Understanding ALUA Multipathing Mechanism for Dual‑Active Storage Systems

ALUA (Asymmetric Logical Unit Access) is a SCSI SPC‑3 defined target port group (TPGS) access model used to manage multi‑port access for Active/Active‑Asymmetric (A/A‑A) and Active/Passive (A/P) storage arrays, effectively serving as a front‑end controller multipathing mechanism.

Storage front‑end multipathing can be categorized into three types: A/A‑Symmetric (all ports active/optimized), ALUA (one port active/optimized, others active/unoptimized), and A/P (only one active/optimized port with others standby). ALUA requires both the multipathing software and the storage array to support the model for optimal path selection.

In dual‑active environments, ALUA enables I/O to be directed only to the preferred controller (Active/Optimized), avoiding latency caused by cross‑datacenter traffic. Path management presents a virtual SCSI device to the host, supports load balancing, and provides transparent failover and failback.

Key ALUA path states are Active/Optimized, Active/Unoptimized, Standby, and Unavailable, each defining the controller’s ability to process commands. Both explicit (EALUA) and implicit (IALUA) modes exist, with explicit allowing configuration of port group states and implicit providing read‑only status.

Major operating systems’ native multipathing support for ALUA varies: Solaris (STMS) supports implicit ALUA; HP‑UX (NMP) supports explicit ALUA with scsimgr management; VMware ESX/ESXi (NMP) supports both explicit and implicit ALUA with configurable path selection plugins; macOS uses ATTO MultiPath Director supporting both IALUA and EALUA.

ALUA’s primary capabilities include I/O path selection (favoring Active/Optimized), internal error handling to hide controller failures, and robust path and controller failover/failback mechanisms, thereby improving I/O performance and reliability in storage arrays that implement the standard.

In summary, enabling ALUA allows third‑party multipathing software to recognize the active controller of a LUN, direct I/O to the optimal path, and achieve higher performance and availability without requiring custom multipathing solutions.