分片技术解决的需求痛点
（1）高数据量和吞吐量的数据库应用会对单机的性能造成较大压力；
（2）大的查询量会将单机的CPU耗尽；
（3）大的数据量对单机的存储压力较大，最终会耗尽系统的内存而将压力转移到键盘IO上
何为分片？
MongoDB分片技术是使用多个服务器存储数据的方法，以支持巨大的数据存储和对数据的操作。当MongoDB存储海量的数据时，一台机器可能不足以存储数据，也可能不足以提供可接受的读写吞吐量。这时，我们就可以通过在多台机器上分割数据，使得数据库系统能存储和处理更多的数据。
MongoDB分片结构
下图展示了MongoDB分片集群结构组成分布：
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上图中主要有如下所述三个主要组件：
用于存储实际的数据块，实际生产环境中一个shard server角色可由几台机器组个一个replica set承担，防止主机单点故障
mongod实例，存储了整个 分片群集的配置信息，其中包括 chunk信息。
客户端由此接入，且让整个集群看上去像单一数据库，前端应用可以透明使用
MongoDB分片优势：
减少单个分片需要处理的请求数，提高群集的存储容量和吞吐量
比如，当插入一条数据时，应用只需要访问存储这条数据的分片
减少单分片存储的数据，提高数据可用性，提高大型数据库查询服务的性能。
当MongoDB单点数据库服务器存储和性能成为瓶颈，或者需要部署大型应用以充分利用内存时，可以使用分片技术
分片群集简单部署：
下面介绍如何在一台CentOS7系统的物理服务器上部署一个简单结构的MongoDB分片集群 。
1台路由实例（端口 27017）
1台配置实例（端口 37017）
2台分片实例（端口 47017， 47018）
本次实验使用手工编译安装mongodb，
百度网盘免费提供安装包：
https://pan.baidu.com/s/1bZXJDBrn9qxBF6r6IF_aLQ
具体步骤如下：
安装支持的软件，解压准备好的MongoDB软件包，注意：本次安装是在64位Linux系统上
yum install openssl-devel -y
tar zxvf mongodb-linux-x86_64-3.2.1.tgz -C /opt/
mv mongodb-linux-x86_64-3.2.1/ /usr/local/mongodb
创建MongoDB四个实例的数据存储目录，日志存储目录/data/logs，日志文件，并修改权限
mkdir -p /data/mongodb/mongodb{1,2,3,4}
mkdir /data/logs
touch /data/logs/mongodb{1,2,3,4}.log
chmod -R 777 /data/logs/*.log
ulimit -n 25000 #大文件 数
ulimit -u 25000 #大进程数
sysctl -w vm.zone_reclaim_mode=0 #永久设置
echo never > /sys/kernel/mm/transparent_hugepage/enabled
echo never > /sys/kernel/mm/transparent_hugepage/defrag
ln -s /usr/local/mongodb/bin/* /usr/bin/ #方便服务管理
部署配置服务器
vim mongodb1.conf
port=37017 #指定服务端口
dbpath=/data/mongodb/mongodb1 #数据存储目录
logpath=/data/logs/mongodb1.log #日志文件
logappend=true #使用追加方式写日志
fork=true #后台运行
maxConns=5000 #设定大同时连接数，默认为2000
storageEngine=mmapv1 #指定存储引擎为内存映射文件
configsvr=true #配置服务
启动配置服务，并进入，可看到配置服务前缀为”configsvr>“，
[root@localhost bin]# mongod -f mongodb1.conf #启动配置服务器
about to fork child process, waiting until server is ready for connections.
forked process: 12594
child process started successfully, parent exiting
[root@localhost bin]# mongo --port 37017 #进入服务
MongoDB shell version: 3.2.1
connecting to: 127.0.0.1:37017/test
Server has startup warnings:
2018-09-15T04:39:24.967+0800 I STORAGE [initandlisten]
2018-09-15T04:39:24.967+0800 I STORAGE [initandlisten] ** WARNING: Readahead for /data/mongodb1 is set to 4096KB
2018-09-15T04:39:24.967+0800 I STORAGE [initandlisten] ** We suggest setting it to 256KB (512 sectors) or less
2018-09-15T04:39:24.967+0800 I STORAGE [initandlisten] ** http://dochub.mongodb.org/core/readahead
2018-09-15T04:39:25.828+0800 I CONTROL [initandlisten] ** WARNING: You are running this process as the root user, which is not recommended.
2018-09-15T04:39:25.828+0800 I CONTROL [initandlisten]
2018-09-15T04:39:25.829+0800 I CONTROL [initandlisten]
2018-09-15T04:39:25.829+0800 I CONTROL [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/enabled is 'always'.
