What are the differences between failover modes on a CLARiiON array?（Diwak Arnitw, 2007）

What are the differences between failover modes on a CLARiiON array?
by Diwak Arnitw, storageknowledgeshare.blogspot.com, July 22, 2007
A CLARiiON array is an Active/Passive device and uses a LUNownership model. In other words, when a LUN is bound it has a defaultowner, either SP-A or SP-B. I/O requests traveling to a port SP-A canonly reach LUNs owned by SP-A and I/O requests traveling to a port onSP-B can only reach LUNs owned SP-B. It is necessary to have differentfailover methods because in certain situations a host will need toaccess 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 inconjunction with the Auto-trespass feature. Auto-trespass is a mode ofoperation that is set on a LUN by LUN basis. If Auto-Trespass isenabled on the LUN, the non-owning SP will report that the LUN existsand is available for access. The LUN will trespass to the SP where theI/O request is sent. Every time the LUN is trespassed a Unit Attentionmessage is recorded. If Auto-trespass is disabled, the non-owning SPwill report that the LUN exists but it is not available for access. Ifan I/O request is sent to the non-owning SP, it is rejected and theLUN’s ownership will not change.
Note:The combination of Failover Mode 0 and Auto-Trespass can be dangerousif the host is sending I/O requests to both SP-A and SP-B because theLUN will need to trespass to fulfill each request. This combination ismost commonly seen on an HP-UX server using PV-Links. The Auto-trespassfeature is enabled through the Initiator Type setting ofHP-AutoTrespass. A host with no failover software should use thecombination of Failover Mode 0 and Auto-trespass disabled.
Failover Mode 1（Passive Not Ready Mode）
In this mode of operation the non-owning SP willreport that all non-owned LUNs exist and are available for access. AnyI/O request that is made to the non-owning SP will be rejected. A TestUnit Ready (TUR) command sent to the non-owning SP will return with astatus of device not ready. This mode is similar to Failover Mode 0with Auto-Trespass disabled.
Note: This mode is most commonly used withPowerPath. To a host without PowerPath, and configured with FailoverMode 1, every passive path zoned, for example, a path to SP-B for a LUNowned by SP-A, will show to the server as Not Ready. This will show aswith offline errors on a Solaris server, SC_DISK_ERR2 errors with sensebytes 0102, 0700, and 0403 on an AIX server or buffer to I/O errors ona Linux server. If PowerPath is installed, these types of messagesshould not occur.
Failover Mode 2（DMP Mode）
In this mode ofoperation the non-owning SP will report that all non-owned LUNs existand are available for access. This is similar to Failover Mode 0 withAuto-trespass Enabled. Any I/O requests made to the non-owning SP willcause the LUN to be trespassed to the SP that is receiving the request.The difference between this mode and Auto-trespass mode is that UnitAttention messages are suppressed.
Note: This mode is used for someVeritas DMP configurations on some operating systems. Because of thesimilarities to Auto-Trespass, this mode has been known to cause“Trespass Storms.” If a server runs a script that probes all paths tothe Clariion, for instance format on a Solaris server, the LUN willtrespass to the non owning SP when the I/O request is sent there. Ifthis occurs for multiple LUNs, a significant amount of trespassing willoccur.
Failover Mode 3（ Passive Always Ready Mode）
In thismode of operation the non-owning SP will report that all non-owned LUNsexist and are available for access. Any I/O requests sent to theNon-owning SP will be rejected. This is similar to Failover Mode 1.However, any Test Unit Ready command sent from the server will returnwith a success message, even to the non-owning SP. Note: This mode isonly used on AIX servers under very specific configuration parametersand has been developed to better handle a CLARiiON non-disruptiveupgrade (NDU) when AIX servers are attached.