File Read Cache - How to purge?

[Hrvoje Niksic]

Nick Craig-Wood <nick at craig-wood.com> writes:

> If you are running linux > 2.6.18 then you can use
> /proc/sys/vm/drop_caches for exactly that purpose.
>
>   http://www.linuxinsight.com/proc_sys_vm_drop_caches.html

That URL claims that you need to run "sync" before dropping the cache,
and so do other resources.  I wonder if that means that dropping the
cache is unsafe on a running system.