MongoDB monitoring with mongotop
Daniel Watrous 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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[…] for monitoring the performance and health of a MongoDB node is mongostat. You’ll recall that mongotop shows the time in milliseconds that a mongo node spent accessing (read and write) a particular […]