VERITAS Volume Manager 3.1 Administrator's Guide

Volume Manager Operations
Dynamic Multipathing (DMP)
Chapter 3 123
Load Balancing
To provide load balancing across paths, DMP follows the balanced path
mechanism for active/active disk arrays. Load balancing makes sure
that I/O throughput can be increased by utilizing the full bandwidth of
all paths to the maximum.
Sequential IOs starting within a certain range will be sent down the
same path in order to optimize IO throughput by utilizing the effect of
disk track caches. However, large sequential IOs that do not fall within
this range will be distributed across paths to take the advantage of IO
load balancing. For active/passive disk arrays, I/Os are sent down the
primary path until it fails. Once the primary path fails, I/Os are then
switched over to the other available primary paths or secondary paths.
Load balancing across paths is not done for active/passive disk arrays to
avoid the continuous transfer of ownership of LUNs from one controller
to another, thus resulting in severe I/O slowdown.
Disabling and Enabling DMP
NOTE After either disabling or enabling DMP, you must reboot the system for
the changes to take effect.
Disabling DMP on an HP System: Step 1.Stop the vxconfigd daemon using the
following command:
# vxdctl stop
Step 2. Save the /stand/system file as /stand/system.vxdmp by executing
the command:
# cp /stand/system /stand/system.vxdmp
Step 3. Save the /stand/vmunix file as /stand/vmunix.vxdmp by executing
the command:
# cp /stand/vmunix /stand/vmunix.vxdmp
Step 4. Edit the /stand/system file and remove the vxdmp entry.