Failover Cluster and AlwaysOn Availability Groups across Data Centers
We'd like to introduce AlwaysOn Availability Groups from ClusterP to ClusterS. I've read multiple guides to this and there is one question I can't really answer. We do NOT have the ability to have cross-datacenter shared storage (no SAN mirroring across datacenters). With this setup, is it possible to setup AlwaysOn Availability Groups?


No, that is not possible.  Outside of a migration, an availability group cannot straddle multiple failover clusters (WSFC).

If you want to leverage an AG, you'll have to ensure that all participating replicas reside within the same WSFC cluster.  As for SQL Server failover cluster instances participating in the AG, that is not a problem.  A FCI can be an AG replica.

You can achieve a geographically dispersed cluster with a few different strategies.  If you need to have a witness vote (which, if you have three node clusters then that wouldn't help and you'd be best with a node majority quorum model) you could utilize a file share witness.

If what you're looking for is your availability group to span two data centers, then yes that is possible.  The only requirement for this situation would be a single WSFC cluster (which is possible to span multiple data centers).