HP HP-UX 11i Administrator'S Manual Download

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Increasing Hardware Path Redundancy Through Multipathing

Your hardware might provide the capability for dual cabling (dual controllers) to the same physical
volume. If so, LVM can be configured with multiple paths to the same physical volume. If the primary
link fails, an automatic switch to an alternate link occurs. Using this type of multipathing (also called
pvlinks) increases availability.

NOTE:

As of HP-UX 11i Version 3, the mass storage stack supports native multipathing without using LVM
pvlinks. Native multipathing provides more load balancing algorithms and path management
options than LVM. HP recommends using native multipathing to manage multipathed devices instead
of using LVM's alternate links.

HP recommends converting volume groups with alternate links to use native multipathing by the
use of persistent DSFs. The

/usr/contrib/bin/vgdsf

script,

vgscan -N

command, or

vgimport -s -N

commands perform this conversion.

For backward compatibility, you can use existing pvlinks. However, you must use legacy device
special files for physical volumes and disable native multipathing for those legacy device special
files using the

scsimgr

command. For more information, see the white paper LVM Migration from

Legacy to Agile Naming Model, available at http://www.hp.com/go/hpux-core-docs.

Setting Up Multipathing to a Physical Volume (LVM PVlinks)

To use an alternate link, you can create a volume group with

vgcreate

, specifying both the

primary link and the alternate link device file names. Both links must represent paths to the same
physical volume. (Do not run

pvcreate

on the alternate link; it must already be the same physical

volume as the primary link.) When you indicate two device file names, both referring to the same
disk using

vgcreate

, LVM configures the first one as the primary link and the second one as the

alternate link.

For example, if a disk has two cables and you want to make one the primary link and the other
an alternate link, enter the following command:

# vgcreate /dev/vg01 /dev/dsk/c3t0d0 /dev/dsk/c5t0d0

To add an alternate link to a physical volume that is already part of a volume group, use

vgextend

to indicate the new link to the physical volume. For example, if

/dev/dsk/c2t0d0

is already

part of your volume group but you want to add another connection to the physical volume, enter
the following command:

# vgextend /dev/vg02 /dev/dsk/c4t0d0

If the primary link fails, LVM automatically switches from the primary controller to the alternate
controller. However, you can also tell LVM to switch to a different controller at any time using the

pvchange

command. For example:

# pvchange -s /dev/dsk/c2t1d0

After the primary link has recovered, LVM automatically switches back from the alternate controller
to the original controller unless you previously instructed it not to by using

pvchange

as follows:

# pvchange -S n /dev/dsk/c2t2d0

NOTE:

You can also disable the automatic switchback by using the

-p

option to

pvchange

disable proactive polling. For more information, see pvchange(1M).

View the current links to a physical volume using

vgdisplay

with the