本文来自《2021MongoDB技术实践与应用案例征集活动》入围案例奖作品
作者：任坤
1. 背景
线上mongo 4分片集群，版本percona 4.2，查看实时qps发现shard1的update很高，而剩余3个shard的update都很低。
--shard1
--shard2
要么是某个分片表的数据分布不均匀，要么就是没有开启分片。
2.诊断
先核查一下大表。登录mongos，切换到该db，执行如下命令，每个表输出一行，分别为表名和size(MB) var collNames = db.getCollectionNames();
代码语言：javascript复制for (var i = 0; i < collNames.length; i ) { var coll = db.getCollection(collNames[i]);
var stats = coll.stats(1024 * 1024); print(stats.ns, stats.storageSize); }找出最大的10个表。执行db.table.getShardDistribution()，发现每个大表都分布均匀。
查看shard1的mongod.log。没有发现任何慢查询，目前慢查询阈值为100ms。
查看shard1的oplog。use local db.oplog.rs.find({ "op":"u" }).sort({$natural: -1}).limit(10) #查询最新的10条
代码语言：javascript复制update oplog { "ts" : Timestamp(1634090487, 1781), "t" : NumberLong(4), "h" : NumberLong(0), "v" : 2, "op" : "u", "ns" : "prod.prod_XXX", "ui" : UUID("22154461-6305-491d-8f1d-9a1630753508"), "o2" : { "_id" : "2021-10-10 18:00:00#505237968" }, "wall" : ISODate("2021-10-13T02:01:27.707Z"), "o" : { "$v" : 1, "$set" : { "__system" : { "pull_time" : "2021-10-13 10:01:27" } } } } { "ts" : Timestamp(1634090487, 1780), "t" : NumberLong(4), "h" : NumberLong(0), "v" : 2, "op" : "u", "ns" : "prod.prod_XXX", "ui" : UUID("22154461-6305-491d-8f1d-9a1630753508"), "o2" : { "_id" : "2021-10-10 18:00:00#505237974" }, "wall" : ISODate("2021-10-13T02:01:27.707Z"), "o" : { "$v" : 1, "$set" : { "__system" : { "pull_time" : "2021-10-13 10:01:27" } } } } { "ts" : Timestamp(1634090487, 1779), "t" : NumberLong(4), "h" : NumberLong(0), "v" : 2, "op" : "u", "ns" : "prod.prod_XXX", "ui" : UUID("22154461-6305-491d-8f1d-9a1630753508"), "o2" : { "_id" : "2021-10-10 18:00:00#505237975" }, "wall" : ISODate("2021-10-13T02:01:27.707Z"), "o" : { "$v" : 1, "$set" : { "__system" : { "pull_time" : "2021-10-13 10:01:27" } } } } { "ts" : Timestamp(1634090487, 1778), "t" : NumberLong(4), "h" : NumberLong(0), "v" : 2, "op" : "u", "ns" : "prod.prod_XXX", "ui" : UUID("22154461-6305-491d-8f1d-9a1630753508"), "o2" : { "_id" : "2021-10-10 18:00:00#505237976" }, "wall" : ISODate("2021-10-13T02:01:27.707Z"), "o" : { "$v" : 1, "$set" : { "__system" : { "pull_time" : "2021-10-13 10:01:27" } } } } { "ts" : Timestamp(1634090487, 1777), "t" : NumberLong(4), "h" : NumberLong(0), "v" : 2,
