What are the differences between failover modes on a CLARiiON array?
A CLARiiON array is an Active/Passive device and uses a LUN ownership model. In other words, when a LUN is bound it has a default owner, either SP-A or SP-B. I/O requests traveling to a port SP-A can only reach LUNs owned by SP-A and I/O requests traveling to a port on SP-B can only reach LUNs owned SP-B. It is necessary to have different failover methods because in certain situations a host will need to access a LUN on the non-owning SP.
The following failover modes apply:
Failover Mode 0 –
Failover Mode 1 – Passive Not Ready Mode In this mode of operation the non-owning SP will report that all non-owned LUNs exist and are available for access. Any I/O request that is made to the non-owning SP will be rejected. A Test Unit Ready (TUR) command sent to the non-owning SP will return with a status of device not ready. This mode is similar to Failover Mode 0 with Auto-Trespass disabled. Note: This mode is most commonly used with PowerPath. To a host without PowerPath, and configured with Failover Mode 1, every passive path zoned, for example, a path to SP-B for a LUN owned by SP-A, will show to the server as Not Ready. This will show as
Failover Mode 2 – DMP Mode In this mode of operation the non-owning SP will report that all non-owned LUNs exist and are available for access. This is similar to Failover Mode 0 with Auto-trespass Enabled. Any I/O requests made to the non-owning SP will cause the LUN to be trespassed to the SP that is receiving the request. The difference between this mode and Auto-trespass mode is that Unit Attention messages are suppressed. Note: This mode is used for some Veritas DMP configurations on some operating systems. Because of the similarities to Auto-Trespass, this mode has been known to cause “Trespass Storms.” If a server runs a script that probes all paths to the Clariion, for instance format on a Solaris server, the LUN will trespass to the non owning SP when the I/O request is sent there. If this occurs for multiple LUNs, a significant amount of trespassing will occur.
Failover Mode 3 – Passive Always Ready Mode In this mode of operation 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. 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. Note: This mode is only used on AIX servers under very specific configuration parameters and has been developed to better handle a CLARiiON non-disruptive upgrade (NDU) when AIX servers are attached.