Administrator Guide

Metro node with active-passive storage
arrays
Topics:
• Active-passive array
• ALUA mode enabled array
• Logical Unit failover execution
• Logical Unit failback
Active-passive array
An active-passive array typically has two controllers and provides active-passive access to a Logical Unit (LU) through a set
of target ports. The access types of these ports are Active (ACT) or Passive (PAS). Active is used for I/O and passive cannot
be used for I/O. When active paths to logical units are lost, the initiator (metro node) can decide to activate passive paths to
perform I/Os by sending vendor specific SCSI commands to the array.
The controller with the active target ports for a specific logical unit is referred to as Active (ACT) controller of that logical unit.
The controller with passive target ports for a specific logical unit is referred to as Passive (PAS) controller of that logical unit.
The controller which is active for a logical unit can be a passive controller for some other logical unit and vice versa.
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.
Logical Unit failover execution
When the logical unit is accessible through all the paths, the active controller becomes the preferred controller, and when no
active paths are available, the passive controller becomes the preferred controller. The failover of the logical unit is triggered
by the master director in the metro node cluster, when the preferred controller is not its active controller. The master director
in the cluster initiates the logical unit failover by sending vendor-specific SCSI commands to the target device to change the
access state of the logical unit. Based on the response received from the target device for the command, the logical unit
failover either succeeds or fails.
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112 Metro node with active-passive storage arrays