Introduction to LVM

LVM (Logical Volume Management) is an extremely handy tool to learn because it allows storage to be handled in a very easy manner. LVM allows you to abstract various pieces of physical storage into groups that can be carved into chunks that form the basis of file systems (virtual partitions if you like). It also allows you to combine physical partitions into groups, resize these groups (grow or shrink), and effectively manage these groups.

Here is a easy diagram shows disk,PV, VG, and LV relationship in LVM

    sda1   sdb1      (PV:s on partitions or whole disks)                        
       \   /                                                                    
        \ /                                                                     
       diskvg        (VG)                                                       
       /  |  \                                                                  
      /   |   \                                                                 
  usrlv rootlv varlv (LV:s)
    |      |     |                                                              
 ext2  reiserfs  xfs (filesystems)                                        


LVM makes things so convience that even small system can get benefit from it. If you are managing a server, or a cluster, then LVM definitely is your choice. Here are some concepts.

Physical Volumes (PV’s) are really partitions on a disk (hence the use of the term “physical”). Above the PV’s is one or more Volume Groups (VG). Then you partition the VG into Logical Volumes (LV’s). These LV’s are used for creating file systems.

LVM Concepts

There are some fundamental concepts in LVM that you need to understand (and master). the diagram above shows the main components in LVM.

Physical Volume

In this case there are two drives, /dev/sdb and /dev/sdb. Each of these drives has two partitions.

Check disk drive partitions, the partition type should be “8e”. When you list the partitions using fdisk, the partitions that are PV’s should read “8e” which corresponds to “Linux LVM” as shown below.

# fdisk -l /dev/sdb

Disk /dev/sdb: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1       30400   244187968+  8e  Linux LVM
/dev/sdb2           30401       60801   244196032+  8e  Linux LVM

Notice the partition type. This has to be done for every partition on the drives being used with LVM.

Once the PV's are created, using LVM is fairly easy by just knowing a few commands. The first command is how to make the physical volumes (PV's) using the command "pvcreate".

# pvcreate /dev/sdb1 /dev/sdc1 
  Physical volume "/dev/sdb1" successfully created
  Physical volume "/dev/sdc1" successfully created

If you like, you can get more extensive information about the PV's with a simple command, "pvdisplay".

# pvdisplay
  "/dev/sdb1" is a new physical volume of "232.88 GB"
  --- NEW Physical volume ---
  PV Name               /dev/sdb1
  VG Name
  PV Size               232.88 GB
  Allocatable           NO
  PE Size (KByte)       0
  Total PE              0
  Free PE               0
  Allocated PE          0
  PV UUID               j4XKT6-OI2y-cPMK-YpNR-gmgc-es67-ey4CV2

  "/dev/sdc1" is a new physical volume of "232.88 GB"
  --- NEW Physical volume ---
  PV Name               /dev/sdc1
  VG Name
  PV Size               232.88 GB
  Allocatable           NO
  PE Size (KByte)       0
  Total PE              0
  Free PE               0
  Allocated PE          0
  PV UUID               qLC1pM-y07j-3Pq4-waiV-BRzc-vySF-KTesHH

Notice that each PV is given a PV UUID. This is a unique address that identifies the PV. Also notice that it tells you if the PV has been assigned to a VG (Volume Group). In this case, none of the PV's have been assigned to a VG.

VG(Volume Group)

The Volume Group is the highest level abstraction used within the LVM. It gathers together a collection of Logical Volumes and Physical Volumes into one administrative unit. You can think Volume Group (VG) as the “virtual hard drive”. It gathers real partitions (PV’s) into a virtual drive (the VG).

The next step is to create the VG (Volume Group) from the PV's using the command "vgcreate".

# vgcreate diskvg /dev/sdb1 /dev/sdc1
  Volume group "diskvg" successfully created

Notice that the first argument to vgcreate is the name of the VG (in this case, diskvg). Similarly to "pvdisplay" there is a "vgdisplay" command.

# vgdisplay
  --- Volume group ---
  VG Name               diskvg
  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               465.76 GB
  PE Size               4.00 MB
  Total PE              119234
  Alloc PE / Size       0 / 0
  Free  PE / Size       119234 / 465.76 GB
  VG UUID               oNH6jk-PBE0-mR0c-aaDi-3Fys-y5SQ-0tVaxX

If you don't happen to remember the name of a VG or you happen upon a new system to administer, then you can use a command called "vgscan" that will tell what VG's are on the system.

# vgscan
  Reading all physical volumes.  This may take a while...
  Found volume group "diskvg" using metadata type lvm2

This command will scan all disks for VG's. This is very useful if you've forgotten the layout of the PV's and VG's. It's also HIGHLY recommended that you keep written notes of the LVM layout in case someone has to pick up administration of the system.

LV(Logic Volume)

The equivalent of a disk partition in a non-LVM system. The LV is visible as a standard block device; as such the LV can contain a file system. You can think of the LV’s as “virtual partitions.” Then you can create file systems or whatever else you need to do with the LV’s.

The next step after creating the VG is to partition it or create LV's using the command "lvcreate". For this example, only a single LV will be created.

# lvcreate --name /dev/diskvg/home_lv --size 300G
  Logical volume "home_lv" created

The name used in the lvcreate command is the full path of the LV (this is another reason it is a good idea to keep really good notes). You can get information on the LV using the command "lvdisplay".

# lvdisplay
  --- Logical volume ---
  LV Name                /dev/diskvg/home_lv
  VG Name                diskvg
  LV UUID                BXXUyW-z8vS-6tgR-N5SW-tttW-j6Yb-6XEyya
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                450.00 GB
  Current LE             115200
  Segments               2
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:0

Notice that it tells you the VG that is used for the LV ("diskvg" in this case). There is lots of other information in the output as well. It's highly recommended you look at the man pages and tutorials on the web to learn this information.

As with VG's, you can always scan for the Logical Volumes using "lvscan".

# lvdisplay
# lvscan
  ACTIVE            '/dev/diskvg/home_lv' [450.00 GB] inherit

The output from the command with the default options is fairly simple but there is a "verbose" option that will give you more information.

File system

At this point, you are ready to create a file system on this LV. In this case, an ext3 file system will be created.

# mkfs.ext3 /dev/diskvg/home_lv
mke2fs 1.41.7 (29-June-2009)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
29491200 inodes, 117964800 blocks
5898240 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=4294967296
3600 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
        4096000, 7962624, 11239424, 20480000, 23887872, 71663616, 78675968,

Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 29 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.

Notice that the "partition" used is the full path to the LV, /dev/diskvg/home_lv. After the file system is created, you can mount the file system as you would any other.

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