized

As you can see from above ,the "bkup_sys" is shrinked to 4GB. Use lvscan to verify it.

[root@localhost ~]# lvscan
  ACTIVE            '/dev/giis_vol_grp/giis_logical_vol' [200.00 MB] inherit
  ACTIVE            '/dev/giis_vol_grp/bkup_sys' [4.00 GB] inherit
  ACTIVE            '/dev/giis_vol_grp/main_sys' [3.00 GB] inherit

Increasing size
Now we increase the "main_sys" to 4GB using lvresize command.

[root@localhost ~]# lvresize -L 4G /dev/giis_vol_grp/main_sys 
  Extending logical volume main_sys to 4.00 GB
  Logical volume main_sys successfully resized

Finally we got the required size,where "main_sys" is increased from 3GB to 4GB and "bkup_sys" size reduced from 5GB to 4GB.

[root@localhost ~]# lvscan
  ACTIVE            '/dev/giis_vol_grp/giis_logical_vol' [200.00 MB] inherit
  ACTIVE            '/dev/giis_vol_grp/bkup_sys' [4.00 GB] inherit
  ACTIVE            '/dev/giis_vol_grp/main_sys' [4.00 GB] inherit