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Successfully addressing these challenges is imperative if you wish to maximize the return on

investment (ROI) for your SAN while continuing to meet the changing needs of the business. To help

you achieve these goals, this paper presents best practices for configuring, tuning, and deploying a

vSphere SAN environment.

Key concepts and features

This section introduces key concepts and features associated with the successful configuration, tuning,

and deployment of a vSphere SAN environment. These include Asymmetric Logical Unit Access

(ALUA) compliance, virtual disk (Vdisk) ownership and access, and Vdisk follow-up.

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 they are automatically mirrored to both controllers’ caches for

enhanced fault tolerance, write requests 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

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