title: What is mongo $mergeCursors?
body: <p>We see a lot of slow queries in mongo logs like below (with pipeline op mergeCursors). We have a shaded mongo with 2 shards with only primaries. What is mergeCursors command? Please let me know if any other information is required.   </p>
  
  <pre><code>{
      "_id" : ObjectId("5571b739f65f7e64bb806362"),
      "op" : "command",
      "ns" : "mongrel.$cmd",
      "command" : {
          "aggregate" : "collection1",
          "pipeline" : [ 
              {
                  "$mergeCursors" : [ 
                      {
                          "host" : "endpoint:27005",
                          "id" : NumberLong(82775337156)
                      }
                  ]
              }
          ]
      },
      "keyUpdates" : 0,
      "numYield" : 0,
      "lockStats" : {
          "timeLockedMicros" : {
              "r" : NumberLong(12),
              "w" : NumberLong(0)
          },
          "timeAcquiringMicros" : {
              "r" : NumberLong(2),
              "w" : NumberLong(2680)
          }
      },
      "responseLength" : 12312,
      "millis" : 6142,
      "execStats" : {},
      "ts" : ISODate("2015-06-05T12:35:40.801Z"),
      "client" : "10.167.212.83",
      "allUsers" : [],
      "user" : ""
  }
  </code></pre>