ALUA --- Asymmetric Logic Unit Access
New failover mode on VNX: ALUA or Asymmetric Active/Active
(ALUA --- Asymmetric Logic Unit Access)
Based on ALUA(part of SCSI SPC-3 specification). In dual SP storage system, I/O can be routed through either SP. For example, if I/O for a LUN is sent to a SP who does not own the LUN, the SP will redirect I/O to the SP who does own the LUN. It is transparent to hosts and trespass is not required at all.
Dual SP storage system which supports ALUA defines two target port groups for each LUN. One target port group is defined for the SP currently owns the LUN, the other port group is defined for the SP who does not own the LUN.
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 usingPV-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(PNR) –
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 with offline errors on a Solaris server, SC_DISK_ERR2 errors with sense bytes 0102, 0700, and 0403 on an AIX server or buffer to I/O errors on a Linux server. If PowerPath is installed, these types of messages should not occur.
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(PAR) –
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, anyTest 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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http://blog.sina.com.cn/s/articlelist_2261389331_0_1.html