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