Too Many Open Files

If you receive the error "too many open files" or "too many open connections" in the mongod log, there are a couple of possible reasons for this.

First, to check what file descriptors are in use, run lsof (some variations shown below):

lsof | grep mongod
lsof | grep mongod | grep TCP
lsof | grep mongod | grep data | wc

If most lines include "TCP", there are many open connections from client sockets. If most lines include the name of your data directory, the open files are mostly datafiles.

ulimit

If the numbers from lsof look reasonable, check your ulimit settings. The default for file handles (often 1024) might be too low for production usage. Run ulimit -a (or limit -a depending on shell) to check.

Use ulimit -n X to change the max number of file handles to X. If your OS is configured to not allow modifications to that setting you might need to reconfigure first. On ubuntu you'll need to edit /etc/security/limits.conf and add a line something like the following (where user is the username and X is the desired limit):

user hard nofile X

Upstart uses a different mechanism for setting file descriptor limits - add something like this to your job file:

limit nofile X

High TCP Connection Count

If lsof shows a large number of open TCP sockets, it could be that one or more clients is opening too many connections to the database. Check that your client apps are using connection pooling.