修改oplog有四种方法：
方法一
步骤如下：
- 停掉所有secondary节点
- 主节点删除local目录下文件，副本节点删除数据目录下所有文件
- 修改所有节点的配置文件，如：oplogSize=1000
- 重启所有节点，包括主节点和副本节点
- 重新配置replca set，副本节点会重新同步数据（initial sync）
优点：操作简单。
缺点：需要停服务，若数据量大，数据同步代价高。
方法二
步骤如下：
- remove其中一个secondary节点，并关闭mongod服务，删除数据目录下所有文件
- 修改此节点的参数文件，如：oplogSize=1000，并启动mongod服务
- 在primary节点执行rs.add()将此节点加入到replica set
- 循环1-3步骤，将所有副本节点全部改完
- primary节点执行rs.stepDown()将主节点降级为副本节点
- 从新的主节点remove掉此节点，并进行步骤1-3
这样逐个修改每个节点，完成oplog修改。
优点：解决了方法一中的停机问题。
缺点：每个节点都要逐个重新同步，时间代价更高。
方法三
The oplog exists internally as a capped collection, so you cannot modify its size in the course of normal operations.另：改变oplog大小，需要在每个节点上执行维护模式。（官方推荐）
1.关闭mongod
关闭mongod实例，如果是primary节点，执行rs.stepDown() 降级
handong1:PRIMARY> rs.stepDown()
{
"ok" : 1,
"$clusterTime" : {
"clusterTime" : Timestamp(1619693040, 1),
"signature" : {
"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
"keyId" : NumberLong(0)
}
},
"operationTime" : Timestamp(1619693040, 1)
}
handong1:SECONDARY>
handong1:SECONDARY>
handong1:SECONDARY>
handong1:SECONDARY> use admin
switched to db admin
handong1:SECONDARY> db.shutdownServer()
2021-04-29T18:44:33.947+0800 I NETWORK [js] DBClientConnection failed to receive message from 127.0.0.1:27017 - HostUnreachable: Connection closed by peer
server should be down...
2021-04-29T18:44:33.950+0800 I NETWORK [js] trying reconnect to 127.0.0.1:27017 failed
2021-04-29T18:44:33.967+0800 I NETWORK [js] reconnect 127.0.0.1:27017 failed failed
2.修改配置文件
修改配置文件，修改端口，注释掉replSet和认证相关的设置
port=27018
fork=true
journal = true
maxConns=500
logappend=true
directoryperdb=true
dbpath=/mongodb/data
logpath=/mongodb/logs/mongodb.log
3.启动mongod实例，并备份oplog
mongod -f /mongodb/conf/mongodb.conf
about to fork child process, waiting until server is ready for connections.
forked process: 31553
child process started successfully, parent exiting
mongodump -d local -c oplog.rs --port 27018 -h 172.16.254.134 -o /mongodb/backup
2021-04-29T18:55:18.167+0800 writing local.oplog.rs to /mongodb/backup/local/oplog.rs.bson
2021-04-29T18:55:18.170+0800 done dumping local.oplog.rs (798 documents)
4.重建oplog
保存oplog最新时间点
> use local
switched to db local
> db.temp.save( db.oplog.rs.find( { }, { ts: 1, h: 1 } ).sort( {$natural : -1} ).limit(1).next() )
WriteResult({ "nInserted" : 1 })
> db.temp.find()
{ "_id" : ObjectId("608a914089abaa981f14e888"), "ts" : Timestamp(1619693066, 1), "h" : NumberLong(0) }
删除旧的oplog
db.oplog.rs.drop()
重建新的oplog，大小为2GB
> db.runCommand( { create: "oplog.rs", capped: true, size: (2 * 1024 * 1024 * 1024) } )
5.插入前面保存的oplog时间点记录
> db.oplog.rs.save( db.temp.findOne() )
> db.oplog.rs.find()
6.关闭mongod实例
> use admin
switched to db admin
> db.shutdownServer()
2021-04-29T19:06:53.745+0800 I NETWORK [js] DBClientConnection failed to receive message from 127.0.0.1:27018 - HostUnreachable: Connection closed by peer
server should be down...
2021-04-29T19:06:53.749+0800 I NETWORK [js] trying reconnect to 127.0.0.1:27018 failed
2021-04-29T19:06:53.749+0800 I NETWORK [js] reconnect 127.0.0.1:27018 failed failed
最后恢复mongodb.conf到初始状态，启动
方法四
如果你的MongoDB版本为4.0以后的版本，可以直接使用replSetResizeOplog修改。
1.查看oplog大小
handong1:SECONDARY> db.getReplicationInfo()
{
"logSizeMB" : 1000,
"usedMB" : 0.17,
"timeDiff" : 6736,
"timeDiffHours" : 1.87,
"tFirst" : "Thu Apr 29 2021 17:19:14 GMT+0800 (CST)",
"tLast" : "Thu Apr 29 2021 19:11:30 GMT+0800 (CST)",
"now" : "Thu Apr 29 2021 19:11:42 GMT+0800 (CST)"
}
2.修改oplog大小
handong1:SECONDARY> db.adminCommand({replSetResizeOplog:1,size:2000})
{
"ok" : 1,
"$clusterTime" : {
"clusterTime" : Timestamp(1619694744, 14),
"signature" : {
"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
"keyId" : NumberLong(0)
}
},
"operationTime" : Timestamp(1619694744, 14)
}
3.验证oplog大小
handong1:SECONDARY> db.getReplicationInfo()
{
"logSizeMB" : 2000,
"usedMB" : 0.18,
"timeDiff" : 6852,
"timeDiffHours" : 1.9,
"tFirst" : "Thu Apr 29 2021 17:19:14 GMT+0800 (CST)",
"tLast" : "Thu Apr 29 2021 19:13:26 GMT+0800 (CST)",
"now" : "Thu Apr 29 2021 19:13:28 GMT+0800 (CST)"
}
4.整理碎片，回收空间（可选）
handong1:SECONDARY> use local
switched to db local
handong1:SECONDARY> db.runCommand({"compact" : "oplog.rs"})
{
"ok" : 1,
"$clusterTime" : {
"clusterTime" : Timestamp(1619694840, 1),
"signature" : {
"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
"keyId" : NumberLong(0)
}
},
"operationTime" : Timestamp(1619694840, 1)
}
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