how do I tell FreeBSD to sync, for real ?

[user]

Sometimes you delete a large batch of files, or you do some other serious
FS operations and the output of `df` does not tell you immediately of the
new disk space, etc.

If you run something like:  sync

Or even:  sync ; sync

it still doesn't show up.  You either have to wait for a while, or you
have to get silly with something like:

sync ; sync ; sync ; sync ; sync ; sync ; sync

(or unmount and remount the FS)

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So my question is, well first off, why does this happen this way ?  But
mostly what I want to know is, is there a more graceful way to tell
FreeBSD "sync for real this time, not just for joke only ?"

Basically, is there a nicer way to get what I want than:

sync ; sync ; sync ; sync ; sync ; sync ; sync

thx.