Dell EMC Metro node 7.0.1 Administrator Guide

ALUA mode enabled array

An Asymmetric Logical Unit Access (ALUA) mode enabled storage array provides active/active access to a logical unit through all the target ports. Based on their bandwidth, these ports are sorted into preferred and non-preferred Target Port Groups (TPG). The higher bandwidth preferred target ports have access state of Active/Active Optimized (AAO) while non-preferred target ports have access state of Active/Non-Optimized (AAN). In the absence of AAO paths, I/Os continue on AAN paths.

The controller with preferred target ports for a specific logical unit, is referred to as Active/Active-Optimized (AAO) controller of that logical unit while the controller with non-preferred target ports for a specific logical unit, is referred to as Active/Non-Optimized (AAN) controller of that logical unit. The controller which is AAO for a logical unit can be a AAN controller for some other logical unit and vice versa. For the purpose of ALUA enabled logical unit failover processing, ALUA access state of Active/Active-Optimized (AAO) equates to an Active (ACT) path and Active/Active-NonOptimized (AAN) equates to a Passive (PAS) path, internally.

Targets advertise their support for ALUA on a per logical unit basis through a standard inquiry response. There are three different modes of operation :

Implicit ALUA The target device can independently change the logical unit access states internally.

Explicit ALUA The target device requires an initiator to change the logical unit access states by sending specific SCSI commands, when necessary.

Implicit-Explicit ALUA Has the advantage of both implicit and explicit ALUA. Targets might support implicit ALUA, explicit ALUA, or implicit-explicit ALUA.