VERITAS Volume Manager 3.1 Administrator's Guide

Volume Manager Operations
Dynamic Multipathing (DMP)
Chapter 3 125
maintenance of controllers attached to the host or a disk array supported
by the Volume Manager. I/O operations to the host I/O controller can be
turned on after the maintenance task is completed. This once again
enables I/Os to go through this controller. This operation can be
accomplished using the vxdmpadm(1M) utility provided with Volume
Manager.
For active/active type disk arrays, Volume Manager uses the balanced
path mechanism to schedule I/Os to a disk with multiple paths to it. As
a result, I/Os may go through any path at any given point in time.
For active/passive type disk arrays, I/Os will be scheduled by Volume
Manager to the primary path until a failure is encountered. Therefore, to
change an interface card on the disk array or a card on the host (when
possible) that is connected to the disk array, the I/O operations to the
host I/O controller(s) should be disabled. This allows all I/Os to be shifted
over to an active secondary path or an active primary path on another
I/O controller before the hardware is changed.
After the operation is over, the paths through these controller(s) can be
put back into action by using the enabled option of the vxdmpadm(1M)
command.
Volume Manager does not allow you to disable the last active
path to the rootdisk.
Displaying DMP Database Information
The vxdmpadm(1M) utility can be used to list DMP database information
and perform other administrative tasks. This command allows the user
to list all the controllers on the systems (connected to disks) and other
related information storedin the DMP database. This information can be
used to locate system hardware and also make a decision regarding
which controllers to enable/disable.
It also provides you with other useful information such as disk array
serial number and the list of DMP devices (disks) that are connected to
the disk array, the list of paths that go through a particular controllers,
etc.