Yahoo: Experiences with MySQL GTID and Multi Threaded Replication

Yahoo Case Study: MySQL GTIDs and Parallel or
Multithreaded Replication
PRESENTED BY Stacy Yuan, Yashada Jadhav October 2015

About Yahoo
▪  Yahoo is focused on making the world’s daily habits inspiring and
entertaining.
▪  By creating highly personalized experiences for our users, we keep people
connected to what matters most to them, across devices and around the
world.
▪  In turn, we create value for advertisers by connecting them with the
audiences that build their businesses
▪  More than 1B monthly active users across Yahoo and Tumblr
▪  More than 575M mobile monthly active users across Yahoo and Tumblr

Ad Products Team
Mission Statement: Delivering scalable and cost efficient data services
through innovation and automation powering Yahoo Products
▪  Thousands of Production Servers
▪  OLTP systems & Data marts
▪  Database Design and Architecture
▪  Capacity Planning and Performance Reviews
▪  24x7 Monitoring and Operational Support

MySQL at Yahoo
▪  MySQL powers many mission-critical products within Advertising
and User space across Desktop and Mobile
▪  Multiple production configurations based on product requirement
▪  Yahoo Sports, Daily Fantasy: Mobile friendly
▪  Flickr: Sharded across thousands of servers
▪  DBaaS setup for multiple products
▪  Hot:Hot, Hot:Warm Configurations
▪  Versions range from Percona Server 5.5 to 5.6 including Percona
XtraDB Cluster
▪  Operating systems running customized RHEL 6.x

About Stacy
▪  14 years of experience on various flavors of relational databases.
▪  Focus on performance tuning, code reviews, database deployment
and infrastructure management for MySQL
▪  In her spare time, she enjoys reading books and doing some
volunteer work.

About Yashada
▪  MySQL DevOps Engineer with a background in database design
and performance tuning.
▪  4+ years of experience on various flavors of relational databases.
▪  Special Skills - Fluency in Sarcasm

What are the next 45 minutes about?
•  GTID Replication
● Advantages and Disadvantages
● Performance when compared to regular replication
▪  Multi threaded slaves
●  Why do we want MTS?
●  MTS vs single threaded replication - Performance tests
▪  Rolling out GTID and MTS to a live system with no downtime
▪  GTID and MTS in Production
●  Operational issues
●  Monitoring and HA
●  Backups using xtrabackup

Why go for GTID and MTS
▪  Slave promotion becomes easier with a global transaction ID
▪  Multitenant database systems suffer from problems like resource
contention due to bad queries, batch jobs etc. that affect replication.
▪  MTS without GTID - replication co-ordinates might no longer be
accurate due to multiple parallel worker threads.
▪  MTS with GTID

File-based Replication
Enables data from one MySQL database server (the master) to be
replicated to one or more MySQL database servers (the slaves) through
MySQL log file and its position
▪  Have replication user, binlog is enabled
▪  Have a copy of master database
▪  Connect to master through master_host, port, replication user,
master log file and its position.
▪  Each slave pulls the data from the master, and execute the events
to the slave.

GTID Replication
A global transaction identifier (GTID) is a unique identifier created and
associated with each transaction committed on the server of origin
(master).
GTID is unique not only to the server on which it originated, but is
unique across all servers in a given replication setup.
GTID = source_id:transaction_id
▪  The source_id identifies the originating server.
▪  The transaction_id is a sequence number determined by the order in
which the transaction was committed on this server.
Example:
▪  5c7401d3-3623-11e5-ae8c-78e7d15fd641:1-13476

GTID Replication Advantage
▪  Replication topology is easy to change - binlog file name and
position are not required any more instead we use
master_auto_position=1
▪  Failover is simplified
▪  Increase performance in relay slave - set sync_binlog=0
▪  Managing multi-tiered replication is easier
Master_log_file=‘mysql-bin.***’
Master_log_pos=****
master_auto_position=1

