Mongostat shows 2 updates on Primaries when insert data When i insert data to a mongo sharded cluster it shows 2 updates on the Primaries (check localhost:55030). Does anyone knows where these updates come from? I only do inserts using for (var i = 1; i &lt;= 1000000; i++) { db.testData.insert( { x : i , y : (i+10)/2}, {z:2},{w:3} ) }

            insert  query update delete getmore command flushes mapped  vsize    res  faults  locked db idx miss %     qr|qw   ar|aw  netIn netOut  conn  set repl       time 
            localhost:55027     *0     *0     *0     *0       0     3|0       0  1.53g  5.58g   871m      0  test:0.0%          0       0|0     0|0   178b     4k     9 rs01  SEC   13:01:28 
            localhost:55028     *0     *0     *0     *0       0     5|0       0  1.53g  5.58g   872m      0  test:0.0%          0       0|0     0|0   452b     4k     9 rs01  SEC   13:01:28 
             localhost:55029        *0     *0     *0     *0       2     3|0       0  1.53g  5.62g   900m      0  test:0.0%          0       0|0     0|0   272b     4k    24 rs01  PRI   13:01:28 
             localhost:55030       427     *0      2     *0     807   801|0       0  1.28g  5.13g   548m     23 admin:0.9%          0       0|0     0|0   276k   239k    25 rs02  PRI   13:01:28 
             localhost:55031      *428     *0     *0     *0       0     1|0       0  1.28g  5.06g   501m     25 local:3.8%          0       0|0     0|0    62b     3k     9 rs02  SEC   13:01:28 
              localhost:55032     *429     *0     *0     *0       0     1|0       0  1.28g  5.06g   501m     25     .:3.4%          0       0|0     0|0    62b     3k     9 rs02  SEC   13:01:28 
               localhost:61027     423      0      0      0       0     424                 2.42g    16m      0                                            57k    24k     3       RTR   13:01:28
I haven't confirmed by looking at code but I'd bet they are updates to the local database regarding your secondaries.  The primary keeps track of the current status of the secondaries in the set (including where they have synced to), so that would be an in-place update to the local.slaves collection.  Similarly, there is local.replset.minvalid which will certainly be changing as you do inserts.

Basically, I wouldn't worry about it, it's not going to be a major factor in your performance (by the way, if you are using 2.6+ then you should read this Q&amp;A on performance of that type of loop in the shell).