//sh.status()
test.sos
    shard key: { "_id": 1 }
    chunks:
      shard1: 20
      shard3: 20
      shard2: 20

//document in the collection      
{ _id: "AH_25a55ce50b8dce490000000001" } 
{ _id: "BH_25a55ce50b8dce490000000002" } 
{ _id: "CH_25a55ce50b8dce490000000003" } 
{ _id: "DH_25a55ce50b8dce490000000004" } 
{ _id: "EH_25a55ce50b8dce490000000005" } 
{ _id: "FH_25a55ce50b8dce490000000006" } 
{ _id: "GH_25a55ce50b8dce490000000007" } 
{ _id: "HH_25a55ce50b8dce490000000008" } 

//mongod.log
$query: { _id: "AH_25a55ce50b8dce490000000001" } } 
planSummary: IDHACK ntoskip:0 nscanned:1 nscannedObjects:1 idhack:1 keyUpdates:0 writeConflicts:0 numYields:0 nreturned:1 reslen:1277 locks:{ Global: { acquireCount: { r: 2 } }, Database: { acquireCount: { r: 1 } }, Collection: { acquireCount: { r: 1 } } } 0ms 
> IDHACK means that normal query plan path (evaluation, caching, re-evaluation) was bypassed for the query shape {_id: <value>} which is supported by the required _id index. 
  this is a performance optimization for the common use case of fetching a document by primary key.