Despite its performance impact, serial mode is important for reliability. In fact, serial mode is the default, because it guards against failures that occur while a write is taking place.
Since serial mode waits until the first write is complete before starting the second write, it is impossible for a single failure to affect both disks. Specifying nonserial mode improves performance, but you risk losing data if a failure occurs that affects both writes.
WARNING! Unless you are sure that your mirrored database system does not need to be absolutely reliable, do not use nonserial mode.