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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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Mpio DMG Manuel de service Page 34

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