Mac OS X Fibre Channel connectivity to the HP StorageWorks Enterprise Virtual Array storage system configuration guide (5697-0025, August 2009)

LUN path information
Table 7 describes the information displayed on the LUN Paths tab.
Table 7 LUN paths information
DescriptionInformation type
Online—All paths are active and available for I/O.
Degraded—One or more paths are offline; however, I/O can continue on the remaining
active paths.
Offline—No paths are available. I/O and disk connectivity is lost.
Status
Pressure—The path with the fewest number of bytes being transferred is selected for
I/O. Pressure load balancing is domain based. The first level of path selection is based
on the domain (adapter channel) with the fewest number of bytes being transferred. If
there are multiple paths to a target on a domain, a second level of selection is used.
Pressure load balancing provides the best (or equal) performance of all load balancing
policies regardless of storage topology.
Queue Depth—The path with the fewest outstanding I/O transactions is selected for I/O.
HP recommends that this target based policy not be used in an environment with mixed
I/O transfers to many devices.
Round Robin—The least used path is selected for I/O. This target based policy uses the
configuration's cumulative I/O count for path selection. For example, if an I/O operation
starts and finishes on one path to a dual ported device, the second path is used when
the next I/O operation starts.
The round robin policy exercises all paths when the application queue depth to a target
is 1. In other words, it ensures that all hardware devices are used evenly. The other
policies select the same path repeatedly when the queue depth is 1.
Load Balance
The number of bytes transferred between the system and the LUN since the system was
booted.
Transfer Count
The WWPN of an EVA controller Fibre Channel port. The associated status icon displays
green for an active path or red if the path is offline.
Target Port
Preferred—The target port is currently managing a vdisk.
Alternate—The target port is currently not managing a vdisk.
The EVA has a pair of active/active controllers that present all LUNs from each controller’s
respective Fibre Channel ports. When you create a vdisk, you can specify which EVA
controller will manage the LUN to distribute vdisk I/O workload between the EVA
controllers.
For optimal performance, the host adapter driver determines which target port manages
the vdisk and automatically sets the read mode to Preferred. A target port that is not
managing the vdisk is set to Alternate. Read requests are serviced on the preferred path
only. Write operations use both preferred and alternate paths.
If a preferred target port fails, I/O reads continue on the alternate path. If a vdisk changes
controllers (for example, if a user changes presentation, or a managing controller fails
or reboots) the EVA notifies the host adapter and the managing EVA target ports are set
to Preferred.
Read Mode
The number of bytes transferred between the system and the LUN target port since the
system was booted.
Transferred
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