As we go through our load test, we start seeing lots of tnscanned and nscannedobjects in the log for our replica set:
2015-08-27T16:45:06.640-0700 I WRITE [conn1171400] update auth.accounts query:

{ id: "accounts:loadtest_adv_user_120817" }

update: { $set: { id: "accounts:loadtest_adv_user_120817", token: "74edd6875a0e809b8cd3e241261494d9011f0aef3d16d66534eeda14496a4d62", account_id: "ef9e1236372512c38e7fd9aa", uuid: "loadtest_adv_user_120817", type: "anonymous", platform: "iOS", locale: "en_US", SNIDS: [], created_date: 1440718756, first_login:

{ fea32fb361e846c3921f495eacfc6c4f: 1440718756 }

, last_login:

{ fea32fb361e846c3921f495eacfc6c4f: 1440719106 }

, casToken: "7de0456af2fd", deviceId: "" } } nscanned:1 nscannedObjects:1 nMatched:1 nModified:1 fastmod:1 keyUpdates:0 writeConflicts:0 numYields:0 locks:{ Global: { acquireCount:

{ r: 2, w: 2 }

}, MMAPV1Journal: { acquireCount:

{ w: 3 }

, acquireWaitCount:

{ w: 1 }

, timeAcquiringMicros:

{ w: 414864 }

}, Database: { acquireCount:

{ w: 2 }

}, Collection: { acquireCount:

{ W: 1 }

, acquireWaitCount:

{ W: 1 }

, timeAcquiringMicros:

{ W: 4285 }

}, oplog: { acquireCount:

{ w: 1 }

} } 419ms
2015-08-27T16:45:06.640-0700 I WRITE [conn1171376] update auth.accounts query:

{ id: "accounts:loadtest_adv_user_75867" }

update: { $set: { id: "accounts:loadtest_adv_user_75867", token: "5fd813afb4436f2cd42bc96b43ed565432c6d846eb1adb011123aa7b1720df3a", account_id: "5de7a7256d6338173d29424e", uuid: "loadtest_adv_user_75867", type: "anonymous", platform: "iOS", locale: "en_US", SNIDS: [], created_date: 1440718756, first_login:

{ fea32fb361e846c3921f495eacfc6c4f: 1440718756 }

, last_login:

{ fea32fb361e846c3921f495eacfc6c4f: 1440719106 }

, casToken: "1b1b7268bdc7", deviceId: "" } } nscanned:1 nscannedObjects:1 nMatched:1 nModified:1 fastmod:1 keyUpdates:0 writeConflicts:0 numYields:0 locks:{ Global: { acquireCount:

{ r: 2, w: 2 }

}, MMAPV1Journal: { acquireCount:

{ w: 3 }

, acquireWaitCount:

{ w: 1 }

, timeAcquiringMicros:

{ w: 412587 }

}, Database: { acquireCount:

{ w: 2 }

}, Collection: { acquireCount:

{ W: 1 }

, acquireWaitCount:

{ W: 1 }

, timeAcquiringMicros:

{ W: 4174 }

}, oplog: { acquireCount:

{ w: 1 }

} } 416ms

2015-08-27T16:45:06.640-0700 I WRITE [conn1170984] update auth.accounts query:

{ id: "accounts:loadtest_adv_user_30791" }

update: { $set: { id: "accounts:loadtest_adv_user_30791", token: "1f199dc363ee77b1b4646c9cac39aaf419e2de68b46d59e43a502f55ca0c5b5a", account_id: "3974b68770a3c183183c9b6d", uuid: "loadtest_adv_user_30791", type: "anonymous", platform: "iOS", locale: "en_US", SNIDS: [], created_date: 1440718754, first_login:

{ fea32fb361e846c3921f495eacfc6c4f: 1440718754 }

, last_login:

{ fea32fb361e846c3921f495eacfc6c4f: 1440719106 }

, casToken: "6ef5c8b4ac8b", deviceId: "" } } nscanned:1 nscannedObjects:1 nMatched:1 nModified:1 fastmod:1 keyUpdates:0 writeConflicts:0 numYields:0 locks:{ Global: { acquireCount:

{ r: 2, w: 2 }

}, MMAPV1Journal: { acquireCount:

{ w: 3 }

, acquireWaitCount:

{ w: 1 }

, timeAcquiringMicros:

{ w: 423119 }

}, Database: { acquireCount:

{ w: 2 }

}, Collection: { acquireCount:

{ W: 1 }

, acquireWaitCount:

{ W: 1 }

, timeAcquiringMicros:

{ W: 3913 }

}, oplog: { acquireCount:

{ w: 1 }

} } 427ms