Last time I checked, multipathd does not and cannot detect faults on the write path as it only performs small reads to check the health of any given path. Checking writes would involve allocating space on the disk for multipathd to safely write to. Maybe someone has changed that in the past decade? I don't know as I'm not involved in anything SAN related anymore (and thank goodness for that!). SAN hardware is particularly awful as the underlying network is essentially hidden from the operating system most of the time. Storage subsystems built 30 years ago were built without any consideration that they might running on top of networks.

These and many other performance issues left me with a particular hatred of SANs.