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

To be added to a DR group, a virtual disk:
• Cannot be a member of another DR group
• Cannot be a snapshot
• Cannot be a mirrorclone
• Must be in a normal operational state
• Must use mirrored cache
CAUTION: Before replicating a Vraid0 source Vdisk or creating a Vraid0 remote copy, consider
the limitations of Vraid0. Although Vraid0 offers the most compact storage of your data, it carries
no data redundancy. If you select a Vraid0 source Vdisk, the failure of a single drive in its disk
group will cause the complete loss of its Vraid0 data, and replication from it will stop. If you choose
to create a Vraid0 destination Vdisk, the failure of a single drive in its disk group will cause the
complete loss of its Vraid0 data, and replication to it will stop.
Implicit LUN transition and HP P6000 Continuous Access
Implicit LUN transition automatically transfers management of a virtual disk to the array controller
receiving the most read requests for that virtual disk. This feature improves performance by reducing
the overhead incurred when servicing read I/Os on the non-managing controller. Implicit LUN
transition is enabled in VCS 4.xxx and all versions of XCS.
When creating a virtual disk, one of the array controllers is selected for managing the virtual disk.
Only the managing controller can issue I/Os to a virtual disk in response to host read and write
requests. If a read I/O request arrives on the non-managing controller, the read request must be
transferred to the managing controller for servicing. The managing controller issues the I/O request,
caches the read data, and mirrors that data to the cache on the non-managing controller, which
then transfers the read data to the host. Because this type of transaction, referred to as a proxy
read, requires additional overhead, it provides less than optimal performance. (There is little impact
on a write request as all writes are mirrored in both controllers’ caches for fault protection.)
With implicit LUN transition, if the array detects that a majority of read requests for a virtual disk
are proxy reads, management of the virtual disk will be transitioned to the non-managing controller.
This improves performance by making the controller that is receiving most of the read requests the
managing controller, which reduces the proxy read overhead for subsequent I/Os.
On XCS 6.100 and later, implicit LUN transition is disabled for all members of an HP P6000
Continuous Access DR group. Because HP P6000 Continuous Access requires that all members of
a DR group be managed by the same controller, it would be necessary to move all members of
the DR group if excessive proxy reads were detected on any virtual disk in the group. This would
impact performance and create a proxy read situation for the other virtual disks in the DR group.
Not implementing implicit LUN transition on a DR group does create the possibility that a virtual
disk in the DR group may be experiencing excessive proxy reads.
DR group name guideline
HP recommends that you assign unique names to DR groups. Duplicate names are supported for
DR groups on different arrays, but they are not supported for DR groups on the same array. DR
group names are case sensitive in HP P6000 Command View but are not case sensitive in HP
P6000 Replication Solutions Manager. In rare cases, this can lead to issues in HP P6000 Replication
Solutions Manager. For example, if you use HP P6000 Command View to create two DR groups,
DRgroupA and drgroupa on the same array, there will be no problem in HP P6000 Command
View; however, actions and jobs in HP P6000 Replication Solutions Manager involving either of
these DR groups may fail or result in unexpected behavior.
Planning DR groups 37