lvm(7)lvm(7)NAMElvm - Logical Volume Manager (LVM)
DESCRIPTION
The Logical Volume Manager (LVM) is a subsystem for managing disk
space. The HP LVM subsystem offers value-added features, such as mir‐
roring (with the optional HP MirrorDisk/UX software), high availability
(with the optional HP Serviceguard software), and striping, that
enhance availability and performance.
Unlike earlier arrangements where disks were divided into fixed-sized
sections, LVM allows the user to consider the disks, also known as as a
pool (or volume) of data storage, consisting of equal-sized extents.
The size of an extent can vary from 1 MB to 256 MB.
An LVM system consists of arbitrary groupings of physical volumes,
organized into A volume group can consist of one or more physical vol‐
umes. There can be more than one volume group in the system. Once
created, the volume group, and not the disk, is the basic unit of data
storage. Thus, whereas earlier one would move disks from one system to
another, with LVM, one would move a volume group from one system to
another. For this reason it is often convenient to have multiple vol‐
ume groups on a system.
Volume groups can be subdivided into virtual disks, called A logical
volume can span a number of physical volumes or represent only a por‐
tion of one physical volume. The pool of disk space that is repre‐
sented by a volume group can be apportioned into logical volumes of
various sizes. The size of a logical volume is determined by its num‐
ber of extents. Once created, logical volumes can be treated just like
disk partitions. Logical volumes can be assigned to file systems, used
as swap or dump devices, or used for raw access.
Commands
LVM information can be created, displayed, and manipulated with the
following commands:
Change logical volume characteristics
Stripe, create logical volume in volume group
Display information about logical volumes
Increase space, increase mirrors for logical volume
Prepare logical volume to be root, primary swap, or dump volume
Display limits associated with a volume group version
Decrease number of physical extents allocated to logical volume
Remove one or more logical volumes from volume group
Remove logical volume link to root, primary swap, or dump volume
Change characteristics of physical volume in volume group
Create physical volume for use in volume group
Display information about physical volumes within volume group
Move allocated physical extents from one physical volume to
other physical volumes
Create or update volume group configuration backup file
Display or restore volume group configuration from backup file
Set volume group availability
Create volume group
Display information about volume groups
Export a volume group and its associated logical volumes
Extend a volume group by adding physical volumes
Import a volume group onto the system
Modify volume group attributes
Move data from old set of disks to a new set of disks
Remove physical volumes from a volume group
Remove volume group definition from the system
Scan physical volumes for volume groups
Migrate a volume group from one volume group version to another
The following commands are also available if the HP MirrorDisk/UX soft‐
ware is installed:
Merge two logical volumes into one logical volume
Split mirrored logical volume into two logical volumes
Synchronize stale mirrors in logical volumes
Synchronize stale logical volume mirrors in volume groups
Device Special Files
Starting with HP-UX 11i Version 3, the Mass Storage Stack supports two
naming conventions for the device special files used to identify
devices (see intro(7)). Devices can be represented using:
· Persistent device special files, or
· Legacy device special file names,
While LVM supports the use of both conventions within the same volume
group, the examples shown in the LVM man pages are all using the legacy
device special file convention.
Alternate Links (PVLinks)
In this release of HP-UX, LVM continues to support Alternate Links to a
device to allow continued access to the device, if the primary link
fails. This multiple link or multipath solution increases data avail‐
ability, but continues disallowing the use of multiple paths simultane‐
ously.
A new feature was introduced in the Mass Storage Subsystem on HP-UX 11i
Version 3 that supports multiple paths to a device and allows simulta‐
neous access to these paths. The Mass Storage Subsystem will balance
the I/O load across the valid paths. Multipathing is the default
unless the command is used to enable legacy multipathing and also the
active path is a legacy device special file. See scsimgr(1M) for
details.
Even though the Mass Storage Subsystem supports 32 multiple paths per
physical volume on this version of HP-UX, LVM does not support more
than eight paths to any physical volume. As a result, commands like
and will not succeed in adding more than eight paths per physical vol‐
ume. Additionally, and cannot write more than eight paths per physical
volume in the file. If users want to use any specific path other than
these eight paths, then they have to one of the alternate paths in the
volume group and add that specific path using
It is no longer required or recommended to configure LVM with alternate
links. However, it is possible to maintain the traditional LVM behav‐
ior. To do so, both of the following criteria must be met:
· Only the legacy device special file naming convention is used
in the volume group configuration.
· The command is used to enable the legacy multipath behavior
for each physical volume in the volume group.
LVM's Volume Group Versions 1.0, 2.0, and 2.1
LVM now has three different volume group version, and The original ver‐
sion of LVM volume group is 1.0. Versions 2.0 and 2.1 volume groups
allows LVM to increase many of the limits constraining the size of vol‐
ume groups, logical volumes, and so on. To see a comparison of limits
for volume groups version 1.0, 2.0, and 2.1, use the command (see
lvmadm(1M)).
The procedures and command syntax for managing volume groups version
1.0 is unchanged.
To take advantage of the improvements in volume groups version 2.0 or
higher, a volume group is declared to be version 2.0 or 2.1 at creation
time using the new option to the command.
The command will create the volume group directory and file if they do
not already exist. This is independent of the volume group version.
There are several differences in the procedure for creating a volume
group which is to be version 2.0 or higher.
· The volume group directory and group file will have a differ‐
ent major/minor number combination. See vgcreate(1M) for
details.