Replication Failover Comparison
Regular Rep Failover
If S1 is bad, S4
S5 need
to be rebuilt.
M M
GTID Rep Failover
Redirect
S4 to M
S2
S3
S1
S4
S5
S2 S1
S3 S4
S5

GTID Replication Limitations
▪  GTID does not provide replication monitoring
▪  SQL_SKIP_SLAVE_COUNTER does not work
▪  Can not force the database to start replication from specific position

GTIDs Replication Caveats
▪  Updates involving non-transactional storage engines.
▪  CREATE TABLE ... SELECT statements is not supported.
▪  Temporary table is not supported inside a transaction
To prevent GTID-based replication to fail: enforce-gtid-consistency

Replication Performance GTID vs Regular Rep
In terms of performance, GTID is almost same as regular replication. It
is slightly slower.
The reasons could be -
▪  GTIDs write more lines into binary log - information about GTID
▪  GTID performs additional checks for transactions

Single threaded replication
▪ Applications multi-threaded parallel write into master
▪ Replication from master to slave is single-threaded, it becomes
bottleneck in a busy system.
Master Slave

Multi-Threaded Slaves (MTS)
▪  Coordinator thread on slave dispatches work across several worker
threads
▪  Each worker thread commit transaction individually.
▪  Multiple active schemas/databases can take advantage of parallel
replication
Master Slave

MTS Prerequisites
▪  MySQL 5.6 above
▪  Transactions are independently based on different databases.
▪  Multitenant databases is the best to enable MTS
▪  N databases, use N parallel workers
slave_parallel_workers = N
▪  Example: 3 databases in MySQL, better to set
slave_parallel_workers =3
Master
db1, db2, db3
Slave
db1, db2, db3

Configure MTS
▪  STOP SLAVE;
▪  SET GLOBAL slave_parallel_workers=3;
▪  START SLAVE;

MTS Execution Gaps and Checkpoint
▪  Events are no longer guaranteed to be consecutive
▪  Execution gaps are tracked
▪  Checkpoints are performed from time to time
Check settings
slave_checkpoint_period default 300 ms
slave_checkpoint_group default 512 trx
▪  Exec_Master_Log_Pos shows the latest checkpoint and not latest
transaction
▪  How to fix execution gaps -
STOP SLAVE; START SLAVE UNTIL SQL_AFTER_MTS_GAPS

Convert MTS to Single-threaded
▪  Run MTS until no more gaps are found in the relay log
▪  Stop Replication
▪  Configure single threaded slave
▪  Start single threaded slave
START SLAVE UNTIL SQL_AFTER_MTS_GAPS;
SET @@GLOBAL.slave_parallel_workers = 0;
START SLAVE;

MTS Advantages and Limitations
Advantages:
▪  Take advantage of multi-core servers
▪  Changes to each schema applied and committed independently by
worker threads
▪  Smaller risk of data loss
Limitations:
▪  START SLAVE UNTIL no longer support
▪  Foreign Keys cross-referencing DBs will disable MTS
▪  No implicit transaction retry after transient failure

MTS Caveats
▪  Enforcing foreign key relationships between tables in different
databases causes MTS to use sequential mode which can have
negative impact on performance
▪  Single database replication, it slows down the replication
performance

MTS without GTID
▪  Exec_Master_Log_Pos in SHOW SLAVE STATUS is misleading.
▪  Skipping replication errors with SQL_SLAVE_SKIP_COUNTER=1 is
dangerous
▪  Backup from slave, either mysqldump and xtrabackup might not get
right position
GTID comes to the rescue

Performance Testing - GTID with MTS Setup
Test scenario:
▪  one master,
▪  two slaves (one is single-threaded replication, another slave is multi-
threaded replication both using GTID
Master
Slave1 Slave2
GTID Rep MTS GTID Rep

Replication Performance Comparison

Replication Performance Comparison
QPS is increased about 3 or 4 times
Load, CPU, and Writes per second are increased as well

GTID with MTS enabled: Things to watch out for
▪  Exec_Master_Log_Pos is no longer reliable
▪  Executed_Gtid_Set is the reliable
▪  SQL_SLAVE_SKIP_COUNTER no longer works
▪  START SLAVE UNTIL is not supported
▪  Slave_transaction_retries is treated as 0, and can not be changed.

