All of these approaches show that you gave these things a lot of thought.
You are worried about any pending changes when running FLUSH TABLES WITH READ LOCK;
.
Think about this: When you issue FLUSH TABLES WITH READ LOCK;
, how is replication affected? Recall that replication has two threads
The IO Thread is responsible for communication between Master and Slave. It downloads binary log entries from the Master and stores them in the Slave's relay logs.
The SQL Thread is responsible for
- reading the next SQL statement from the Slave's relay logs and processing them
- maintain are temp tables created within the session of the SQL Thread
When you run FLUSH TABLES WITH READ LOCK;
, only the SQL Thread gets affected because it needs to connect to tables. The IO Thread can still collect binary log entries from the Master and store them in the Slave's relay logs. Any replication lag will simply be caught off guard as is. In light of this, STOP SLAVE;
should be faster than FLUSH TABLES WITH READ LOCK;
. If you are concerned about pending changes, then use STOP SLAVE SQL_THREAD;
instead of STOP SLAVE;
. That way, whatever is last executed on each Master should be checked.
When you do SHOW SLAVE STATUS\G
look for two lines
- Relay_Master_Log_File (line 10)
- Exec_Master_Log_Pos (line 22)
This tells you what was the SQL statement downloaded to the Slave that was last executed.
Knowing this, you could try the following
- Step 01 : On M1 and M2,
STOP SLAVE SQL_THREAD;
- Step 02 : Run
SHOW MASTER STATUS;
on M1 and M2
- Step 03 : Run
SHOW SLAVE STATUS\G
on M1 and M2
- Step 04 : Evaluate this condition
- Does M1's File = M2's Relay_Master_Log_File ?
- Does M2's File = M1's Relay_Master_Log_File ?
- Does M1's Position = M2's Exec_Master_Log_Pos ?
- Does M2's Position = M1's Exec_Master_Log_Pos ?
- Step 05 : If any one of the four conditions in Step 04 is not met
- On M1 and M2,
START SLAVE SQL_THREAD;
- SELECT SLEEP(30);
- Go Back to Step 01
If you get past Step 05 with all four conditions in Step 04, M1 and M2 are in sync.
Once M1 and M2 are frozen simultaneously
- S1 should match M1
- Wait until S1's Seconds_Behind_Master = 0
- M1's File = S1's Relay_Master_Log_File
- M1's Position = S1's Exec_Master_Log_Pos
- S2 should match M2
- Wait until S2's Seconds_Behind_Master = 0
- M2's File = S2's Relay_Master_Log_File
- M2's Position = S2's Exec_Master_Log_Pos
- No need to run
STOP SLAVE;
on S1 or S2
I hope this helps
UPDATE 2012-05-11 17:30 EDT
Once S1 and S2 match up with their respective Master, you could STOP SLAVE;
if you want to. Since M1 and M2 are frozen, no other changes can reach S1 or S2. Thus, STOP SLAVE;
is not a requirement but you do so anyway.
UPDATE 2012-05-11 21:29 EDT
Your Comment
M1/M2 are frozen from receiving updates from one another but not from receiving a legit update from an external client/application, no?
Are you still accepting incoming feeds? You did say in the original question
As I try thinking this out I keep running into gotchas that won't quite work out.
That would certainly be one gotcha. Therefore, discontinue incoming feeds.
Since you want to do FLUSH TABLES WITH READ LOCK;
to M1 and M2, I have one recommendation. Please set this one hour before syncing everything:
SET GLOBAL innodb_max_dirty_pages_pct = 0;
This will clear all dirty pages from the InnoDB Buffer Pool. That way, the time for FLUSH TABLES WITH READ LOCK;
is as fast as possible. When all syncing is done, set it back to 90 (if running MySQL 5.5) or 75 (otherwise).
Your Comment
I could see how M1/M2 were locked if they flushed w/ read lock but it seemed your steps were not including such a step
I was not including such a step because I was under the impression you would disable outside feeds.
The simple answer is No.
In Mysql replication, Master copies the bin log files to slaves, and after that, it's work is over. Now the Slave will run the bin files and execute them, but there won't be any performance on Master.
There might be scenario where you are using full synchronous replication, in which master will wait for the slave to execute the query, but again it won't impact the performance in terms of memory or CPU, but the master will wait for the query to be executed.
Also, for your second question, Phil already answered it, that ssh sends data through encryption which uses a lot of CPU, hence if you want other ways, use the other methods which are described by Phil.
Best Answer
1 Stop the slave on the first master
If the second master becomes a read only then the first master should no longer replicate from it.
2 Change the 2nd master's configuration to read only.
Add read_only=1 to my.cnf for the next restart
Read Only variable
3 Unlock the tables on the second master
You don't need to lock tables on the second master (now slave)
4 Secure privileges
Redefine privileges on the 2nd master, follow this answer here : Should a MySQL replication slave be set to read only?
5 Verify slave status on 2nd Master, now slave.
If the second master is not replicating then you may have to reinitialize the replication to the first master.
A. On first master, identify current log and position
B. On second master now slave, reinitalize replication
C. Show status on second master(now slave)