HP StorageWorks SAN Virtualization Services Platform Manager user guide (AG781-96017, March 2009)

12 Working with Data Path Modules
A Data Path Module (DPM) is a device deployed in a split-path architecture that is responsible for the
data flow between the host application and the storage systems. The DPM is both a SCSI target that
presents virtual disks to the hosts and a SCSI initiator for routing all I/O from the virtual disks to their
physical destinations. A VSM fabric agent running on the DPM is responsible for getting metadata
from the VSM and providing it to the DPM.
Up to two DPMs present to the hosts any VSM storage elements, such as virtual disks or snapshots,
for which particular hosts have permission. If a virtual disk or snapshot is assigned to a host that is
communicating with two DPMs, the virtual disk or snapshot can be serviced by either DPM.
Every virtual disk or snapshot that can be serviced by two DPMs has a primary and a secondary DPM
defined within a DPM group. By default, the primary DPM in the DPM group services the virtual disk.
The secondary DPM in the DPM group can take over the servicing of the virtual disk if the primary
DPM fails. Failover between DPMs must be supported by multipath software installed on the host. See
the section in the product release notes for operating systems and their supported multipath drivers.
While failover takes place automatically in case of failure, you can force failover of all virtual disks
from a DPM within each DPM group. This enables you to perform maintenance on the DPM. After
failover, you can switch back virtual disks from their active secondary DPMs to their primary DPMs.
You can view the primary and secondary DPM definitions and the currently active DPM in the Data
Path Module/VSM tab for a virtual disk or snapshot (see
“Virtual disk information--Data Path Module tab” on page 105 and
“Snapshot information--Data Path Module tab” on page 209).
Primary and secondary DPM definitions are configurable per virtual disk (see
“Changing the primary and secondary DPM” on page 120) and a preferred primary DPM can be set
per host (overridden by a per virtual disk configuration) (see
“Changing the preferred path” on page 153). If neither configuration is performed, VSM distributes
virtual disks evenly between DPMs.
When you create a virtual disk group (VDG), all VDG members always reside on the same DPM. You
therefore configure the primary/secondary DPM setting per VDG and not per individual VDG member.
You can:
• View the DPMs list
• View DPM information
• Fail over virtual disks from a DPM
• Fail back virtual disks from a DPM
• Replacing a DPM
• Delete DPMs
Viewing the DPMs list
The DPM list displays all the DPMs exposed to the local SVSP domain.
To view the DPMs list:
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