MySQL57 GA
▪  Parallel replication improvement
Slave can apply transaction in parallel with single database/schema with --slave-parallel-
type=LOGICAL_CLOCK.
▪  GTID improvements:
●  Automatically tracks the replication position in replication stream.
●  Enable/disable GTID can be online without MySQL restart

MySQL57 SLAVE_PARALLEL_TYPE STUDY
Master
:
slave-‐parallel-‐type
DATABASE
LOGICAL_CLOCK

Master
generated
binary
logs(MB)
3924
3690

read/write
requests
18704820
17587168

read/write
requests/per
sec
20783.1
19541.25

response
Sme
AVG
ms
1.54
1.64

95
percenSle
2.21
2.36

15
mins
work
81
38

Slave
QPS
4614.648
9466.198

Rolling out GTID and MTS to production

Online Rollout GTID with MTS in Percona Server
▪  MySQL56 requires downtime to enable GTID, it is not acceptable
▪  With Percona server 5.6, with almost no downtime
The variable GTID_DEPLOYMENT_STEP plays an important role

Database Servers Setup
Dual masters setup
▪ Masters setup cross different colos.
▪ Each master carries one slave DNS
Prod master BCP master
Prod slave BCP slave

Enable GTID without downtime
Enable GTID in BCP side
1. Make sure BCP master and
BCP slave are sync
2. Stop mysqld in BCP master and BCP slave,
add gtid_deployment_step=on,
gtid_mode=ON,
enforce-gtid-consistency into my.cnf
Restart mysqld in both servers.
3. Replication from prod master to
BCP master is good.
DNS
Prod master
BCP master
GTID_deployme
nt_step=on
Prod slave
BCP slave
GTID_deployme
nt_step=on

Promote BCP master to Prod master
4. Prod master: set global read_only=on
5. BCP master:
set global gtid_deployment_step = off;
set global read_only=off;
6. The replication from BCP master to
Prod master is broken.
DNS
Prod master
BCP master
GTID_deployme
nt_step=off
Prod slave
BCP slave
GTID_deployme
nt_step=on

Enable GTID in Prod master
7. Enable GTID on old prod master and prod slave
8. Fix replication from BCP master to prod master
CHANGE MASTER TO
MASTER_AUTO_POSITION = 1;
START SLAVE;
9. Enable GTID replication from
Prod master to BCP master
10. Enable MTS in all servers
stop slave;
set global slave_parallel_workers=16;
start slave;
DNS
Prod master
GTID enabled
BCP master
GTID_deployme
nt_step=off
Prod slave
GTID enabled
BCP slave
GTID_deployme
nt_step=on

Switch back
10. Perform switchover in Prod master
Disable gtid_deployment_step across all servers.
DNS
Prod master
GTID enabled
BCP master
GTID_deployme
nt_step=off
Prod slave
GTID enabled
BCP slave
GTID_deployme
nt_step=off

Switchover Steps

•  Enable global read_only=on in prod master
•  Sanity check to make sure BCP master catch up its master
(WAIT_UNTIL_SQL_THREAD_AFTER_GTIDS)
•  Disable read_only in BCP master. BCP master becomes prod
master
Failover:
•  If prod master is unreachable, it will be failover without step 1 and 2.

GTID and MTS in Production : MySQL Ops

GTID and MTS in production : What did we learn?
▪  Errant Transactions
▪  Replication Monitoring
▪  Building slaves using xtrabackup

Errant Transaction
The errant transactions are:
They are only executed in slaves.
▪  Could result from a mistake
▪  Could be intentionally by design, such as report tables
▪  Why they cause problem
When the slave becomes the master during failover, it exchanges its own
set of executed GTIDs, then send any missing transactions to the slaves.

