Mongodb – error when starting up the mongodb cluster

The resident memory size represents the number of pages in memory actually touched by the mongod process. If that is significantly lower than the available memory and data exceeds the available memory (yours does), then it could be a case of simply not having actively touched enough pages yet.

To determine if this is the case, you should run free -m, the output should look something like this:

free -m
             total       used       free     shared    buffers     cached
Mem:          3709       3484        224          0         84       2412
-/+ buffers/cache:        987       2721
Swap:         3836        156       3680

In my example, cached is not close to the total, which means that not only has mongod not touched enough pages, the filesystem cache has not yet even been filled by pages being read from the disk in general.

A quick remedy for this would be the touch command (added in 2.2) - it should be used with caution on large data sets as it will attempt to load everything into RAM even if the data is far too large to fit (causing a lot of disk IO and page faults). It will certainly fill up the memory effectively though :)

If your cached value is close to the total available, then your issue is that a large number of pages being read into memory from disk are not relevant to (and hence not touched by) the mongod process. The usual candidate for this kind of discrepency is readahead. I've already covered that particular topic elsewhere in detail, so I'll just link those two answers for future reading if necessary.

First of all when you run mongod as a normal user you need to make sure that the particular user has the following permissions:

1. Access to network interface ( open socket ) in order to be able to listen to connections
2. Access to the database folder. The default is accessible only by root or mongod user (/data/db)
3. Access to logging folder /var/log/mongod

All of the above are automatically set and granted for the user mongodb (mongod in earlier versions). If you take a look at /etc/init.d/mongodb you will see that the service will be stared under the user mongodb DAEMONUSER=${DAEMONUSER:-mongodb} and not as root.