As a SA, it not uncommon to have regularly requests about big differences between the du and df outputs on a UFS file system. (For ZFS specific considerations, please see the ZFS FAQ.)

The du utility reports the sum of space allocated to all files in the file hierarchy rooted in the directory plus the space allocated to the directory itself. The df utility reports the amount of disk space occupied by a mounted file system.

When a file is remove from the file system, i.e. is unlinked (the hard link count goes to zero), the space belonging to this file is accounted against the du tool, but is not visible to the df utility until all references to it (open file descriptors) are closed. In order to find the guilty process, one can follow the information found in the SunManagers Frequently Asked Questions. Here is an example of such finding, but using a slightly different method to get the process currently holding the open descriptor to the deleted file.

Find the file which has been unlinked through the procfs interface:

# find /proc/*/fd \( -type f -a ! -size 0 -a -links 0 \) -print | xargs \ls -li
 415975 --w-------   0 user  group  2125803025 Oct 15 23:59 /proc/1252/fd/3

Eventually, get more detail about it:

# pargs -c 1252
1252:   rvd.basic -reliability 5 -listen tcp:9876 -logfile /path/to/log/rvd_9876.l
argv[0]: rvd.basic
argv[1]: -reliability
argv[2]: 5
argv[3]: -listen
argv[4]: tcp:9876
argv[5]: -logfile
argv[6]: /path/to/log/rvd_9876.log

Check to see if you can understand what is the content of the unlinked file:

# tail /proc/1252/fd/3
2008-10-15 23:59:32.002116 - [MSG], line 792 (thread 25087:4)
[4060] Sent a heartbeat
------------------------------------------------------------------------------- [4111] No activity detected. Send a Heartbeat message
2008-10-15 23:59:32.134829 - [MSG], line 1138 (thread 25087:4)
[4065] Heartbeat acknowledged by Bloomberg

You can correlate the size of the removed, but always referenced, file to the space accounted from the du and df tools:

# df -k /path/to
Filesystem            kbytes    used   avail capacity  Mounted on
/dev/md/dsk/d5       6017990 5874592   83219    99%    /path/to
# du -sk /path/to
3791632 /data
# echo "(5874616-3791632)*1024" | bc

So, we now found the ~2GB log file which was always opened (used) by a process. Now, there are two solutions to be able to get back the freed space:

  1. Truncate the unlinked file (quick workaround).
  2. Simply restart properly the corresponding program (better option).

Use the solution which fits the best your need in your environment.