Errant Transaction Detection and Fix
Detect: GTID_SUBSET(slave-Executed_Gtid_Set, master-Executed_Gtid_Set)
If it returns true(1), no errant trx.
If it returns false(0), it does have errant trx.
Identify:GTID_SUBTRACT(slave-Executed_Gtid_Set, master-Executed_Gtid_Set)
It returns the errant GTID.
Fix: Inject empty transaction on all other servers or its master.
If the transaction must be executed in slave only, use
set sql_log_bin=0;

Inject Empty Transaction
Sql_skip_slave_counter=n no longer works
Execute a fake trx with the GTID that you want to skip
For example: GTID=68fb0071-299b-11e5-9cd6-78e7d15dbe38:501
STOP SLAVE;
SET GTID_NEXT="68fb0071-299b-11e5-9cd6-78e7d15dbe38:501";
BEGIN; COMMIT;
SET GTID_NEXT="AUTOMATIC";
START SLAVE;
SHOW SLAVE STATUSG # Verification

MySQL Replication Monitoring

•  Seconds_Behind_Master
A good approximation of how late the slave is only when the slave
actively processes updates.
If the network is slow or not much updates in the master, this is NOT
a good measurement.

MySQL Replication Monitoring at Yahoo
▪  MySQL Health Heartbeat
1. Master generates heartbeat by updating timestamp (last_update)
2. Slave checks the difference between current time and last_update

GTID MTS Monitoring Challenge
▪  SHOW SLAVE STATUS
▪  Seconds_Behind_Master is still a good indication of the
replication lag
▪  Retrieved_Gtid_Set: List of GTIDs received by the I/O thread,
cleared after a server restart
▪  Executed_Gtid_Set: List of GTIDs executed by the SQL thread
▪  Auto_position: 1 if GTID-based replication is enabled
▪  5.7 is using performance_schema

Build Slaves Using Xtrabackup
▪  Start Xtrabackup from either master or slave
If the backup is taken from the master,
Please check the file xtrabackup_binlog_info in the backup folder
If the backup is from slave,
Please check the file xtrabackup_slave_info
$ cat xtrabackup_slave_info
SET GLOBAL gtid_purged='ffee1ff8-363f-11e5-af47-9cb654954cac:1-29123533';
CHANGE MASTER TO MASTER_AUTO_POSITION=1

Build Slave Using Xtrabackup
Enable Replication in Slave
Issue
mysql> SET GLOBAL gtid_purged='ffee1ff8-363f-11e5-af47-9cb654954cac:1-29123533';
ERROR 1840 (HY000): @@GLOBAL.GTID_PURGED can only be set when
@@GLOBAL.GTID_EXECUTED is empty.
How to fix
▪  RESET MASTER;
▪  SET GLOBAL gtid_purged='ffee1ff8-363f-11e5-af47-9cb654954cac:1-29123533’;
▪  CHANGE MASTER TO MASTER_HOST="mastername", master_user='rep_user',
master_password='rep_password', MASTER_AUTO_POSITION = 1;
▪  START SLAVE;

Build Slave Using Xtrabackup
Still issue?
mysql> start slave;
ERROR 1872 (HY000): Slave failed to initialize relay log info structure from the repository
RESET SLAVE;
START SLAVE;

Summary
▪  GTID
▪  MTS
▪  GTID with MTS performance comparison
▪  GTID with MTS online rollout
▪  Things to watch out
▪  Rebuild slave

We would love to talk more ..
mysqlatyahoo.tumblr.com
Yashada Jadhav
yashada@yahoo-inc.com
Stacy Yuan
syuan@yahoo-inc.com

Yahoo: Experiences with MySQL GTID and Multi Threaded Replication

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Yahoo: Experiences with MySQL GTID and Multi Threaded Replication