Some time ago I reported that after I deleted
some large amount of space from one of my xfs volumes,
'df' still showed the original, pre-delete space, though
'du' only showed the expected amount.
Mentioned that I had tried 'sync' to no avail, and had
only found umount/mount to cause the figures to synchronize.
Someone suggested cat [1|3] >/proc/sys/vm/drop_caches.
That works as a 1 time event, but I've found that doing
so only works once/system uptime (if you 'cat' drop_caches'
it retains the last value you put there, and doesn't accept
a new value with fewer bits set than what you echo'd to it
originally (you can change from 1>3, but then not from 3>1 or
(3|1)>0. So not so useful.
Prob w/unmounting was inuse file descriptors (being exported
by samba to clients being the most likely culprit).
This might not be wise if the FS was actively being written
to for a backup, but temporarily doing a
mount -o remount,ro /backups && mount -o remount,rw /backups
seemed to do the trick and cause the disk space to update without
me having to stop processes with FD's open on the vol.