[lug] du vs df

[Michael J. Hammel]

I ran into an interesting problem today.  We have hg configured on
a /home partition and a user was reporting out of disk space when using
it.  I checked with df -m and sure enough the partition was full.

So I figured someone screwed up and filled up their home directory with
junk.  I did a du -sm /home and found only about 40% of the partition
size of data.  Du reported plenty of disk space.  df said the partition
was full.  I don't expect them to be identical, just close.  du was 60%
less than df (roughly) on a partition with about 220GB of total space.

I'm assuming this was some kind of sparse file, though I'm not clear why
it showed itself on /home.  /home is just for user data, including our
mercurial and internal web repositories.  

While researching this a bit, I watched as df showed the partition being
cleared of the extra usage.  Lots of space being freed up very fast.  du
still showed the same thing. 

Any thoughts on what could make df think the partition was full but du
not think that?  I'm trying to figure out what got us into that
situation to begin with.  I suppose I could move the scm and web stuff
to another partition but I'd like to know *how* this all came about.

One possible event during this was a junior engineer was running a find
on the entire filesystem looking for files with IP addresses in them
(the boss asked if we logged web accesses, and that was his first idea
to find out.  He has been properly flogged.).  I suppose if he was
saving the output to a file, then deleted the file while the process
ran, something bad might have happened.  Even that seems a bit of a
stretch.  

Other than that, I'm not clear what might have been running to cause
this.

-- 
Michael J. Hammel <mjhammel at graphics-muse.org>