All Virtual Servers Are in Read-Only Mode or Showing IO Errors

Question

The virtual servers on one of my data stores are getting IO errors when trying to start. Any virtual servers, which are still up, are in read-only mode. 

Answer

Such an error appears often when there is a connection issue with the SAN. Checking the status of ISCSI sessions or whichever storage you are using reveal the source of the problem.

Use /var/log/messages on your compute resources to find out the cause of the issue.