Understanding ALUA Multipath Mechanism and Its Application in Dual‑Active Storage Systems

ALUA (Asymmetric Logical Unit Access) is defined in the SPC‑3 SCSI standard as a target port‑group (TPGS) access model that enables differentiated handling of Active/Active‑Asymmetric (A/A‑A) and Active/Passive (A/P) storage arrays, effectively serving as a front‑end controller multipath mechanism.

The primary storage front‑end multipath models are:

Active/Active‑Symmetric (A/A‑S): all controller ports are active/optimized, providing automatic load balancing without host‑side multipath software.

ALUA (Active/Active‑Asymmetric): only one controller’s port group is active/optimized while others are active/unoptimized, requiring host multipath software to direct I/O to the preferred controller.

Active/Passive (A/P): a single controller is active/optimized; other ports are standby, and I/O selection relies on host multipath policies.

In dual‑active storage deployments, ALUA allows the host to identify the optimal controller for each LUN, avoiding costly I/O redirection between data centers. Multipath software provides three core capabilities: path management to present a virtual device, I/O load‑balancing based on policies, and transparent failover/failback when paths or controllers fail.

ALUA distinguishes four target‑port states—Active/Optimized, Active/Unoptimized, Standby, and Unavailable—governed by the SPC‑3 standard, which dictate whether a LUN can be accessed through a given port at any moment.

ALUA implementations include Explicit ALUA (EALUA) and Implicit ALUA (IALUA). EALUA allows both querying and setting port‑group states, while IALUA only queries the asymmetric state and relies on the host to select the optimal path.

Native OS support varies:

Solaris 9/10 (STMS) supports implicit ALUA; Solaris 10 Update 3+ adds full ALUA.

HP‑UX 11’s NMP supports explicit ALUA with scsimgr configuration.

VMware ESX/ESXi (4.x+) includes native NMP plugins that support both explicit and implicit ALUA, using SATP and PSP modules for path selection.

macOS relies on HBA‑provided multipath drivers (e.g., ATTO MultiPath Director) that support both EALUA and IALUA, handling path states according to SCSI definitions.

Key ALUA functions include I/O path selection (preferring Active/Optimized paths), internal error handling by abstracting controller failures, and coordinated path and LUN failover/failback mechanisms that maintain performance and reliability.

In summary, enabling ALUA on storage arrays allows third‑party multipath software to identify the active controller for each LUN, direct I/O to Active/Optimized paths, and thus improve I/O performance and availability without requiring custom host multipath solutions.