Storage Multi-Pathing choices in HP-UX Serviceguard environments, August 2009

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Abstract
Serviceguard protects users and applications from a variety of failures (e. g. software, system, and
network failure to name a few) by forming a cluster of multiple nodes and providing sufficient
redundancy to remove single points of failure from the configuration. In general, Serviceguard directly
provides the failure protection while in specific areas it relies on other technology to add redundancy.
Highly available storage is usually provided by hardware mirroring with storage arrays or software
mirroring functionality of volume managers, while Serviceguard coordinates the access to the highly
available storage among the cluster nodes. When physically redundant hardware paths between the
cluster nodes and the storage devices are present, multi-pathing software provides protection against
path failure. The focus of this whitepaper is to provide the reader with an overview of what multi-
pathing solutions are available for various HP-UX Serviceguard environments and decision points for
choosing between them if more than one multi-pathing solution is feasible in a specific configuration.
Executive summary
Serviceguard clusters rely on multi-pathing software to protect against storage path failures. Either the
Operating System, as with HP-UX 11i v3 and Native Multi-Pathing, or additional software provides
this functionality. Customers deploying Serviceguard clusters on HP-UX have several choices to
configure systems that are resilient to storage path failures. Depending on the version of HP-UX –
either 11i v2 or 11i v3 – those choices are different.
HP-UX 11i v2 itself does not offer a built-in multi-pathing solution, so customers need to utilize
additional multi-pathing software that protects against storage path failures and often also provides
performance improvements by dynamically balancing the I/O load between the available paths.
These solutions are either volume manager based or storage platform specific:
Table 1.
Volume manager based Storage platform specific
LVM Alternate Links (PVlinks) HP StorageWorks SecurePath®
VxVM/CVM Dynamic Multipathing (DMP) EMC PowerPath®
These multi-pathing solutions can generally coexist on the same server and storage system, but multi-
pathing functionality to any individual LUN can only be provided by one solution.
HP-UX 11i v3 provides built-in native multi-pathing that generally makes add-on multi-pathing solutions
redundant.
 While LVM PVlinks are still supported with 11i v3, due to Native Multi-Pathing being bundled with
11i v3 it is no longer required nor recommended to configure LVM alternate links (PVLinks).
 DMP
– with VxVM 4.1 coexists with native multi-pathing; it acts in pass-through mode allowing HP-UX to
perform multi-pathing and load-balancing.
– with VxVM 5.0 coexists with native multi-pathing;
o will handle the multi-pathing by default (native multi-pathing is pass-through)
o can be configured in pass-through mode (native multi-pathing is active)
– CVM 5.0 volumes require DMP to handle multi-pathing (native multi-pathing is in pass-through
mode)
 HP add-on AutoPath® and SecurePath® products are no longer supported with HP-UX 11i v3.
 EMC PowerPath® is supported on HP-UX 11i v3 with restrictions.