It is if they’re in the same physical datacenter. Usually the way this is done is to wait for at least M replicas to fsync, but only require the data to be in memory for the rest. It smooths out the tail latencies, which are quite high for SSDs.

You can push the safety envelope a bit further and wait for your data to only be in memory in N separate fault domains. Yes, your favorite ultra-reliable cloud service may be doing this.