ANATOMY OF LVM

LVM Anatomy

  • VG (Volume Group): gathers together LVs and PVs into one administrative unit.

  • PV (Physical Volume): a PV is typically a hard disk (partition), raid device, etc...

  • LV (Logical Volume): the equivalent of a disk partition in a non-LVM system.

  • PE (Physical Extent): each PV is divided in chunks of data, known as PEs. These extents have the same size as the LEs.

  • LE (Logical Extent): each LV is split into chunks of data, known as LEs.

Mapping modes (linear/striped)

Mapping nodes

  • We now can create a LV (LV1).

  • LV1 can be any size between 1 and 347 (99+248) extents.

  • When LV1 is created, a mapping is defined between LEs and PEs (eg. LE[i] could map onto PE[i] of PV1 and PE[i] of PV2).

  • Strategies for mapping LEs onto PEs:

    1. Linear (similar to RAID Linear) mapping will assign a range of PEs to an area of an LV in order (eg., LE[1 - 99] map onto PV1 and LE[100 - 248] map onto PV2).

    2. Striped (similat to RAID0):

      LE[1] --> PV1[1]

      LE[2] --> PV2[1]

      LE[3] --> PV1[2]

      and so on.

Snapshots

  • Snapshots allows the administrator to create a new block device which is an exact copy of a LV, frozen at some point in time.

  • Used when for instance, we need to perform a backup on the LV, but you don't want to halt a live system that is changing the data.

Boot time script 

  • The startup of LVM requires just the following two commands:
# vgscan 

	vgscan – reading all physical volumes (this may take a while...) 

	vgscan – "/etc/lvmtab" and "/etc/lvmtab.d" successfully created

	vgscan – WARNING: This program does not do a VGDA backup of your volume group

# vgchange -ay
  • And the shutdown only one:
# vgchange -an

Startup script (/etc/init.d/lvm)

#!/bin/sh 

case "$1" in

	start)  /sbin/vgscan 
	
			/sbin/vgchange -ay ;; 

	stop)   /sbin/vgchange -an ;; 

	restart|force-reload)
	
			/sbin/vgchange -an && /sbin/vgscan && /sbin/vgchange -ay ;; 

esac 

exit 0

LVM Files

  • /etc/lvmtab

    Informs about the activated VGs

  • /proc/lvm/*

    Inform about the VGs structure

  • /dev/my_VG[1-n]/*

    LVM device files