AlwaysOn is actually a marketing term which covers Failover Cluster Instances (FCIs) and Availability Groups (AGs). Allan Hirt (@sqlha | blog) is a strong proponent of ensuring that people understand what this actually means. So much so that he even ranted about it a little.
I’ve used FCIs for years, going back to the active/passive clustering days of old, and I’ve used Availability Groups in the last few months. They are both great, and both have limitations: FCIs with their shared storage and AGs with some network and quorum oddities.
Both of them will do a fine job for you if you have the time, patience, and in the case of AGs, money to get them up and running. They still will not allow you to meet your RPO/RTO though.
Critical to your business and your users is your up time, and that’s where the Recovery Time Objective (RTO) and Recovery Point Objective (RPO) come into play. They reflect amount of time it will take to get your services back up and running, as well as the level of data loss that you are willing to accept.
Where FCI/AG win
The key problem with FCI/AG is that they do everything that they can to ensure that transactions are kept as up to date as possible.
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