MongoDB can't set 'maxIncomingConnections' - mongodb

Question

I have a problem setting the maximum incoming connections for my MongoDB.
I ran ulimit -n 1000000 and restarted mongo, the last ping in my MMS dashboard shows:
"maxIncomingConnections": 1000000,
however:
"connections": {
"current": 701,
"totalCreated": 712,
"available": 118
},
as you can see current + available is 819 which is the default (80% from 1024) from system.
Any ideas?

Answer 1

I don't know as which user you ran the ulimit command, but keep in mind that this only is valid for the current user in the current environment.
A better approach is to set the open file limit in /etc/security/limits.conf like this:
# Max is 64k anyway, and there is a hard limit
# of 20k connection in MongoDB anyway
# 40k open files should be more than enough
# unless you have _very_ large disks and a _shitload_ of datafiles
mongodb soft nofiles 40000
mongodb hard nofiles 64000
# Make sure we don't get throttled CPU wise
mongodb soft cpu unlimited
mongodb hard cpu unlimited
# This is kind of useless, since the maximum size
# a file created by MongoDB is 2GB for now
# but it is save and the docs say to do so
mongodb soft fsize -1
mongodb hard fsize -1
# We don't want our resident stack to be limited...
mongodb soft rss -1
mongodb hard rss -1
# ... nor the address space
mongodb soft as -1
mongodb hard as -1
# Last but not least, we want the number of processes at a reasonable limit
mongodb soft noproc 32000
mongodb hard noproc 32000
However, this is only a fallback in case you start MongoDB manually, since the upstart script should set the according limits. After adding these values, a reboot is needed iirc. The number of available connections should increase, then.
Note: Keep in mind that each connection gets about 1MB of stack allocated on the server, which then can not be used for holding indices and data within RAM.