FYI: better workaround for updating 'df' info after 'rm' on xfs-vols

[Dave Chinner]

On Mon, Feb 25, 2013 at 05:41:09PM -0800, Linda Walsh wrote:
> 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.

Sure, because unlinked files might not have their second phase of
processing (which releases the disk space) done immediately.

> Mentioned that I had tried 'sync' to no avail, and had
> only found umount/mount to cause the figures to synchronize.

sync doesn't cause unlinked inodes to be reclaimed and processed,
unlike remount,ro, unmount or freeze.

> Someone suggested cat [1|3] >/proc/sys/vm/drop_caches.

echo, not cat. It does work every time, whether you see anything
obvious or not. And if you want to reclaim inodes, then you want
"echo 2 > ..."

> 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.

freeze/thaw should do what you want without affecting anything
running on the fs at all.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com