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Re: Primary vs Alternate Link

 
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Nazmal Kanji
Occasional Contributor

Primary vs Alternate Link

There are 2 FC10's on seperate controllers mirrorred to each other.
One FC10 is being used for primary and the other is the Alternate in the vg configuration.
Does it make any difference which FC10 contains the primary copy of the data and which contains the mirror? or more generally, Since LVM mirroring can read from either FC10 - How does HPUX decide which disk to utilize?

Any comments would be greatly appreciated.
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Patrick Wessel
Honored Contributor

Re: Primary vs Alternate Link

Nazmal,

Primary and Alternate link has nothing to do with MirrorUX. If you see a primary and an alternate link in your VG-configuration are there two physical links established to your FC10 (usually you have two Loop Controller Cards (LCC) installed). If the LVM is not able to access the disk drives using the primary link, it will switch to the alternate. This might be the case whe the fibre channel cable is broken or the FC host bus adapter fails.
The LVm uses the alternate link just if the request over the primary timed out
There is no good troubleshooting with bad data
Dave Wherry
Esteemed Contributor
Solution

Re: Primary vs Alternate Link

As Patrick said, your situation is mirroring, not primary and alternate links.
About the rest of your question, when using mirroring a write goes to the primary and then a second write goes to the mirror. In a write intensive environment you can pay a penalty for having to do two writes. The good news is that you have spread your primary and alternate on different controllers.
For reads, each disk has a queue for read requests. A new read request will be put on the shorter queue. So, in a read intensive environment you get a performance gain.