Check number of file descriptors opened by process

To check on the file descriptors opened by process:

lsof -p {PID} | awk '$4 ~ /^[0-9]/ {print $4}' | wc -l


ls -1 /proc/{PID}/fd | wc -l

To check all file descriptors opened by a user:

lsof -u {user} | awk '$4 ~ /^[0-9]/ {print $4}' | wc -l

By default the hard and soft limits for a single process are set to 1024 on linux systems. To check on the limits:

ulimit -Hn
ulimit -Sn

To increase edit the "/etc/security/limits.conf" file for corresponding user:

{user} soft nofile 4096
{user} hard nofile 4096

To check if the new limits has been applied type `ulimit -n` after you get a new shell.

Resolving df and du reporting different output

If df and du give different output of disk usage. Then, most probably it is due to "open file descriptors".

Run `lsof +L1` to get a listing of open files that have been unlinked or removed. Note the file size and kill or restart the respective service.