reads with 'available' readConcern will call setIgnorePrepared() in read_concern.cpp. For other storage engines, setIgnorePrepared() is not implemented, so a 'CommandNotSupported' exception will be thrown. A possible fix would be to make setIgnorePrepared a noop for other storage engines. Another would be to have a boolean called 'supportPrepare' for each storage engine and do setIgnorePrepared() only when it's true.