2018-09-15T04:39:25.829+0800 I CONTROL [initandlisten] ** We suggest setting it to 'never'
2018-09-15T04:39:25.829+0800 I CONTROL [initandlisten]
2018-09-15T04:39:25.829+0800 I CONTROL [initandlisten] ** WARNING: /sys/kernel/mm/transparent_hugepage/defrag is 'always'.
2018-09-15T04:39:25.829+0800 I CONTROL [initandlisten] ** We suggest setting it to 'never'
2018-09-15T04:39:25.829+0800 I CONTROL [initandlisten]
configsvr>
部署分片服务器
编辑mongodb2.conf配置文件，端口号为47017，设置shardsvr=true，
vim mongodb2.conf
port=47017
dbpath=/data/mongodb/mongodb2
logpath=/data/logs/mongodb2.log
logappend=true
fork=true
maxConns=5000
storageEngine=mmapv1
shardsvr=true
同样编辑mongodb3.conf配置文件，端口号为47018，
vim mongodb3.conf
port=47018
dbpath=/data/mongodb/mongodb3
logpath=/data/logs/mongodb3.log
logappend=true
fork=true
maxConns=5000
storageEngine=mmapv1
shardsvr=true
[root@localhost bin]# mongod -f mongodb2.conf
about to fork child process, waiting until server is ready for connections.
forked process: 12810
child process started successfully, parent exiting
[root@localhost bin]# mongod -f mongodb3.conf
about to fork child process, waiting until server is ready for connections.
forked process: 12825
child process started successfully, parent exiting
启动路由服务器
通过./mongos --help命令可以查看启动路由相关参数信息。chunkSize为数据块大小，默认为200MB，为了便于测试这里将值设置为1
[root@localhost bin]# ./mongos --port 27017 --fork --logpath=/usr/local/mongodb/bin/route.log --configdb 192.168.30.55:37017 --chunkSize 1
2018-09-15T05:03:06.217+0800 W SHARDING [main] Running a sharded cluster with fewer than 3 config servers should only be done for testing purposes and is not recommended for production.
about to fork child process, waiting until server is ready for connections.
forked process: 12934
child process started successfully, parent exiting
启用分片服务器
连接到路由实例后，可以通过sh.status()命令查看分片状态信息
[root@localhost bin]# mongo #进入路由实例
MongoDB shell version: 3.2.1
connecting to: test
Server has startup warnings:
2018-09-15T16:00:19.746+0800 I CONTROL [main] ** WARNING: You are running this process as the root user, which is not recommended.
2018-09-15T16:00:19.746+0800 I CONTROL [main]
mongos> sh.status()
--- Sharding Status ---
sharding version: {
"_id" : 1,
"minCompatibleVersion" : 5,
"currentVersion" : 6,
"clusterId" : ObjectId("5b9cbc14b4c77895df796bac")
}
shards: #shards下为空，没有分片服务器
active mongoses:
"3.2.1" : 1
balancer:
Currently enabled: yes
Currently running: no
Failed balancer rounds in last 5 attempts: 0
Migration Results for the last 24 hours:
No recent migrations
databases:
通过sh.addShard命令添加两个分片服务器，
mongos> sh.addShard("192.168.30.55:47017")
{ "shardAdded" : "shard0000", "ok" : 1 }
mongos> sh.addShard("192.168.30.55:47018")
{ "shardAdded" : "shard0001", "ok" : 1 }
再次查看分片信息，可以看到shards：选项下已经显示刚刚添加的分片服务器
mongos> sh.status()
--- Sharding Status ---
sharding version: {
"_id" : 1,
"minCompatibleVersion" : 5,
"currentVersion" : 6,
"clusterId" : ObjectId("5b9cbc14b4c77895df796bac")
}
shards:
{ "_id" : "shard0000", "host" : "192.168.30.55:47017" }
{ "_id" : "shard0001", "host" : "192.168.30.55:47018" }
active mongoses:
"3.2.1" : 1
balancer:
Currently enabled: yes
Currently running: no
Failed balancer rounds in last 5 attempts: 0
Migration Results for the last 24 hours:
No recent migrations
