Daniel Watrous on Software Engineering

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MongoDB monitoring with mongotop

In the process of tuning the performance of a MongoDB replica set, it’s useful to be able to observe mongod directly, as opposed to inferring what it’s doing by watching the output of top, for example. For that reason MongoDB comes with a utility, mongotop.

The output of mongotop indicates the amount of time the mongod process spend reading and writing to a specific collection during the update interval. I used the following command to run mongotop on an authentication enabled replica set with a two second interval.

[watrous@d1t0156g ~]# mongotop -p -u admin 2
connected to: 127.0.0.1
Enter password:
 
                              ns       total        read       write           2013-01-11T23:41:51
                          admin.         0ms         0ms         0ms
            admin.system.indexes         0ms         0ms         0ms
         admin.system.namespaces         0ms         0ms         0ms
              admin.system.users         0ms         0ms         0ms
 coursetracker.system.namespaces         0ms         0ms         0ms
document_queue.system.namespaces         0ms         0ms         0ms

The output doesn’t refresh in the same way top does. Instead it aggregates, similar to running tail -f. When I began my experiment I could immediately see the resulting load:

                              ns       total        read       write           2013-01-11T23:41:19
                   documents.nav        60ms        60ms         0ms
               documents.product        53ms        53ms         0ms
                          admin.         0ms         0ms         0ms
            admin.system.indexes         0ms         0ms         0ms
         admin.system.namespaces         0ms         0ms         0ms
              admin.system.users         0ms         0ms         0ms
 coursetracker.system.namespaces         0ms         0ms         0ms
 
                              ns       total        read       write           2013-01-11T23:41:21
                   documents.nav        82ms        82ms         0ms
               documents.product        54ms        54ms         0ms
                          admin.         0ms         0ms         0ms
            admin.system.indexes         0ms         0ms         0ms
         admin.system.namespaces         0ms         0ms         0ms
              admin.system.users         0ms         0ms         0ms
 coursetracker.system.namespaces         0ms         0ms         0ms
 
                              ns       total        read       write           2013-01-11T23:41:23
                   documents.nav        63ms        63ms         0ms
               documents.product        45ms        45ms         0ms
                          admin.         0ms         0ms         0ms
            admin.system.indexes         0ms         0ms         0ms
         admin.system.namespaces         0ms         0ms         0ms
              admin.system.users         0ms         0ms         0ms
 coursetracker.system.namespaces         0ms         0ms         0ms

A related performance utility is mongostat.

Verified load balancing

Before running the experiment I set the ReadPreference to nearest. As a restult I expected to see a well balanced, but asymmetrical distribution between nodes in my replica set with all hosts responding to queries. That’s exactly what I saw.

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