Since vSphere 4.0 there has been a large increase in the available alarms which not only come pre-configured but are also available to be created and you can pretty much now create an alarm for almost anything within vCenter.
It still surprises me though why some quite essential monitoring areas are not included within the default set of pre-configured alarms. One such alarm is the Storage Path Redundancy alarm will let you know when you have lost paths to your SAN storage and what datastores this will be affecting etc. This is a very simple alarm to setup but also pretty essential to virtually all vSphere implementations these days I'd imagine.
To set up the alarm select the vCenter server in the vSphere client and then go to the 'Alarms' tab.
Select 'Definitions' to see a list of all currently configured alarms and then right click in the section to create a new alarm.
Give the alarm a name ('Degraded Storage Paths' for example) and change the Monitor to 'Hosts' and then choose 'Monitor for specific events occurring on this object, for example, VM powered On'.
On the 'Triggers' tab click 'Add' and then change the Event type to 'Degraded Storage Path Redundancy'.
Next select the 'Actions' tab and Add an action to be performed when this event occurs. This can either be an email alert perhaps to the storage team or even a task for the ESXi host to perform.
Once set, click 'OK' and the alarm is set.
It's also worth creating another alarm to go along with this once which alerts when one of the ports goes offline too. That way you get notifications of path redundancy lost or a full port connectivity loss which will help in troubleshooting the issue being experienced.
To set this up, simply create another rule as above but this time set the trigger to be 'Lost Storage Path Redundancy' and set whatever actions you would like.
There are many other good alarms to set depending on what monitoring solutions you may or may not have in place for your virtual environment so its always good to have a look through the list of available alarms and just check that you have everything you need configured before you need it....they're not going to do that much if you've created them after the event!