Ah, good old sync(2) :)

> ;;; Amazingly bogus syscall -- doesn't *actually* sync the filesys.
> (import-os-error-syscall sync-file-system () "scsh_sync")

> ;;; Amazingly bogus syscall -- doesn't *actually* sync the filesys.
> (import-lambda-definition-2 %sync-file-system (fd) "scsh_fsync")

Does anyone lurking here know what's the story with sync() now that SSDs
are gradually taking over?

IIRC sync() was always kind of a weird syscall because it basically
promises safety, but its behavior is so vague and complex that there's
no clear criteria for how applications should rely on that promise. In
practice, people resort to wishful thinking and do things like write a
hundred files calling sync() after each file, or calling sync() then
waiting a few seconds then calling sync() again to be sure. I've never
met anybody who can lay out a principled approach for how to use it.

The thing is, kernels typically sync() by themselves every 30-60 seconds
anyway, even if applications do not request it.