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April 4th, 2012 19:00
do we have a standard definition for Failover Mode ?
Hi all,
We know that different hosts might have different Failover Mode settings with the storage array. But do we have a a standard definition for Failover Mode to better understand how does it work?
thanks!
AnkitMehta
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April 5th, 2012 10:00
In simple term, we may consider Failover refer to load balancing. 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. Hence, the concept of Failover Mode.
There are different different failover modes, which are explained in detail in Primus emc157830.
zhouzengchao
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April 5th, 2012 18:00
thank you, AnkitM. This doc is great!
AnkitMehta
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April 5th, 2012 19:00
A CLARiiON array can be set up to perform Automatic Trespass operations. Change the system parameters through Navisphere to enable this feature. When auto-trespass is enabled on a unit, a Trespass command automatically occurs whenever the storage processor (SP) that does NOT currently have it, accesses the unit assigned. This behaves exactly as if the SP had actually received a Trespass command. This feature allows a system to have high-availability access to the disk units without implementing the Trespass command.
What I mean here is auto-trespass can be set from the Storage side. However, a host can request a LUN trespass in different different scenario like load balancing, que length is high, connectivity issue from the host. (One path is down to any SP).
For More information: Refer Primus emc166260.
JFYI:
Not just that, if the array has M/V configured, for eg. if a LUN on Primary SP trespasses in that case a LUN on Secondary will trespass however, neither SP asked nor Host Requested but only because of 1 Rule of Mirrorview that Soruce and the target LUN should be owned by the same SP.
If this has answered your query. Please remember to mark your question Answered.