· It is no longer necessary to set maximums for physical vol‐
umes, logical volumes, or extents per physical volume.
Instead the command expects a maximum size for the volume
group. This size of a volume group is the sum of the user
data space on all physical volumes assigned to the volume
group.
· Extent size is now a required parameter. For volume groups
version 1.0, the default extent size is 4MB. For volume
groups version 2.0 or higher, extent size must be specified.
Volume group versions 2.0 and higher do not support root, boot, swap,
or dump. Additionally, volume groups version 2.0 or higher do not sup‐
port spare physical volumes.
The maximum number of 1.0 version volume groups per system is 256. The
maximum number of 2.0 version volume groups per system is 512. The
maximum combined 2.0 and 2.1 volume groups is 2048.
The vgversion(1M) command allows the migration between any two sup‐
ported volume group versions, with the exception of moving back to ver‐
sion 1.0.
Extent Sizing for Volume Group Version 2.0 and Higher
In volume groups version 1.0, LVM metadata is required to fit into a
single physical extent. If large values for maximum physical volumes,
logical volumes, and extents per physical volume were chosen, then a
large extent size is required.
In volume groups version 2.0 and higher, metadata is not restricted to
an extent. There is an implementation limit to the number of extents
in a volume group (see lvmadm(1M)), so the larger the extent size the
larger the maximum volume group size which can be supported. The
amount of space taken up on each physical volume by LVM metadata is
dependent on the physical extent size and the maximum volume group size
specified when the volume group is created. LVM metadata for volume
groups version 2.0 and higher may consume more space than on volume
groups version 1.0.
The command has a new option which will show the relationship between
extent size and maximum volume group size.
A smaller extent size allows finer granularity in assigning space to
logical volumes. It also means that smaller blocks of data are marked
stale when IOs to a mirror copy fail. For small logical and physical
volumes, a smaller extent size may result in less wasted space.
Since there are limits to the number of extents in a logical or physi‐
cal volume, a small extent size will limit the total size of a logical
or physical volume. Conversely a larger extent size allows creation of
larger logical volumes and use of larger physical volumes.
Auto Boot Disk Migration
This feature is intended to allow users to configure how LVM handles
situations where the physical location of the boot disk changes between
reboots. This situation can occur during hardware configuration
changes or if boot disk images are cloned. In those situations, Auto
Migration of Boot Disk will automatically update stale configuring
entries for the root volume group in LVM configuration file and the
Boot Data Reserved Areas for each bootable physical volume in the root
volume group. The configuration files are synchronized with the infor‐
mation from the kernel at the time of boot.
The Auto Boot Disk Migration feature (defined by the AUTO_BOOT_MIGRATE
flag in the file) is turned on by default on the system. When the fea‐
ture is turned on, any mismatch between the entries and the on-disk
metadata structures for the root volume group in the kernel will be
automatically fixed during the boot process.
The Auto Boot Disk Migration feature can be turned off by editing the
file and setting the flag AUTO_BOOT_MIGRATE to 0. In those situations,
users need to check the file post boot activity and follow the instruc‐
tions logged to the file, if any.
EXAMPLES
The basic steps to take to begin using LVM are as follows:
· Identify the disks to be used for LVM.
· Create an LVM data structure on each identified disk (see
pvcreate(1M)).
· Collect all the physical volumes to form a new volume group
(see vgcreate(1M)).
· Create logical volumes from the space in the volume group (see
lvcreate(1M)).
· Use each logical volume as if it were a disk section (create a
file system, or use for raw access).
To configure disk as part of a new volume groups version 1.0 named
First, initialize the disk for LVM with the command.
Then, create the pseudo device file that is used by the LVM subsystem.
The directory and file will be created automatically. Optionally,
these files can be created before doing the vgcreate, as follows:
The minor number for the file should be unique among all the volume
groups on the system. It has the format where NN ranges from to
Create the volume group, containing the physical volume, with the com‐
mand.
You can view information about the newly created volume group with the
command.
Create a logical volume of size 100 MB, named on this volume group with
the command.
This creates two device files for the logical volume, which is the
block device file, and which is the character (raw) device file.
You can view information about the newly created logical volume with
the command.
Any operation allowed on a disk partition is allowed on the logical
volume. Thus, you can use to hold a file system.
To use a volume group version or higher in the above example only few
changes are required.
The volume group directory and file are created automatically in all
supported versions.
The command would be changed. The following creates the volume group
with an extent size of 32 megabytes and a maximum volume group size of
32 terabytes (see vgcreate(1M)).
or
SEE ALSOlvchange(1M), lvcreate(1M), lvdisplay(1M), lvextend(1M), lvlnboot(1M),
lvmadm(1M), lvreduce(1M), lvremove(1M), lvrmboot(1M), pvchange(1M),
pvcreate(1M), pvdisplay(1M), pvmove(1M), vgcfgbackup(1M), vgcfgre‐
store(1M), vgchange(1M), vgcreate(1M), vgdisplay(1M), vgexport(1M),
vgextend(1M), vgimport(1M), vgmodify(1M), vgmove(1M), vgreduce(1M),
vgremove(1M), vgscan(1M), vgversion(1M), intro(7).
If HP MirrorDisk/UX is installed: lvmerge(1M), lvsplit(1M), lvsync(1M),
vgsync(1M).
If HP Serviceguard is installed: cmcheckconf(1M), cmquerycl(1M),
lvm(7)
