lvm - Linux Logical Volume Manager

lvm is a logical volume manager for Linux. It enables you to concatenate several physical volumes (hard disks etc.) into a so called volume group (VG, see pvcreate(8) and vgcreate(8) ) forming a storage pool, like a virtual disk. IDE, SCSI disks as well as multiple devices (MD) are supported. The storage capacity of a volume group can be divided into logical volumes (LVs), like virtual disk partitions. The size of a logical volume is in multiples of physical extents (PEs, see lvcreate(8) ).

The size of the physical extents can be configured at volume group creation time. If a logical volume is too small or too large you can change its size at runtime ( see lvextend(8) and lvreduce(8) ). lvcreate(8) can be used to create snapshots of existing logical volumes (so called original logical volumes in this context) as well.

Creating a snapshot logical volumes grants access to the contents of the original logical volume it is associated with and exposes the read only contents at the creation time of the snapshot. This is useful for backups or for keeping several versions of filesystems online.

If you run out of space in a volume group it is possible to add one or more pvcreate'd disks to the system and put them into an existing volume group ( see vgextend(8) ). The space on these new physical volumes can be dynamically added to logical volumes in that volume group ( see lvextend(8) ).

To remove a physical volume from the system you can move allocated logical extents to different physical volumes ( see pvmove(8) ). After the pvmove the volume group can be reduced with the vgreduce(8) command.

Inactive volume groups must be activated with vgchange(8) before use. vgcreate(8) automatically activates a newly created volume group.

PV for physical volume, PE for physical extent, VG for volume group, LV for logical volume, and LE for logical extent.

All command names corresponding to physical volumes start with pv, all the ones concerned with volume groups start with vg and all for logical volumes with lv. General purpose commands for the lvm as a whole start with lvm.

The volume group descriptor area (or VGDA for short) holds the necessary metadata to handle the LVM functionality. It is stored at the beginning of each pvcreate'd disk. It contains four parts: one PV descriptor, one VG descriptor, the LV descriptors and several PE descriptors. LE descriptors are derived from the PE ones at vgchange(8) time. Automatic backups of the VGDA are stored in files in /etc/lvmconf/ (please see vgcfgbackup(8)/vgcfgrestore(8) too). Take care to include these files in your regular (tape) backups as well.

Currently up to 99 volume groups with a grand total of 256 logical volumes can be created. The limit for the logical volumes is not caused by the LVM but by Linux 8 bit device minor numbers.

This means that you can have 99 volume groups with 1-3 logical volumes each or on the other hand 1 volume group with up to 256 logical volumes or anything in between these extreme examples.

Depending on the physical extent size specified at volume group creation time (see vgcreate(8) ), logical volumes of between a maximum of 512 Megabytes and 1 Petabyte can be created. Actual Linux kernels on IA32 limit these lvm possibilities to a maximum of 2 Terabytes per logical and per physical volume as well. This enables you to have as much as 256 Terabytes under LVM control with all possible 128 scsi disk subsystems. You can have up to 65534 logical extents (on IA32) in a logical volume at the cost of 1 Megabyte in kernel memory. Physical volumes can have up to 65534 physical extents.

The operational state of active volume groups with their physical and logical volumes can be found in the /proc/lvm/ directory. /proc/lvm/global contains a summary of all available information regarding all VGs, LVs and PVs. The two flags for PV status in brackets mean A/I for active/inactive and A/N for allocatable or non-allocatable. The four flags for LV status in brackets mean A/I for active/inactive, R/W for read-only or read/write, D/C for discontiguous or contiguous and L/S for linear or striped. S can optionally be followed by the number of stripes in the set. At /proc/lvm/VGs/ starts a subdirectory hierarchy containing information about every VG in a different subdirectory named /proc/lvm/VGs/VolumeGroupName where VolumeGroupName stands for an arbitrary VG name. /proc/lvm/VGs/VolumeGroupName/ in turn holds a file group containing summary information for the VG as a total. /proc/lvm/VGs/VolumeGroupName/LVs/LogicalVolumeName holds information for an arbitrary LV named LogicalVolumeName /proc/lvm/VGs/VolumeGroupName/PVs/PhysicalVolumeName contains information for an arbitrary PV named PhysicalVolumeName. All of the information in the files below /proc/lvm/VGs/ is presented in attribute/value pairs to be easyly parsable.

We have disk partitions /dev/sda3, /dev/sdb1 and /dev/hda2 free for use and want to create a volume group named "test_vg". Steps required:

1. Change partition type for these 3 partitions to 0x8e with fdisk. (see pvcreate(8): 0x8e identifies LVM partitions)

2. pvcreate /dev/sda3 /dev/sdb1 /dev/hda2

3. vgcreate test_vg /dev/sda3 /dev/sdb1 /dev/hda2

With our volume group "test_vg" now online, we can create logical volumes. For example a logical volume with a size of 100MB and standard name (/dev/test_vg/lvol1) and another one named "my_test_lv" with size 200MB striped (RAID0) across all the three physical volumes.

Steps required:

1. lvcreate -L 100 test_vg

2. lvcreate -L 200 -n my_test_lv -i 3 test_vg

Now let's rock and roll. For example create a file system with "mkfs -t ext2 /dev/test_vg/my_test_lv" and mount it with "mount /dev/test_vg/my_test_lv /usr1"

e2fsadm(8), lvchange(8), lvcreate(8), lvdisplay(8),

lvextend(8), lvmchange(8), lvmdiskscan(8),

lvmcreate_initrd(8), lvmsadc(8), lvmsar(8),

lvreduce(8), lvremove(8), lvrename(8),

lvscan(8), pvchange(8), pvcreate(8), pvdata(8),

pvdisplay(8), pvmove(8), pvscan(8), vgcfgbackup(8),

vgcfgrestore(8), vgchange(8), vgck(8), vgcreate(8),

vgdisplay(8), vgexport(8), vgextend(8), vgimport(8),

vgmerge(8), vgmknodes(8), vgreduce(8), vgremove(8),

vgrename(8), vgscan(8), vgsplit(8)

Heinz Mauelshagen <Linux-LVM@Sistina.com>