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 !

MySQL : Replication Error 1062

It's been a pretty long time without posts. So, there's a lot to blog it!

One of the common error in MySQL Replication is 'Duplicate Entry' - Error 1062. This pop-ups only if there are any manual intervention.
The SQL Thread stops with this error. Just check the record in the error on both master and slave. If you understand the exact cause, you may delete the record in slave and start replication. Or else, just skip ignore the insert statement using the below command:


mysql> STOP SLAVE;
mysql> SET GLOBAL SQL_SLAVE_SKIP_COUNTER = 1; 
mysql> START SLAVE; 

This will skip 1 statement and continues replication. If you're getting lot of such errors, again and again, just add the below entry in Slave's my.cnf and restart MySQL:

[mysqld]

slave_skip_errors    = 1062


This will keep on skipping duplicate entries whenever it occurs.

Please note that, you can use this for any other error, as well.