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SAN Performance Troubleshooting


OK, first of all, I'm not a SAN expert.  I hold the SNIA certification but that just means I have a rudimentary knowledge of the topic and/or am a good test taker.  Having said that, I want to tackle SAN performance troubleshooting from the DBA/Performance Engineer perspective. 
Whenever any performance problem pops up, first ask, what changed? 
Next, look for compatibility problems.  Not all hardware vendors interoperate and play nice with others' equipment.  Make sure your SAN guy checks this vigorously.  Else, get a list/inventory from your SAN guy of switch/firmware/hardware/software/etc.  Then do your own research on vendor websites.  They all have a support matrix. 
Next, look at capacity.  Almost all SAN vendors can look at a storage port and show %free/%busy.  The problem is this is difficult to monitor on ISLs (interswitch links).  Fan-in/fan-out could be the problem too, and you may never know.  You really need to know your environment. 
Could you have a cabling problem?  It's tough to diagnose this.  Take a look for anything obvious and consider swapping some cables when possible. 
Host configuration...lots can go wrong here.  HBA/firmware/q depth/etc etc etc.  Look here. 

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