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

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Active/Passive Storage Systems
With Active/Passive storage systems, one controller is assigned to a LUN as primary controller (owner
of the LUN) and handles all the I/O requests to it. The other controller – or multiple other controllers, if
available – acts as standby controller. The standby controller of a LUN only issues I/O requests to it,
if their primary controller failed. To achieve load balancing it makes sense to distribute the LUNs
evenly among the controllers. In the configuration example above, each of the two controllers would
“own” two LUNs.
Most active/passive storage systems require a controller failover to switch between controllers. A
failover can be caused by simply accessing a LUN via the controller that is not currently the owner.
This is not a big issue for clusters which use LVM or VxVM with exclusive activation by only one node
at a time. However for concurrent access from multiple nodes with SLVM, CVM or CFS, all nodes must
coordinate primary LUN ownership so they use the same controller for the same LUNs. HP SecurePath
does this coordination for EVA active/passive models (EVA 3000 [HSV100] and EVA 5000
[HSV110]). EMC PowerPath coordinates primary LUN ownership among the cluster nodes for
CLARiiON arrays.
Active/Active Storage Systems
With Active/Active storage systems, multiple controllers can issue I/O requests to an individual LUN
concurrently. This enables multi-pathing software to provide load balancing between the paths in
addition to path failure protection. The multi-pathing solutions offer different load balancing policies.
Depending on the type of array and specific I/O pattern of the application, users can choose an
appropriate policy.
Active/Active Storage Systems further divide into two categories:
 Asymmetrical Active/Active (AAA) storage systems;
With theses type of arrays, one controller is assigned to each LUN as a preferred controller. Each
controller can have multiple ports – our example above showed 2 ports per controller. I/O requests
reaching the storage system through ports of the preferred controller of a LUN will be sent directly
to the LUN. I/O requests arriving at the non-preferred controller of a LUN will be first forwarded to
the preferred controller of the LUN. These arrays are also called Asymmetrical Logical Unit Access
(ALUA) compliant devices.
The EVA 4x00/6x00/8x00 and the EVA3000/5000 (HSV101/111) with active/active firmware
are ALUA compliant arrays. Multi-pathing software can query ALUA compliant arrays to load
balance only between paths connected to the preferred controller and use the paths to the non-
preferred controller for automatic path failover if all of the paths to the primary controller fail.
Many of the EMC CLARiiON arrays have new firmware releases that provide support for ALUA
with these arrays. HP-UX 11i v3 update 1 (0709) integrated ALUA support into the mass storage
stack so that it can natively manage multi-pathing and optimize load balancing for ALUA compliant
arrays, HP and non-HP.
 Symmetrical Active/Active (SAA) storage systems;
These type of arrays do not have a primary or preferred controller per LUN. I/O requests can be
issued over all paths mapped to a LUN. The current models of the HP StorageWorks XP Disk Array
family are symmetrical active/active arrays.
Most storage systems implement a read-ahead mechanism to improve sequential read performance by
efficiently using the array’s cache and internal components. Key for those read-ahead mechanisms is
to successfully identify sequential read operations. If the disk array recognizes sequential access
patterns and reads ahead, subsequent requests may be satisfied from data pre-read into cache.