User guide

Technical white paper | HP Enterprise Virtual Array Storage and VMware vSphere 4.x and 5.x configuration best practices
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When configuring HP EVA with iSCSI Modules, it must be noted that I/O will be routed slightly differently than with a Fibre
Channel configuration. With an HP EVA iSCSI configuration, I/O is still routed to the same controller that is the optimal path
to Vdisks 1-4 however the data flows through the iSCSI Module in the alternate controller before being routed back to the
LUNs owning controller. Figure 15 below illustrates this.
Figure 15. EVA iSCSI connectivity with vSphere 4.x/5.x
Paths 1 and 3 are the optimal paths to Vdisks 1-4. However path 3 has a connection through the iSCSI Module that is
embedded in controller B. Internal to the iSCSI Module, I/O coming through path 3 is routed to controller A.
Note
From the perspective of Vdisk access, you can easily balance vSphere 4.x with an EVA array. However, it may be desirable to
capture performance metrics at the EVA and assess data access to/from each controller to ensure that the workload is truly
balanced between the two controllers.
Using Fixed_AP I/O path policy
Because it is ALUA-aware, Fixed_AP I/O path policy can extend the functionality of Fixed I/O path policy to active-passive and
ALUA-aware arrays. In addition, Fixed_AP can identify the preferred controller for a Vdisk.
Despite being ALUA-aware, Fixed_AP’s primary path selection attribute is the preferred controller setting of a Vdisk and not
just its access state.
Note that the preferred controller for accessing a Vdisk in an ALUA-capable array is defined in SCSI by the PREF bit, which is
found in byte 0, bit 7 of the target port descriptor format
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. If the PREF bit is set to one, this indicates that the Vdisk is
preferred to the controller the target port group request was sent to; if it is set to zero, this indicates the Vdisk is not
preferred to the controller the target port group request was sent to. Thus, the controller preference in an EVA array is
equivalent to setting a Vdisk access path in Command View EVA to Path A/B-Failover/failback, as described in Vdisk
provisioning.
Primary use case
The primary use case for Fixed_AP is based on its ability to automatically return to a balanced, pre-configured environment
after Vdisk access has become unbalanced following an event such as a controller failure or restore. After a controller
failure, all Vdisks would migrate to the remaining controller and become optimized for that controller; however the logical
unit’s initial controller preference does not change and can point to the restored controller. Fixed_AP causes all Vdisks with
a preference for the restored controller to migrate back to that controller.
Consider the uses cases shown in Figure 16.
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For more information, refer to SCSI Primary Commands (SPC) standard.