The entire noSQL family of servers is based on relaxing the reliability constraints of the classic ACID protections that SQL databases provided.

Look at the comment you're replying to :-) In early Pick systems I believe it was possible for a single item to be larger than available memory ...

Okay, it laid the original systems wide open to serious problems if something went wrong, but as far as users were concerned Pick systems didn't have disk. It was just "permanent memory". And Pick was designed to "store all its data in ram and treat the disk as a huge virtual memory". I believe they usually got round any problem by flushing changes from core to disk as fast as possible, so in a crash they could just restore state from disk.

Cheers,
Wol