databases:
实现分片功能
mongos> show dbs
config 0.031GB
mongos> use school #进入并创建数据库school
switched to db school
mongos> for (var i=1;i<=50000;i++)db.info.insert({"id":i,"name":"tom"+i}) #创建集合info，并使用循环插入50000条数据
WriteResult({ "nInserted" : 1 })
使用sh.enableSharding("school")命令启用school数据库分片
mongos> sh.enableSharding("school")
{ "ok" : 1 }
mongos> db.info.createIndex({"id":1})
{
"raw" : {
"192.168.30.55:47017" : {
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 1,
"numIndexesAfter" : 2,
"ok" : 1
}
},
"ok" : 1
}
mongos> sh.shardCollection("school.info",{"id":1})
{ "collectionsharded" : "school.info", "ok" : 1 }
mongos> sh.status()
--- Sharding Status ---
sharding version: {
"_id" : 1,
"minCompatibleVersion" : 5,
"currentVersion" : 6,
"clusterId" : ObjectId("5b9cbc14b4c77895df796bac")
}
shards:
{ "_id" : "shard0000", "host" : "192.168.30.55:47017" } #两个分片服务器信息
{ "_id" : "shard0001", "host" : "192.168.30.55:47018" }
active mongoses:
"3.2.1" : 1
balancer:
Currently enabled: yes
Currently running: no
Failed balancer rounds in last 5 attempts: 0
Migration Results for the last 24 hours:
5 : Success
databases:
{ "_id" : "school", "primary" : "shard0000", "partitioned" : true } #数据库school的分片信息
school.info
shard key: { "id" : 1 }
unique: false
balancing: true
chunks: #可以看到chunks均匀分布到两个分片上
shard0000 6
shard0001 5
{ "id" : { "$minKey" : 1 } } -->> { "id" : 4682 } on : shard0001 Timestamp(2, 0)
{ "id" : 4682 } -->> { "id" : 9364 } on : shard0001 Timestamp(3, 0)
{ "id" : 9364 } -->> { "id" : 14046 } on : shard0001 Timestamp(4, 0)
{ "id" : 14046 } -->> { "id" : 18728 } on : shard0001 Timestamp(5, 0)
{ "id" : 18728 } -->> { "id" : 23410 } on : shard0001 Timestamp(6, 0)
{ "id" : 23410 } -->> { "id" : 28092 } on : shard0000 Timestamp(6, 1)
{ "id" : 28092 } -->> { "id" : 32774 } on : shard0000 Timestamp(1, 6)
{ "id" : 32774 } -->> { "id" : 37456 } on : shard0000 Timestamp(1, 7)
{ "id" : 37456 } -->> { "id" : 42138 } on : shard0000 Timestamp(1, 8)
{ "id" : 42138 } -->> { "id" : 46820 } on : shard0000 Timestamp(1, 9)
{ "id" : 46820 } -->> { "id" : { "$maxKey" : 1 } } on : shard0000 Timestamp(1, 10)
mongos> sh.addShardTag("shard0000","abc01")
mongos> sh.addShardTag("shard0001","abc02")
mongos> sh.status()
--- Sharding Status ---
sharding version: {
"_id" : 1,
"minCompatibleVersion" : 5,
"currentVersion" : 6,
"clusterId" : ObjectId("5b9cbc14b4c77895df796bac")
}
shards:
{ "_id" : "shard0000", "host" : "192.168.30.55:47017", "tags" : [ "abc01" ] }
{ "_id" : "shard0001", "host" : "192.168.30.55:47018", "tags" : [ "abc02" ] }
MongoDB分片服务器管理
根据需求可以添加或删除sharding server。
[root@localhost bin]# cp mongodb3.conf mongodb4.conf
[root@localhost bin]# vim mongodb4.conf
port=47019
logpath=/data/logs/mongodb4.log
dbpath=/data/mongodb4
fork=true
logappend=true
maxConns=5000
storageEngine=mmapv1
shardsvr=true
[root@localhost bin]# mongod -f mongodb4.conf
about to fork child process, waiting until server is ready for connections.
forked process: 52634
child process started successfully, parent exiting
[root@localhost bin]# mongo
MongoDB shell version: 3.2.1
connecting to: test
Server has startup warnings:
2018-09-15T16:00:19.746+0800 I CONTROL [main] ** WARNING: You are running this process as the root user, which is not recommended.
