MongoDB CPU佔用率高的原因分析

1. 通過查找Stack Overflow找到相關問題的解答
2. 跟索引相關
3. 使用db.currenyOp();命令得到正在執行的命令（增刪改查）以及通過ns來查看是否正在使用索引'ns':'collections.system.indexes';            
4. 同時，查看MongoDB log日誌 MongoDB日誌器會打印索引查詢時間超過100ms的操作，發現當前數據庫打印的數據查詢超過400m
5. 2018-06-28T16:51:08.275+0800 I COMMAND  [conn30] command bws.block command: find { find: "block", filter: { blockhash: "378bc0ec06bc22534f2967db5b2b1855dc9776596a7e6b46ad76805aad71d6e3" }, sort: { blockheight: -1 }, limit: 1 } planSummary: COLLSCAN keysExamined:0 docsExamined:376638 hasSortStage:1 cursorExhausted:1 numYields:2944 nreturned:1 reslen:4594 locks:{ Global: { acquireCount: { r: 5890 } }, Database: { acquireCount: { r: 2945 } }, Collection: { acquireCount: { r: 2945 } } } protocol:op_query 396ms
   
6. 從日誌得出查詢時間較長的語句，單獨執行db.collections.find({條件}).sort({value:-1}).limit(1).explain('executionStatus');得出查詢詳細信息。其中totalDocsExamined指的是MongoDB掃描的文檔數量。
7. {
   "queryPlanner" : {
   "plannerVersion" : 1,
   "namespace" : "bws.block",
   "indexFilterSet" : false,
   "parsedQuery" : {
   "blockhash" : {
   "$eq" : "378bc0ec06bc22534f2967db5b2b1855dc9776596a7e6b46ad76805aad71d6e3"
   }
   },
   "winningPlan" : {
   "stage" : "SORT",
   "sortPattern" : {
   "blockheight" : -1
   },
   "limitAmount" : 1,
   "inputStage" : {
   "stage" : "SORT_KEY_GENERATOR",
   "inputStage" : {
   "stage" : "COLLSCAN",
   "filter" : {
   "blockhash" : {
   "$eq" : "378bc0ec06bc22534f2967db5b2b1855dc9776596a7e6b46ad76805aad71d6e3"
   }
   },
   "direction" : "forward"
   }
   }
   },
   "rejectedPlans" : [ ]
   },
   "executionStats" : {
   "executionSuccess" : true,
   "nReturned" : 1,
   "executionTimeMillis" : 205,
   "totalKeysExamined" : 0,
   "totalDocsExamined" : 403964,
   "executionStages" : {
   "stage" : "SORT",
   "nReturned" : 1,
   "executionTimeMillisEstimate" : 171,
   "works" : 403969,
   "advanced" : 1,
   "needTime" : 403967,
   "needYield" : 0,
   "saveState" : 3156,
   "restoreState" : 3156,
   "isEOF" : 1,
   "invalidates" : 0,
   "sortPattern" : {
   "blockheight" : -1
   },
   "memUsage" : 4502,
   "memLimit" : 33554432,
   "limitAmount" : 1,
   "inputStage" : {
   "stage" : "SORT_KEY_GENERATOR",
   "nReturned" : 1,
   "executionTimeMillisEstimate" : 161,
   "works" : 403967,
   "advanced" : 1,
   "needTime" : 403965,
   "needYield" : 0,
   "saveState" : 3156,
   "restoreState" : 3156,
   "isEOF" : 1,
   "invalidates" : 0,
   "inputStage" : {
   "stage" : "COLLSCAN",
   "filter" : {
   "blockhash" : {
   "$eq" : "378bc0ec06bc22534f2967db5b2b1855dc9776596a7e6b46ad76805aad71d6e3"
   }
   },
   "nReturned" : 1,
   "executionTimeMillisEstimate" : 161,
   "works" : 403966,
   "advanced" : 1,
   "needTime" : 403964,
   "needYield" : 0,
   "saveState" : 3156,
   "restoreState" : 3156,
   "isEOF" : 1,
   "invalidates" : 0,
   "direction" : "forward",
   "docsExamined" : 403964
   }
   }
   }
   },
   "serverInfo" : {
   "host" : "MwalletDB",
   "port" : 27017,
   "version" : "3.4.2",
   "gitVersion" : "3f76e40c105fc223b3e5aac3e20dcd026b83b38b"
   },
   "ok" : 1
   }
   
8. 由於是生產環境無法停止數據庫的訪問，所以準備後臺構建索引。命令爲：db.collections.createIndex({value:1},{background:true})