HP StorageWorks SAN Virtualization Services Platform Best Practices Guide (5697-0935, May 2011)

In the SVSP GUI, there are two personality options used with Create VMware Host:
1. OS Type: VMWARE
2. Personality: HP-EVA 3000 or ALUA
If you select HP-EVA 3000, the SVSP performs as an active/passive array with product ID HSV100.
Active/passive means a LUN can only be operational from one DPM or another, but not both.
Using this personality, the Target Port Group Support (TPGS) bit, which indicates whether this array
adheres to Asymmetric Logical Unit Access (ALUA) specification, is not set (reference SCSI standard
SPC-3).
The ALUA personality has product ID HSVX700. The SVSP firmware is not active/active, so this
personality changes the product ID and the TPGS bit is turned on. A host can now use the ALUA
specification to access the LUN. Read the SPC-3 specification to clearly understand what ALUA
does. This provides an advantage of using ALUA defined transitions to move the LUN from one
state to another, and also a bit defined as the PREF BIT. For example, an ALUA-compliant host
such as ESX4.0 and ESX4.1 can acquire a LUN for its ALUA access states and more intelligently
access the LUN through its optimal path.
CAUTION: After choosing one of these two personalities for a specific server, or set of servers,
and once the LUNs have been presented to these servers, do not change the personality for those
hosts after creating ESX datastores. This will cause all of the datastores (LUNs with VMFS) on these
ESX servers to be deemed as snapshots and no longer accessible by your ESX server until they are
either re-signatured or force mounted. The personality change may be backed out if a re-signature
or force mount is not an option at the time. Each situation may be different, therefore the impact
of force mount or re-signature should be evaluated. If you do want to proceed with resignaturing
or force mount, be sure you understand the impact.
HP recommends using the ALUA AP (HP-EVA 3000 or ALUA) personality for a new installation
and remaining on the previous personality for customer using SVSP 3.0.2 and earlier (even round
robin MPIO may not be worth the re-signature/force mount issues on a system running well. One
reason you may want to change is that with the ALUA personality, ESX can benefit from using
round robin load balancing and queue I/O to more than one DPM port for each LUN. The increased
host port bandwidth for each LUN is beneficial.
The following describes various load balancing options available with ESX:
• MRU—With the OS type option, only a single path is used for I/O to a LUN. If the path fails,
the Most Recently Used (MRU) attempts to find another path for the I/O. If the initial path is
restored, the MRU will not move back to the initial path.
With the HP-EVA 3000 or ALUA personality, when a path fails, the MRU attempts to find an
active/optimized path as the next choice for I/O queuing, and only if the MRU cannot find
a path would it use another path. If the failed path is restored, the MRU will remain on the
initial path.
• Fixed—With either personality option, Fixed sends I/O down a preferred path. When that
path is unavailable, it will fail over to the next available path. If the failed path is restored,
Fixed will fail back to its preferred path. Fixed has no understanding of ALUA and ignores it.
• Round Robin—Round Robin sends I/O to all active/optimized paths in a round robin fashion.
If a path fails, I/O continues to be sent down all the remaining active ports. When the failed
path is restored, I/O is added back to the round robin. Round Robin is ALUA-compliant, and
by default will only send I/O down the active/optimized path. For an array that is not
ALUA-compliant (using the VMware OS type AP personality), Round Robin may try and send
I/O down all available paths regardless if they are active or passive, and I/O errors would
occur from the passive paths. The errors are because without ALUA, Round Robin has no sense
of where to route I/O. Using Round Robin is desired with HP-EVA3000 ALUA to get increased
target port bandwidth of the I/O processing.
Operating system and application considerations 35