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 –
LUN Based Trespass Mode This failover mode is the default and works in conjunction with the Auto-trespass feature. Auto-trespass is a mode of operation that is set on a LUN by LUN basis. If Auto-Trespass is enabled on the LUN, the non-owning SP will report that the LUN exists and is available for access. The LUN will trespass to the SP where the I/O request is sent. Every time the LUN is trespassed a Unit Attention message is recorded. If Auto-trespass is disabled, the non-owning SP will report that the LUN exists but it is not available for access. If an I/O request is sent to the non-owning SP, it is rejected and the LUN’s ownership will not change.
Note: 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. This combination is most commonly seen on an HP-UX server using PV-Links. The Auto-trespass feature is enabled through the Initiator Type setting of HP-AutoTrespass. A host with no failover software should use the combination of Failover Mode 0 and Auto-trespass disabled.
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.
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