There's an even better reason for databases to not write to memory mapped pages: Pages get synched out to disk at the kernel's leisure. This can be ok for a cache but it's definitely not what you want for a database!
Yes, for almost all databases, although there was a cool paper from the University of Wisconsin Madison a few years ago that showed how to design something that could work without write barriers, and under the assumption that disks don't always fsync correctly:
"the No-Order File System (NoFS), a simple, lightweight file system that employs a novel technique called backpointer based consistency to provide crash consistency without ordering writes as they go to disk"
Does that generalize to databases? My understanding is that file systems are a restricted case of databases that don’t necessarily support all operations (eg transactions are smaller, can’t do arbitrary queries within a transaction, etc etc).
You can do write/sync/write/sync in order to achieve that. It would be nicer to have FUA support in system calls (or you can open the same file to two descriptors, one with O_SYNC and one without).