Logical Volume Management

LVM is an optional subsystem that is a beefed up version of disk partitioning. Essentially, you can group disks together into a Volume Group (VG). The OS is then presented the VG as a block device.

LVM Benefits

• More efficient use of disk storage
• Can move LVs among different physical devices
• Grow and shrink volumes on-the-fly
• Take snapshots of whole filesystems

Physical Volumes(pv)

Physical volumes can be :

• disks
• disk paritions
• Raid objects

Volume Groups (vg)

LVM combines physical volumes into storage pools known as volume groups. Volume groups abstract the characteristics of the underlying devices and function as a unified logical device with combined storage capacity of the component physical volumes. (digital ocean)

Logical Volume (lv)

• A slice of a vg (kind of like a partition, but more flexible)

Extent

An extent is a fixed-size-chunk within a volume group. They can be either a physical extent(PE) or logical extent(LE). An LV is just mapping between a PE and LE.

LVM Anatomy

    +-- Volume Group --------------------------------+
    |                                                |
    |    +----------------------------------------+  |
    | PV | PE |  PE | PE | PE | PE | PE | PE | PE |  |
    |    +----------------------------------------+  |
    |      .          .          .        .          |
    |      .          .          .        .          |
    |    +----------------------------------------+  |
    | LV | LE |  LE | LE | LE | LE | LE | LE | LE |  |
    |    +----------------------------------------+  |
    |            .          .        .         .     |
    |            .          .        .         .     |
    |    +----------------------------------------+  |
    | PV | PE |  PE | PE | PE | PE | PE | PE | PE |  |
    |    +----------------------------------------+  |
    |                                                |
    +------------------------------------------------+

Another view

        hda1   hdc1      (PV:s on partitions or whole disks)                        
           \   /                                                                    
            \ /                                                                     
           diskvg        (VG)                                                       
           /  |  \                                                                  
          /   |   \                                                                 
      usrlv rootlv varlv (LV:s)
        |      |     |                                                              
     ext2  reiserfs  xfs (filesystems) 

Example

See here

LV More

• You can grow/shrink a supported filesystem while mounted. (Parted can grow/shrink but only if not in use)
• Can expand to use any free space in the VG (parted can only do contiguous)

Snapshots

You can freeze an existing LV in time, even while system is running. You can continue to use the original volume normally, but the snapshot volume appears to be an image of the original, frozen in time at the moment you created it. You can use this to get a consistent filesystem image to back up, without shutting down the system. You can also use it to save the state of the system, so that you can later return to that state if you mess things up. You can even mount the snapshot volume and make changes to it, without affecting the original.

Logical Volume Management commands

• pvcreate - This writes the LVM header to the partition, which identifies it as a Physical Volume, and sets up a small area to hold the metadata describing everything about the Volume Group, and the the rest of the partition as unused Physical Extents.

Logical Volume Management commands

• vgcreate - Creates the volume group
• lvcreate - creates the logical volume in a volume group by allocating logical extents from the free physical extent pool of that volume group. If there are not enough free physical extents then the volume group can be extended with other physical volumes or by reducing existing logical volumes of this volume group in size

Logical Volume Management commands

• show disks to add to volumes
  • lvmdiskscan
• register disks as physical volumes
  • pvcreate /dev/sda5 /dev/sda6 /dev/sda7 /dev/sda8
• create vg with 3 disks
  • vgcreate myvolumegroup /dev/sda5 /dev/sda6 /dev/sda7
• create a lvm
  • lvcreate -L 30M -n thirty myvolumegroup –type raid1
• Add a new disk to the volume group
  • vgextend myvolumegroup /dev/sda8
• Extend an lvm and resize the fs
  • lvresize -r –size 250M /dev/myvolumegroup/fifty