I ran across an uncommon scenario whereby a file system on a Linux server was showing it was nearly full, but I could only account for about half of the space usage.
[root@web18 /]# df -h / Filesystem Size Used Avail Use% Mounted on /dev/sda2 165G 135G 21G 87% /
After some research, it was determined that a stray process had a lock in place on files that were previously deleted. Since this lock was in place, the file system would not show the space as available.
[root@web18 /]# ls -l /proc/*/fd/* | grep deleted l-wx------ 1 nginx nginx 64 Aug 1 09:39 /proc/55627/fd/15 -> /var/log/nginx/access_log-20130731 (deleted) l-wx------ 1 nginx nginx 64 Aug 1 09:39 /proc/55627/fd/16 -> /var/log/nginx/error_log-20130731 (deleted)
After killing the stale nginx process (killall -9 nginx), the space started showing its normal consumption.
[root@web18 /]# df -h / Filesystem Size Used Avail Use% Mounted on /dev/sda2 165G 67G 90G 43% /
I hope this helps someone who suffers from a similar problem