Friday, November 21, 2014

Starting replication for a very large MySQL database on Amazon EC2

In this post, I will outline steps used to start replication of a very large MySQL database from scratch.

Here is the problem that we faced:
  1. I had set up Master-Slave replication for a MySQL database in the beginning (using the simple steps outlined here)
  2. On both the Master and the Slave server, I had /var/lib/mysql point to an, IOPS optimized, EBS disk
  3. Since this data was very critical, I had set up a cron job to backup the database on the slave server
  4. I was monitoring the Master MySQL server with NewRelic and set up alerts when CPU, Memory or Disk consumption reached certain thresholds
  5. There were three mistakes that I did (for those of you setting this up from scratch):
    • Not setting up NewRelic on the Slave Server
    • Performing the backup on the same EBS disk as the DB partition
    • Not "cleaning up" or archiving older backups
  6. As a result of these mistakes, the Slave EBS disk became full and replication stopped
  7. I was blissfully unaware of this until a few days passed
  8. I cleaned up the Slave EBS disk, but I did not know where or when the replication stopped. So, I could not just resume the replication
At this time, here were the constraints that we were dealing with:
  1. There were already over 10m records in some tables and 60m records in some other tables.
  2. This was a live, production, system - so shutting it down (with a READ LOCK) was not an option
  3. Performing the replication and doing the database back up remained to be critical
  4. This task had to be completed in a few hours - because without it we ran the risk of being exposed
These constraints ruled out:
  1. Following the replication steps in the beginning of this post - since we could not do a READ LOCK
  2. Backing up the Database (on the Master) and restoring on the Slave - this would take too much time
Given these requirements and constraints, all the documented methods for resuming the Master Slave replication were ruled out. Here are steps taken to accomplish this task:

On the Master server:
  1. Edit my.cnf

  2.  server-id=1  
     binlog-format= mixed  
     binlog-do-db={DB to replicate}  

  3. Restart Master MySQL server

  4. In the mysql command line on the Master, run the following commands:

  5.  CREATE USER replicant@{SlaveDBServerIP};  
     GRANT REPLICATION SLAVE ON *.* to 'replicant'@'{SlaveDBServerIP}' IDENTIFIED BY 'r3pl_us3r';  
     GRANT RELOAD ON *.* TO 'replicant'@{SlaveDBServerIP};  
     GRANT SUPER ON *.* TO 'replicant'@{SlaveDBServerIP};  

  6. Take a back up of the database in the Master MySQL  server using:

  7.  mysqldump -u {user} -p{password} --skip-lock-tables --single-transaction --flush-logs --hex-blob --master-data=2 -A > dump.sql  

  8. Get the master_log_file and MASTER_LOG_POS from the MySQL dump:

  9.  head dump.sql -n80 | grep "MASTER_LOG_POS"  

  10. On your AWS console, take a Snapshot of the Volume housing your Master MySQL DB:
  11. Now create a Volume using the Snapshot-Id from above in the same AZ as the SlaveDB

  12. Attach this Volume to the SlaveDB instance
On the Slave Server:
  1. Point /var/lib/mysql to the mysql directory in the newly attached volume

  2. Start MySQL and verify that the MasterDB is now available on the slave server. Note that some records (towards the end) may be missing. That is ok. The steps that we will perform below will sync the DBs.

  3. Edit my.cnf

  4.  server-id=101  
     binlog-format = mixed  
     log-bin = /var/lib/mysql/mysql-bin.log  
     relay-log = /var/lib/mysql/mysql-relay-bin  
     log-slave-updates = 1  
     slave-skip-errors = all  
     read-only = 1  
     binlog-do-db={DBs to be replicated}  

  5. Restart the Slave

  6. In the mysql command line, run the following commands

  7.  change master to master_host={IP.Address.Of.Master},  
     STOP SLAVE;  

    The Output will look similar to this:

     Slave_IO_State: Waiting for master to send event  
     Master_Host: master_ip  
     Master_User: rep_user  
     Master_Port: 3306  
     Connect_Retry: 60  

    It may take a while to reach this state - depending on how many records need to be synced

No comments: