I have a MongoDB v2.4 replica set on AWS and have been monitoring my stats using MMS and dbStats(). Yesterday I saw an increase in both mapped and virtual memory usage, which correlated with an increased data fileSize and looked completely normal...except that the increase occurred on the secondaries a full two hours before it occurred on the primary (all of these members being located in the same data center).
I vaguely recall that not all members of a replica set will necessarily have the same organization of data in their data files, and I know that you can use the compact() command to defragment the files.
The only difference between the primary and the secondaries in this replica set is that, at one time, the primary was taken offline for roughly 20 minutes. It was then brought back online and re-elected as the primary.
My question is: Is there any reason to be alarmed that the primary seemed to lag behind the secondaries when increasing its mapped & virtual memory usage?