2018-09-15T16:00:19.746+0800 I CONTROL [main]
使用sh.addShard("192.168.30.55:47019")命令添加一个新的分片服务器，再次查看分片信息，发现分片均匀地分布到3个分片服务器上
mongos> sh.addShard("192.168.30.55:47019") #添加分片服务器
{ "shardAdded" : "shard0002", "ok" : 1 }
mongos> sh.status() #查看分片服务信息
--- Sharding Status ---
sharding version: {
"_id" : 1,
"minCompatibleVersion" : 5,
"currentVersion" : 6,
"clusterId" : ObjectId("5b9cbc14b4c77895df796bac")
}
shards:
{ "_id" : "shard0000", "host" : "192.168.30.55:47017" }
{ "_id" : "shard0001", "host" : "192.168.30.55:47018" }
{ "_id" : "shard0002", "host" : "192.168.30.55:47019" }
active mongoses:
"3.2.1" : 1
balancer:
Currently enabled: yes
Currently running: no
Failed balancer rounds in last 5 attempts: 0
Migration Results for the last 24 hours:
8 : Success
databases:
{ "_id" : "school", "primary" : "shard0000", "partitioned" : true }
school.info
shard key: { "id" : 1 }
unique: false
balancing: true
chunks:
shard0000 4
shard0001 4
shard0002 3
{ "id" : { "$minKey" : 1 } } -->> { "id" : 4682 } on : shard0002 Timestamp(9, 0)
{ "id" : 4682 } -->> { "id" : 9364 } on : shard0001 Timestamp(9, 1)
{ "id" : 9364 } -->> { "id" : 14046 } on : shard0001 Timestamp(4, 0)
{ "id" : 14046 } -->> { "id" : 18728 } on : shard0001 Timestamp(5, 0)
{ "id" : 18728 } -->> { "id" : 23410 } on : shard0001 Timestamp(6, 0)
{ "id" : 23410 } -->> { "id" : 28092 } on : shard0002 Timestamp(7, 0)
{ "id" : 28092 } -->> { "id" : 32774 } on : shard0002 Timestamp(8, 0)
{ "id" : 32774 } -->> { "id" : 37456 } on : shard0000 Timestamp(8, 1)
{ "id" : 37456 } -->> { "id" : 42138 } on : shard0000 Timestamp(1, 8)
{ "id" : 42138 } -->> { "id" : 46820 } on : shard0000 Timestamp(1, 9)
{ "id" : 46820 } -->> { "id" : { "$maxKey" : 1 } } on : shard0000 Timestamp(1, 10)
使用db.runCommand({"removeshard":"192.168.30.55:47019"})命令可以删除新添加的分片服务器
mongos> use admin # 注：在admin db下执行命令。
switched to db admin
mongos> db.runCommand({"removeshard":"192.168.30.55:47019"})
{
"msg" : "draining started successfully",
"state" : "started",
"shard" : "shard0002",
"note" : "you need to drop or movePrimary these databases",
"dbsToMove" : [ ],
"ok" : 1
}
mongos> db.runCommand({"removeshard":"192.168.30.55:47019"})
{
"msg" : "removeshard completed successfully",
"state" : "completed",
"shard" : "shard0002",
"ok" : 1
}
“注意：该命令至少执行两次才能成功删除，执行到state为completed才真正删除，否则就是没有删除成功，该分片处于"draining" : true状态，该状态下不但该分片没有删除成功，而且还影响接下来删除其他分片操作，遇到该状态再执行一次removeshard即可，最好就是删除分片时一直重复执行删除命令，直到state为completed
mongos> sh.status()
--- Sharding Status ---
sharding version: {
"_id" : 1,
"minCompatibleVersion" : 5,
"currentVersion" : 6,
"clusterId" : ObjectId("5b9cbc14b4c77895df796bac")
}
shards:
{ "_id" : "shard0000", "host" : "192.168.30.55:47017" }
{ "_id" : "shard0001", "host" : "192.168.30.55:47018" }
active mongoses:
"3.2.1" : 1
balancer:
Currently enabled: yes
Currently running: no
Failed balancer rounds in last 5 attempts: 0
Migration Results for the last 24 hours:
11 : Success
databases:
{ "_id" : "school", "primary" : "shard0000", "partitioned" : true }
school.info
shard key: { "id" : 1 }
unique: false
balancing: true
chunks:
shard0000 6
shard0001 5
{ "id" : { "$minKey" : 1 } } -->> { "id" : 4682 } on : shard0000 Timestamp(10, 0)
{ "id" : 4682 } -->> { "id" : 9364 } on : shard0001 Timestamp(9, 1)
{ "id" : 9364 } -->> { "id" : 14046 } on : shard0001 Timestamp(4, 0)
{ "id" : 14046 } -->> { "id" : 18728 } on : shard0001 Timestamp(5, 0)
{ "id" : 18728 } -->> { "id" : 23410 } on : shard0001 Timestamp(6, 0)
{ "id" : 23410 } -->> { "id" : 28092 } on : shard0001 Timestamp(11, 0)
{ "id" : 28092 } -->> { "id" : 32774 } on : shard0000 Timestamp(12, 0)
{ "id" : 32774 } -->> { "id" : 37456 } on : shard0000 Timestamp(8, 1)
{ "id" : 37456 } -->> { "id" : 42138 } on : shard0000 Timestamp(1, 8)
{ "id" : 42138 } -->> { "id" : 46820 } on : shard0000 Timestamp(1, 9)
{ "id" : 46820 } -->> { "id" : { "$maxKey" : 1 } } on : shard0000 Timestamp(1, 10)
连接配置服务器查看信息
配置服务器存储了MongoDB数据库集合分片的详细信息，可以通过以下命令查看
mongo --port 37017
configsvr> use config
configsvr> show collections
....
collections
chunks
databases
....
configsvr> db.chunks.findOne()
configsvr> db.collections.find()
configsvr> db.databases.find()
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