chunksize was 64, but autosplit would be triggered before data size touch it.
In my test, autosplit would be triggered after 16 times update with upsert flag.
When autosplit, 'err' was None but n was 0 in return message and "u'updatedExisting': False" is miss

===Python Test Script===

from pymongo import Connection
_SYSTEM_INDEX_COLLECTION = u'system.indexes'
def get_conn():
    return Connection( "P1.trend.lava.tw" )
def main():
    db = "testsplit"
    coll = "coll_5"
    conn = get_conn()
    colls = conn[db].collection_names()
    if _SYSTEM_INDEX_COLLECTION in colls:
        colls.remove(_SYSTEM_INDEX_COLLECTION)
    for coll in colls:
        conn[db].drop_collection(coll)
    admin = conn['admin']
    admin.command('enablesharding', db, allowable_errors=['already enabled'])
    admin.command('shardcollection', "%s.%s"%(db,coll),allowable_errors=['already sharded'], key={"shardkey2":1,"shardkey1":1}, unique=False)
    for id in range(1,16):
        mesg = conn[db][coll].update({"shardkey1":"test%s"%id,"shardkey2":"test%s"%id}, {"$set":{"test_upsert":"new_upsert_value3"}}, safe=True, upsert=True, multi=False)
        print "mesg=%s" % mesg
if __name__ == '__main__':
    main()

===Test Script Output===
mesg={u'ok': 1.0, u'err': None, u'shards': [u'P1.trend.lava.tw:27019,P2.trend.lava.tw:27019,P3.trend.lava.tw:27019', u'P1.trend.lava.tw_P2.trend.lava.tw_P3.trend.lava.tw/P1.trend.lava.tw:27018,P3.trend.lava.tw:27018,P2.trend.lava.tw:27018'], u'n': 1, u'updatedExisting': False, u'shardRawGLE': {u'P1.trend.lava.tw:27019,P2.trend.lava.tw:27019,P3.trend.lava.tw:27019':

, u'P1.trend.lava.tw_P2.trend.lava.tw_P3.trend.lava.tw/P1.trend.lava.tw:27018,P3.trend.lava.tw:27018,P2.trend.lava.tw:27018':

}}
mesg=

mesg=

mesg=

mesg=

mesg=

mesg=

mesg=

mesg=

mesg=

mesg=

mesg=

mesg=

mesg=

mesg={u'shardRawGLE': {u'P1.trend.lava.tw:27019,P2.trend.lava.tw:27019,P3.trend.lava.tw:27019':

, u'P1.trend.lava.tw_P2.trend.lava.tw_P3.trend.lava.tw/P1.trend.lava.tw:27018,P3.trend.lava.tw:27018,P2.trend.lava.tw:27018': {u'connectionId': 48862, u'lastOp': 5743692655800352785L, u'ok': 1.0, u'err': None, u'n': 0}}, u'ok': 1.0, u'shards': [u'P1.trend.lava.tw:27019,P2.trend.lava.tw:27019,P3.trend.lava.tw:27019', u'P1.trend.lava.tw_P2.trend.lava.tw_P3.trend.lava.tw/P1.trend.lava.tw:27018,P3.trend.lava.tw:27018,P2.trend.lava.tw:27018'], u'err': None, u'n': 0}

===The changelog after test===
{ "_id" : "p1.trend.lava.tw-2012-05-18T02:20:58-142", "server" : "p1.trend.lava.tw", "clientAddr" : "N/A", "time" : ISODate("2012-05-18T02:20:58.095Z"), "what" : "dropCollection.start", "ns" : "testsplit.coll_5", "details" : { } }
{ "_id" : "p1.trend.lava.tw-2012-05-18T02:20:58-143", "server" : "p1.trend.lava.tw", "clientAddr" : "N/A", "time" : ISODate("2012-05-18T02:20:58.242Z"), "what" : "dropCollection", "ns" : "testsplit.coll_5", "details" : { } }
{ "_id" : "p3.trend.lava.tw-2012-05-18T02:20:58-21", "server" : "p3.trend.lava.tw", "clientAddr" : "172.16.2.149:41322", "time" : ISODate("2012-05-18T02:20:58.665Z"), "what" : "split", "ns" : "testsplit.coll_5", "details" : { "before" : { "min" : { "shardkey2" :

, "shardkey1" :

}, "max" : { "shardkey2" :

, "shardkey1" :

}, "lastmod" :

