Logical Volume Management
#########################

.. note::

    Not using lvm in the modern Linux environment requires an explanation.
..

Why LVM
=======

* This is especially important when creating cloud-served templates, AMIs, etc. to ensure disk addressability after deployment.
* LVM allows dynamism not only in size, but specificity in mounts.
* Disks can reorder, UUIDs might change as you replace disks and drives.
* LVM allows you to manage your storage with name spaces that stay consistent...

  * ...across architectures.
  * ...across installations.
  * ...across verions.
  * ...across operating systems.

* This is a risk reduction measure that also applies to changing disk allocation requirements.

LVM Basics
==========

* For this example, we will create...

  * ...a disk-filling file system called **data**
  * ...in a namespace called **squirrel**
  * on physical drive **/dev/sdb**
  * to mount into **/mnt**

.. code-block:: bash
  :linenos:

  # Pick a disk from the list.  For this example, we will use /dev/sdb
  cat /proc/partitions
  # Set our variables for the example
  mydisk=/dev/sdb # we do not want to rely on a particular disk order
  namespace=squirrel # this is just an example name
  partname=data # this is also an example
  fstype=ext4 # alternately, xfs or anything you like
  # Prepare a physical volume
  /usr/sbin/pvcreate ${mydisk}
  # Create a volume group.  Think of this as a namespace.
  /usr/sbin/vgcreate ${namespace} ${mydisk}
  # Create a logical volume in that volume group.
  #  100%FREE is a special size that fills all available space
  /usr/sbin/lvcreate -n ${partname} -l 100%FREE ${namespace}
  # Create a file system on that logical volume
  mkfs.${fstype} /dev/${namespace}/${partname}
  # Mount the logical volume where you want it to be
  mount /dev/${namespace}/${partname} /mnt
  # Make that mount appear automatically at boot
  echo "/dev/${namespace}/${partname} /mnt ext4 rw,defaults 0 0" >> /etc/fstab
..

* That is it! You never have to think about whether your disk appeared as /dev/xvfdb, /dev/sdb, or /dev/sdx.