ESXi Multipathing Decoded: MRU, Fixed, and Round Robin
When you present a LUN to an ESXi host, the Native Multipathing (NMP) engine automatically assigns a policy based on the type of storage array detected. However, as an admin, you need to understand why a policy was chosen—and when you should manually intervene.
1. Most Recently Used (MRU)
Best For: Active/Passive Arrays. MRU selects the first working path it finds at boot. If that path fails, it switches to a standby path.
- Key Behavior: It does not fail back. Even if the original path becomes healthy again, the host stays on the current path. This prevents “path thrashing” on Active/Passive arrays where switching controllers is an expensive operation.
2. Fixed
Best For: Active/Active Arrays. The Fixed policy uses a specific “Preferred Path.” If the preferred path fails, it moves to an alternative.
- Key Behavior: It does fail back. As soon as that designated preferred path is back online, the host immediately switches back to it.
3. Round Robin (RR)
Best For: Load Balancing (Active/Active or ALUA). Round Robin rotates through all available paths to distribute the I/O load.
- Active/Active: Uses every available path.
- Active/Passive: Only uses all paths leading to the active controller.
Note: For Microsoft Failover Clusters (MSCS), Round Robin is only supported on ESXi 5.5 and later.
4. Fixed with Array Preference (FIXED_AP)
Introduced in ESXi 4.1 for ALUA-capable arrays, this policy lets the storage array tell the host which path is the “optimal” one.
- Note: This was removed in ESXi 5.0 in favor of letting the NMP automatically select MRU or Fixed based on the array’s ALUA response.
⚠️ Critical Warnings for Admins
- Don’t Fight the NMP: VMware generally warns against manually changing a LUN from Fixed to MRU. The host chooses the policy based on the hardware it detects; forcing a change can lead to instability.
- Verify Vendor Support: Round Robin is powerful but not supported by every array. Always check the VMware Compatibility Guide before making it your default.
- MSCS Limitations: If you are virtualizing SQL clusters or other failover clusters, double-check your ESXi version before toggling Round Robin, or you risk losing disk heartbeat connectivity.
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