"op" : "u", "ns" : "prod.prod_XXX", "ui" : UUID("22154461-6305-491d-8f1d-9a1630753508"), "o2" : { "_id" : "2021-10-10 18:00:00#505238113" }, "wall" : ISODate("2021-10-13T02:01:27.707Z"), "o" : { "$v" : 1, "$set" : { "__system" : { "pull_time" : "2021-10-13 10:01:27" } } } } { "ts" : Timestamp(1634090487, 1776), "t" : NumberLong(4), "h" : NumberLong(0), "v" : 2, "op" : "u", "ns" : "prod.prod_XXX", "ui" : UUID("22154461-6305-491d-8f1d-9a1630753508"), "o2" : { "_id" : "2021-10-10 18:00:00#505720693" }, "wall" : ISODate("2021-10-13T02:01:27.707Z"), "o" : { "$v" : 1, "$set" : { "__system" : { "pull_time" : "2021-10-13 10:01:27" } } } } { "ts" : Timestamp(1634090487, 1775), "t" : NumberLong(4), "h" : NumberLong(0), "v" : 2, "op" : "u", "ns" : "prod.prod_XXX", "ui" : UUID("22154461-6305-491d-8f1d-9a1630753508"), "o2" : { "_id" : "2021-10-10 18:00:00#505726096" }, "wall" : ISODate("2021-10-13T02:01:27.707Z"), "o" : { "$v" : 1, "$set" : { "__system" : { "pull_time" : "2021-10-13 10:01:27" } } } } { "ts" : Timestamp(1634090487, 1774), "t" : NumberLong(4), "h" : NumberLong(0), "v" : 2, "op" : "u", "ns" : "prod.prod_XXX", "ui" : UUID("22154461-6305-491d-8f1d-9a1630753508"), "o2" : { "_id" : "2021-10-10 18:00:00#505726320" }, "wall" : ISODate("2021-10-13T02:01:27.707Z"), "o" : { "$v" : 1, "$set" : { "__system" : { "pull_time" : "2021-10-13 10:01:27" } } } } { "ts" : Timestamp(1634090487, 1773), "t" : NumberLong(4), "h" : NumberLong(0), "v" : 2, "op" : "u", "ns" : "prod.prod_XXX", "ui" : UUID("22154461-6305-491d-8f1d-9a1630753508"), "o2" : { "_id" : "2021-10-10 18:00:00#505750437" }, "wall" : ISODate("2021-10-13T02:01:27.707Z"), "o" : { "$v" : 1, "$set" : { "__system" : { "pull_time" : "2021-10-13
10:01:27" } } } } { "ts" : Timestamp(1634090487, 1772), "t" : NumberLong(4), "h" : NumberLong(0), "v" : 2, "op" : "u", "ns" : "prod.prod_XXX", "ui" : UUID("22154461-6305-491d-8f1d-9a1630753508"), "o2" : { "_id" : "2021-10-10 18:00:00#505750438" }, "wall" : ISODate("2021-10-13T02:01:27.707Z"), "o" : { "$v" : 1, "$set" : { "__system" : { "pull_time" : "2021-10-13 10:01:27" } } } } { "ts" : Timestamp(1634090487, 1771), "t" : NumberLong(4), "h" : NumberLong(0), "v" : 2, "op" : "u", "ns" : "prod.prod_XXX", "ui" : UUID("22154461-6305-491d-8f1d-9a1630753508"), "o2" : { "_id" : "2021-10-10 18:00:00#505750439" }, "wall" : ISODate("2021-10-13T02:01:27.706Z"), "o" : { "$v" : 1, "$set" : { "__system" : { "pull_time" : "2021-10-13 10:01:27" } } } }发现都是针对prod_XXX表的update，而该表没有分片。
和开发核对后，对其_id列创建hash索引并开启分片。
登录mongos，切换到该db，执行：
代码语言：javascript复制db.prod_XXX.ensureIndex({_id: "hashed"}, {background: true}) sh.shardCollection("prod.prod_XXX", { _id : "hashed" } )update以肉眼可见的速度均衡，问题解决。
--shard1
--shard2
3.小结
本次案例很简单也很常见，mongo分片如果tps不均衡，可以参照上述方法快速定位并解决。
用惯了mysql的人刚转手mongo会很不习惯，尤其是很多sql语法根本记不住，比如本文的查询集合大小以及查看oplog的命令，最好是记个笔记用到的时候直接翻出来看。
关于作者：
任坤，现居珠海，先后担任专职 Oracle 和 MySQL DBA，现在主要负责 MySQL、MongoDB、Redis和Clickhouse 维护工作
