在停止balancer之後，備份整個分片庫，然後啓用balancer，刪除分片集合並重建。遇到以下報錯：
mongos> db.businessInfo.drop();
2016-08-14T09:06:58.324+0800 E QUERY Error: drop failed: {
"code" : 13331,
"ok" : 0,
"errmsg" : "exception: collection's metadata is undergoing changes. Please try again."
}
at Error (<anonymous>)
at DBCollection.drop (src/mongo/shell/collection.js:620:15)
at (shell):1:12 at src/mongo/shell/collection.js:620
理了一下邏輯，應該先drop sharding collection然後再啓用balancer.