User guide

Technical white paper | HP Enterprise Virtual Array Storage and VMware vSphere 4.x and 5.x configuration best practices

ALUA compliance

All EVA storage solutions – models P6x00, EVA8x00/6x00/4x00 – are dual-controller asymmetric active-active arrays that

are compliant with the SCSI ALUA standard for Vdisk access/failover and I/O processing.

Note

ALUA is part of the SCSI Primary Commands – 3 (SPC-3) standard.

While the active-active nature of the array allows I/O requests to a Vdisk to be serviced by either controller, the array’s

asymmetry forces the optimal access path to the Vdisk to be used (that is, the I/O path to the controller that requires less

processing overhead).

The controller with the optimal path to the Vdisk – managing controller – can issue I/Os directly to the Vdisk, whereas the

non-managing controller – proxy controller – can receive I/O requests but must pass them to the managing controller to

initiate fulfillment.

The following example shows how a read I/O request sent to the non-managing controller is processed:

1. The non-managing controller transfers the request (proxy) to the managing controller for the Vdisk.

2. The managing controller issues the I/O request to the Vdisk and caches the resulting data.

3. The managing controller then transfers the data to the non-managing controller, allowing the request to be fulfilled via

the controller/server ports through which the server initiated the request.

Thus, a proxy read – a read through the non-managing controller – generates processing overhead.

Note

Since write requests are automatically mirrored to both controllers’ caches for enhanced fault tolerance, they are not

affected by proxy processing overhead. Thus, the managing controller always has a copy of the write request in its local

cache and can process the request without the need for a proxy from the non-managing controller.

Vdisk controller ownership and access

The ability to identify and alter Vdisk controller ownership is defined by the ALUA standard.

EVA arrays support the following ALUA modes:

• Implicit ALUA mode (implicit transition) – The array can assign and change the managing controller for the Vdisk

• Explicit ALUA mode (explicit transition) – A host driver can set or change the managing controller for the Vdisk

EVA arrays also support the following ALUA access types:

• Active-Optimized (AO) – The path to the Vdisk is through the managing controller

• Active-Non-Optimized (ANO) – The path to the Vdisk is through the non-managing controller

ALUA compliance in vSphere 4.x and 5.x

ALUA compliance was one of the major features added to the vSphere 4 SCSI architecture and remains standard in vSphere

4.1 and 5.x. The hypervisor can detect whether a storage system is ALUA-capable; if so, the hypervisor can optimize I/O

processing and detect Vdisk failover between controllers.

vSphere 4.x and 5.x support all four ALUA modes:

• Not supported

• Implicit transitions

• Explicit transitions

• Both implicit and explicit transitions

In addition, vSphere 4.x supports all five ALUA access types:

• AO

• ANO