> If I understand mount(8) (http://man.openbsd.org/mount) right, FFS
> mounts have a metadata I/O mode and a data I/O mode. By default,
> metadata is accessed synchronously and data is accessed
> "-o sync" will force both to synchronous mode, and "-o softdep" would
> change the metadata I/O mode to the alternative softdep access mode.
No. softdep and async are different concepts.
Default: metadata, data accordingly: sync, async
not that there is async - not softdep
You can do the oposite by options: nosync, noasync
It would mean asynchronous mode for metadata and synchronous for data - which
is stupid (slow and dangerous), so don't.
I think default are quite good for SSD. Maybe add noatime for some partitions.