Have you been receiving Storage Disconnects alarms from your vCenter quite often? Here are some quick commands to see if there are any dead paths.
esxcfg-mpath -l | grep -i dead
Following command does the same thing but lists extra information
esxcfg-mpath -b | grep -i dead
If resulting output contains dead paths and you want to be sure that it's not from some earlier known issues in your environment, you can run the following command to rescan all your targets
esxcfg-rescan vmhbaXX , where XX is the last two digits your HBA under Storage Adapters in vCenter.
Run esxcfg-mpath -l | grep -i dead or esxcfg-mpath -b | grep -i dead again and see if it results in any dead paths, if it does then there may be some serious connectivity or storage issues with your SAN.
Use vmkernel ping to verify you can ping all your iSCSI targets.
vmkping 192.168.x.x, where 192.168.x.x is the IP of target IP of your SAN.
For Fiber Channel, make sure your SAN software sees all FC HBAs as registered and that are no zoning issues.