Because RDS is built on EBS, any slight issue with EBS manifests itself as a nastier issue for RDS. Interestingly, EBS will never return an I/O error up to the

Because RDS is built on EBS, any slight issue with EBS manifests itself as a nastier issue for RDS. Interestingly, EBS will never return an I/O error up to the attached OS, which is likely a good decision as most OSes choke on disk errors. What this means, however, is that if something even get slow within EBS (let alone stuck), applications that are dependent on it will suffer. Most of these applications (such as databases) have connection/response timeouts for their clients, so while EBS might just be running slowly, a service like RDS will throw up connection errors instead of waiting even a bit more. You can imagine the cascading errors that might result from such a situation (instance looks dead, start failover...etc)