July 09, 2015

Asymmetric Logical Unit Access (ALUA)

ALUA

“A storage controller manages the flow of data between the server and the LUN, assigning two paths, in case one of the paths becomes unavailable.”

An Active controller is available at all times. A passive controller sits idle until the active controller becomes unavailable.

A dictionary definition of asymmetric is “not identical on both sides of a central line”. An Asymmetric Logical Unit Access (ALUA) suggests unequal paths between the server to the LUN.
ALUA is implemented on active/active controllers. There are two types of active active controllers:
  • Asymmetric Active Active
  • Symmetric Active Active
In an Asymmetric Active/Active storage controller architecture (also known as ALUA compliant devices), there is a path to the LUN via either controller and both controllers are defined as “active”, however only one of the paths is defined as an optimal (direct) path. This controller is also referred to as the preferred controller.

IO requests arriving at the preferred controller are sent directly to the LUN. The other path is unoptimized (indirect) and is available only as a standby path, in case the optimized path becomes unavailable. IO requests arriving at the non-preferred controller are first forwarded to the preferred controller before being sent to the LUN.

In a Symmetric Active/Active storage controller architecture, there are no preferred or primary controllers.

ALUA
  • ALUA (Asymmetric Logical Unit Access)
  • Is a SCSI standard.
  • Typically implemented on mid-range storage arrays.
  • The LUN is reachable across both storage processors at the same time
  • All controllers are defined as “active”, however only one controller provides an optimal path to the LUN, this is the controller that owns the LUN
  • Rebalancing across the controllers as workloads change is a manual task
  • ALUA compliance required at the array and at the host multipathing layer
  • Multipathing software can query ALUA compliant arrays to load balance and failover
With ALUA, any given LUN is accessible via both storage processors however only one of these storage processors owns the LUN. This owning controller provides the optimal path to the LUN. The other controller provides the unoptimized path to the LUN. Paths for the non-owning storage processor transit data across the internal interconnect architecture of the mid-range arrays.

Terms to consider: Optimized vs. unoptimized paths; Direct vs. indirect path; Storage processor; Owned; asymmetric active-active architecture vs. symmetric active-active architecture.

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