HP P6000 Continuous Access Implementation Guide (T3680-96431, August 2012)

Selecting a preferred controller
The following are important points to remember when selecting a preferred controller for DR group
members:
• The same preferred controller is used for all members of a DR group. The preferred controller
value used for each virtual disk is set automatically based on the value assigned to the first
DR group member. Ensure that the first member you add to the DR group is set to the desired
preferred controller.
• On XCS 6.200 or later and 09003000 or later, the preferred controller value used for the
DR group source virtual disks is automatically used for all destination virtual disks. On earlier
versions of controller software, you should manually set the DR group destination virtual disks
to the same preferred controller value used for the source.
• If the preferred controller value for the DR group source is set to No preference, different
preferred controllers may be used for the DR group source and destination.
• If the DR group source and destination use different preferred controllers, the DR relationship
will function properly but the load may not be balanced for optimal performance.
• Changing the preferred controller for an existing DR group member automatically changes
all DR group members to the new value. On XCS 6.200 or later and 09003000 or later,
changes made to the DR group source members are automatically propagated to the DR group
destination members. However, changes made to the DR group destination members are not
propagated to the source members.
NOTE:
• HP P6000 Command View refers to the preferred controller as Preferred path/mode and the
controller that currently owns the virtual disk as Managing Controller. The HP Storage System
Scripting Utility refers to the controller that currently owns the virtual disk as online controller.
• Setting the Preferred path/mode for a virtual disk enables transition of controller ownership.
To transition a virtual disk to the other controller, select the appropriate Preferred path/mode
for the controller Path A (or B)-Failover/failback and save the change. The presented virtual
disk will transition to the new controller after the change is saved. For the EVA4400 controller,
HP P6000 Command View displays Controller 1 or Controller 2 as the Managing Controller
for the selection of Path A or Path B respectively.
• Because of the effort required to transition a DR group and associated members between
controllers, HP recommends that the Preferred path/mode of a DR group member LUN only
be changed during light host loads. The Preferred path/mode can also be changed during a
copy or when a merge is in progress.
Using the failover/failback setting
The recommended setting for virtual disks that are members of a DR group is failover/failback.
This setting ensures that following a controller state change that results in a LUN transition, the
original managing controller regains ownership of the LUN. This behavior maintains the DR group
load balancing configured by the user. If the failover/failback setting is not used for DR group
member virtual disks, the user must manually transition the virtual disks back to the original managing
controller to re-establish DR group load balancing. HP Storage System Scripting Utility scripts can
also be created for rebalancing the DR groups.
The user must determine if the multipathing solution on the host the virtual disk is presented to has
the ability to react to Unit Attentions generated by a LUN transition. There are some instances of
LUN transition that do not generate a Unit Attention. The multipathing solution should provide a
mechanism to handle this occurrence. Consult the appropriate operating system multipathing
documentation to determine if this functionality is available.
Creating and presenting source virtual disks 97