MongoDB: Change opLog Size

If you've MongoDB Replication setup, you must be aware of opLog..! 

Just as Transaction Log in MS SQL Server or Binary log in MySQL, all those transactions on MongoDB shard will be written into a separate file, called opLog. This opLog will have a fixed size, which means, as soon as it reaches the max size, it will be overwritten. 

So, if there are huge number of transactions, you may need to increase the size of it.
To check your current opLog size:

rs.printReplicationInfo()


To change it's size, below are the plan of execution:

* If the shard in Primary, make it secondary
* Shutdown this secondary shard 
* Start it with different port -- basically, we're isolating it from replica set
* Take dump of opLog -- for safety purpose
* Create a temporary collection 
* Insert the last entry of opLog to this temporary collection
* Drop the existing opLog
* Create a new opLog with custom size
* Insert the record from temporary collection into this new opLog
* You're all set to go..!
* Re-start shard with original port

Here are the steps with commands:

Check size:
> rs.printReplicationInfo()


Make primary as secondary:
> use admin
> rs.stepDown()


Shutdown the server:
> use admin

> db.shutdownServer()


Start with dummy port:
# ./mongod --port 37010 --dbpath /paath/data/rs --logpath /path/logs/rs.log


Create Dump of opLog:
# ./mongodump --db local --collection 'oplog.rs' --host localhost --port 37010 


Create a temp collection:
> use local

> db = db.getSiblingDB('local')
local

> db.temp.drop()

false

> db.temp.save( db.oplog.rs.find( { }, { ts: 1, h: 1 } ).sort( {$natural : -1} ).limit(1).next() )

> db.temp.find()
{ "_id" : ObjectId("5436532b1cd0a62fea32625b"), "ts" : Timestamp(1412841535, 2), "h" : NumberLong("6772852693461355875") }



Drop and Create a new opLog(I'm creating here for 20 GB):
> db = db.getSiblingDB('local')

local

> db.runCommand({create: "oplog.rs", capped: true, size:(20 * 1024 * 1024 * 1024)})
{ "ok" : 1 }

> db.oplog.rs.save(db.temp.findOne())

> db.oplog.rs.find()

{ "_id" : ObjectId("5436532b1cd0a62fea32625b"), "ts" : Timestamp(1412841535, 2), "h" : NumberLong("6772852693461355875") }


Re-start as earlier:
> use admin;

> db.shutdownServer()


That's all !