}, "left" : { "min" : { "shardkey2" :

, "shardkey1" :

}, "max" :

, "lastmod" :

}, "right" : { "min" :

, "max" : { "shardkey2" :

, "shardkey1" :

}, "lastmod" :

} } }

===Log of mongos.log after test===
Fri May 18 03:02:37 [mongosMain] connection accepted from 172.16.2.60:50953 #8299
Fri May 18 03:02:37 [conn8299] DROP: testsplit.coll_5
Fri May 18 03:02:37 [conn8299] about to log metadata event: { _id: "p1.trend.lava.tw-2012-05-18T03:02:37-144", server: "p1.trend.lava.tw", clientAddr: "N/A", time: new Date(1337310157712), what: "dropCollection.start", ns: "testsplit.coll_5", details: {} }
Fri May 18 03:02:37 [conn8299] distributed lock 'testsplit.coll_5/p1.trend.lava.tw:27017:1287157484:1804289383' acquired, ts : 4fb5bbcd0cf1d1e54a301387
Fri May 18 03:02:37 [conn8299] about to log metadata event: { _id: "p1.trend.lava.tw-2012-05-18T03:02:37-145", server: "p1.trend.lava.tw", clientAddr: "N/A", time: new Date(1337310157971), what: "dropCollection", ns: "testsplit.coll_5", details: {} }
Fri May 18 03:02:38 [conn8299] distributed lock 'testsplit.coll_5/p1.trend.lava.tw:27017:1287157484:1804289383' unlocked.
Fri May 18 03:02:38 [conn8299] CMD: shardcollection: { shardcollection: "testsplit.coll_5", unique: false, key:

}
Fri May 18 03:02:38 [conn8299] enable sharding on: testsplit.coll_5 with shard key:

Fri May 18 03:02:38 [conn8299] created new distributed lock for testsplit.coll_5 on P1.trend.lava.tw:27019,P2.trend.lava.tw:27019,P3.trend.lava.tw:27019 ( lock timeout : 900000, ping interval : 30000, process : 0 )
Fri May 18 03:02:38 [conn8299] ChunkManager: time to load chunks for testsplit.coll_5: 0ms sequenceNumber: 147 version: 0|0
Fri May 18 03:02:38 [conn8299] going to create 1 chunk(s) for: testsplit.coll_5
Fri May 18 03:02:38 [conn8299] warning: version 0 found when reloading chunk manager, collection 'testsplit.coll_5' initially detected as sharded
Fri May 18 03:02:38 [conn8299] created new distributed lock for testsplit.coll_5 on P1.trend.lava.tw:27019,P2.trend.lava.tw:27019,P3.trend.lava.tw:27019 ( lock timeout : 900000, ping interval : 30000, process : 0 )
Fri May 18 03:02:38 [conn8299] ChunkManager: time to load chunks for testsplit.coll_5: 0ms sequenceNumber: 148 version: 1|0
Fri May 18 03:02:38 [conn8299] SyncClusterConnection connecting to [P1.trend.lava.tw:27019]
Fri May 18 03:02:38 [conn8299] SyncClusterConnection connecting to [P2.trend.lava.tw:27019]
Fri May 18 03:02:38 [conn8299] SyncClusterConnection connecting to [P3.trend.lava.tw:27019]
Fri May 18 03:02:38 [conn8299] created new distributed lock for testsplit.coll_5 on P1.trend.lava.tw:27019,P2.trend.lava.tw:27019,P3.trend.lava.tw:27019 ( lock timeout : 900000, ping interval : 30000, process : 0 )
Fri May 18 03:02:38 [conn8299] ChunkManager: time to load chunks for testsplit.coll_5: 0ms sequenceNumber: 149 version: 1|2
Fri May 18 03:02:38 [conn8299] autosplitted testsplit.coll_5 shard: ns:testsplit.coll_5 at: P1.trend.lava.tw_P2.trend.lava.tw_P3.trend.lava.tw:P1.trend.lava.tw_P2.trend.lava.tw_P3.trend.lava.tw/P1.trend.lava.tw:27018,P3.trend.lava.tw:27018,P2.trend.lava.tw:27018 lastmod: 1|0 min:

max:

on:

(splitThreshold 921)
Fri May 18 03:02:38 [conn8299] update will be retried b/c sharding config info is stale, left:4 ns: testsplit.coll_5 query:

Fri May 18 03:02:38 [conn8299] end connection 172.16.2.60:50953