[sepin4]

While a SAN/NAS is overall an expensive hardware in both price and manageability, there are entry level devices. You can even get direct SCSI devices in some cloud services(Azure has "shared disks"). For Oracle RAC the data is accessed from same location, it's not distributed/replicated across nodes. It's literally the same disks attached to all nodes. Also ASM works with raw block devices which bypasses the need for file system abstraction and all the latency that comes with it. You could also deploy a cluster on a cluster/network file system, but this would generally not be a good idea since you get a lot of overhead.

Postgre clusters can be configured in similar fashion.

I'm not saying that Mongo doesn't have it's strengths. However using the full potential of your environment requires much more planing and has a lot more pitfalls.

I was particularly discussing the implications on what the relative immutability of the shard key has on the data distribution and that it is not that easy to change.