From the qu manpage:

       The qu utility displays the file system block usage for each
       directory in the file hierarchy rooted in each directory
       argument owned by the current process owner (or 'user' if
       specified). If no directory is specified, the block usage of
       the hierarchy rooted in the current directory usage is
       displayed. Usage is only calculated for files and directories
       owned by the process owner (i.e., you), making this useful for
       calculating real quota usage (hence qu).

What this means is that qu can show you where all your disk space
went. Consider this scenario: you log in one morning to find you're
way over your disk quota.

    server% quota
    Disk quotas for user server (uid 1234): 
	 Filesystem  blocks   quota   limit   grace ...
	       /usr  218783* 204800  225280   7days ...

How do you track down where all your space went? You know that your
logs are rotated every night, so that's not the problem. You've tried
du but du shows you all disk space used, even by root, and does not
take into account hard linked files.

Let's try qu at our shell prompt:

    % qu
    
    Disk Usage for /usr/home/server/
	     Disk blocks   Directory Path
	     ===========   ==============
		      65   /
		       3   /.cpan/
		     292   /.cpan/CPAN/
		   11164   /.cpan/build/
		    2991   /.cpan/sources/
		       1   /.ssh/
		       2   /Mail/
		     319   /checkbot-1.64/
		      65   /checkbot-1.64/blib/
		     292   /etc/
		       1   /etc/.restore/
		      24   /formmail/
		       3   /ftp/
		    5722   /ftp/upload/
		     122   /tmp/
		      10   /usr/
		   90921   /usr/home/
		       2   /usr/lib/
		       1   /usr/libexec/
		  104712   /usr/local/
		       9   /usr/share/
		       5   /var/
		    1857   /var/log/
		       6   /var/mail/
		       8   /var/run/
		       1   /var/spool/
    Blocks:       218598
    
    Total Blocks: 218598

Hmmm... let's try that again:

    server% qu --sort=blocks_reverse
    
    Disk Usage for /usr/home/server/
	     Disk blocks   Directory Path
	     ===========   ==============
		  104712   /usr/local/
		   90921   /usr/home/
		   11164   /.cpan/build/
		    5722   /ftp/upload/
		    2991   /.cpan/sources/
		    1857   /var/log/
		     319   /checkbot-1.64/
		     292   /.cpan/CPAN/
		     292   /etc/
		     122   /tmp/
		      65   /
		      65   /checkbot-1.64/blib/
		      24   /formmail/
		      10   /usr/
		       9   /usr/share/
		       8   /var/run/
		       6   /var/mail/
		       5   /var/
		       3   /.cpan/
		       3   /ftp/
		       2   /Mail/
		       2   /usr/lib/
		       1   /.ssh/
		       1   /etc/.restore/
		       1   /usr/libexec.foo/
		       1   /var/spool/
    Blocks:       218598
    
    Total Blocks: 218598

Looks like I've got two directories that might be bigger than I want.
100 megs for ~/usr/local seems about right, ~/usr/home seems fishy.

Just to make sure about ~/usr/local, let's see where that space is
really going:

    server% qu --sort=blocks_reverse --levels=1 /usr/local

    Disk Usage for /usr/home/server/usr/local/
	     Disk blocks   Directory Path
	     ===========   ==============
		   31424   /pgsql-7.1.2/
		   18801   /mysql.old/
		   15728   /etc/
		   10353   /mysql-3.23.41/
		    8676   /lib/
		    2905   /man/
		    2893   /curl-7.7.1/
		    2192   /curl-7.8/
		    1878   /mailman/
		    1854   /analog-5.03/
		    1791   /urchin3309d/
		    1556   /analog-5.01/
		    1040   /bin/
		     925   /majordomo-1.94.5/
		     703   /awstats-2.23q/
		     669   /expat-1.95.1/
		     577   /sendmail/
		     347   /swish-e-2.0.5/
		     312   /swish++/
		      34   /frontpage/
		      26   /
		      26   /include/
		       3   /libexec/
    Blocks:       104713
    
    Total Blocks: 104713

Nothing out of the ordinary there. Let's check that big home directory
we saw earlier:

    server% qu --sort=blocks_reverse --levels=1 /usr/home
    
    Disk Usage for /usr/home/server/usr/home/
	     Disk blocks   Directory Path
	     ===========   ==============
		   80872   /foo/
		    5505   /bob/
		    4541   /joe/
		       4   /
    Blocks:        90922
    
    Total Blocks: 90922

Ouch! The 'foo' directory is much bigger than I had thought. Let's
see what's going on there:

    server% cd usr/home/foo
    server% ls -l
    total 80872
    -rw-r--r--  1 server  vuser  31603990 Aug 27 06:28 bar.log
    -rw-r--r--  1 server  vuser  51136330 Aug 27 06:28 foo.log

So! It looks like some logs that I had forgotten about. Now I can simply:

    server% rm *.log
    server% quota
    Disk quotas for user server (uid 1234): 
	 Filesystem  blocks   quota   limit   grace ...
	       /usr  137911  204800  225280         ...

and I'm back in the saddle again.

To find out more about qu type 'man qu' at your shell prompt.