Failover modes in clariion

Beginners post!

Note: With release 26 of FLARE, regardless of the Failover Mode for a given host, when the SP that owns the LUN cannot access that LUN due to a back-end failure, I/O is redirected through the other SP by the lower redirector. In this situation, the LUN is trespassed by FLARE to the SP that can access the LUN. After the  failure is corrected, the LUN is trespassed back to the SP that previously owned the LUN.  See the “Enabler for masking back-end failures” section for more information.

Failover Mode 0 – LUN Based Trespass Mode

1. Default mode, works in conjunction with the Auto-trespass feature.
2. The combination of Failover Mode 0 and Auto-Trespass can be dangerous if the host is sending I/O requests to both SP-A and SP-B because the LUN will need to trespass to fulfill each request.



Failover Mode 1 – Passive Not Ready Mode

1. This mode is similar to Failover Mode 0 with Auto-Trespass disabled.
2. This mode is most commonly used with PowerPath.

Failover Mode 2 – DMP Mode

1. This is similar to Failover Mode 0 with Auto-trespass Enabled.
2. The difference between this mode and Auto-trespass mode is that Unit Attention messages are suppressed.
3. This mode is used for some VERITAS DMP configurations on some operating systems.

Failover Mode 3 – Passive Always Ready Mode

1.The non-owning SP will report that all non-owned LUNs exist and are available for access.  Any I/O requests sent to the Non-owning SP will be rejected.
2.This is similar to Failover Mode 1. However, any Test Unit Ready command sent from the server will return with a success message, even to the non-owning SP.

Failover mode 4 - Active/Active Asymmetric Logical Unit Access (ALUA)  Mode

1. Asymmetric Active/Active introduces a new initiator Failover Mode (Failover mode 4) where initiators are permitted to send I/O to a LUN regardless of which SP actually owns the LUN.
2.  When a I/O arrives from an ALUA initiator on the surviving SP, FLARE initiates an internal trespass operation. This operation changes ownership of the target LUN to the surviving SP since its peer SP is dead. Hence, the host (failover software) must have access to the secondary SP so that it can issue an I/O under these circumstances.