Important Caveats When Using pt-online-schema-change

 

Using pt-online-schema-change is a great way to change a table without downtime or switching over to a secondary database first.  While using it on production systems, we found some interesting issues to consider.

 

1) There were foreign_keys referencing the table we were altering.  There are two options to handle this.  They are specified with "--update-foreign-keys-method rebuild_constraints|drop_old_table".  One is to rebuild the tables with foreign keys.  This does lock the tables, and will block reads and writes while it is happening.  I chose to use drop_old_table.  This method disables foreign key checks, then drops the original table and renames the new table before reenabling foreign key checks.  Normally, pt-online-schema-change just renames the tables, then drops the old table, so locking is minimal.  We did find that there was locking while dropping the original table and renaming the new table.  This is something to be cautious with, especially with large tables.  In theory, you could run an online change to drop the foreign key constraints on the child tables prior to working on the parent table.

 

2) When testing, this ran in 15 minutes against an unladen database and took over 24 minutes against production.  You could add --sleep n, where n is a number of seconds, if you need it to add less load to production.  

 

3) One limit we also saw is that pt-online-schema-change will only work on tables with at least one unique single column index, such as a single column primary key.  Often partitioned tables don't have a unique single column index.

 

 

Here is some of the output of a sample pt-online-schema-change...

 
 
## Install pt-online-schema-change and pre-requisites...
wget percona.com/get/pt-online-schema-change
chmod 755 pt-online-schema-change
yum install perl-DBI
yum install perl-DBD-MySQL
 
time ./pt-online-schema-change h=127.0.0.1,D=my_schema,t=my_table -P3306 --alter "add column signature varbinary(255) NULL DEFAULT NULL; add column signature_version varchar(4) NULL DEFAULT NULL" --child-tables fk_table1,fk_table2 --update-foreign-keys-method drop_old_table -pXXXX --bin-log
# 2011-10-14T03:20:28 ./pt-online-schema-change started
# 2011-10-14T03:20:28 USE `my_schema`
# 2011-10-14T03:20:28 Alter table my_table using temporary table __tmp_my_table
# 2011-10-14T03:20:28 Checking if table my_table can be altered
# 2011-10-14T03:20:28 SHOW TRIGGERS FROM `my_schema` LIKE 'my_table'
# 2011-10-14T03:20:28 Table my_table can be altered
# 2011-10-14T03:20:28 Chunk column id, index PRIMARY
# 2011-10-14T03:20:28 Chunked table my_table into 25685 chunks
# 2011-10-14T03:20:28 User-specified child tables: fk_table1, fk_table2
# 2011-10-14T03:20:28 Starting online schema change
# 2011-10-14T03:20:28 CREATE TABLE `my_schema`.`__tmp_my_table` LIKE `my_schema`.`my_table`
# 2011-10-14T03:20:28 ALTER TABLE `my_schema`.`__tmp_my_table` add column signature varbinary(255) NULL DEFAULT NULL
# 2011-10-14T03:20:29 ALTER TABLE `my_schema`.`__tmp_my_table`  add column signature_version varchar(4) NULL DEFAULT NULL
# 2011-10-14T03:20:30 Shared columns: id, col1, col2, col3, col4, col5
# 2011-10-14T03:20:30 Calling OSCCaptureSync::capture()
# 2011-10-14T03:20:30 CREATE TRIGGER mk_osc_del AFTER DELETE ON `my_schema`.`my_table` FOR EACH ROW DELETE IGNORE FROM `my_schema`.`__tmp_my_table` WHERE `my_schema`.`__tmp_my_table`.id = OLD.id
# 2011-10-14T03:20:30 CREATE TRIGGER mk_osc_upd AFTER UPDATE ON `my_schema`.`my_table` FOR EACH ROW REPLACE INTO `my_schema`.`__tmp_my_table` (id, col1, col2, col3, col4, col5) VALUES (NEW.id, NEW.col1, NEW.col2, NEW.col3, NEW.col4, NEW.col5)
# 2011-10-14T03:20:30 CREATE TRIGGER mk_osc_ins AFTER INSERT ON `my_schema`.`my_table` FOR EACH ROW REPLACE INTO `my_schema`.`__tmp_my_table` (id, col1, col2, col3, col4, col5) VALUES(NEW.id, NEW.col1, NEW.col2, NEW.col3, NEW.col4, NEW.col5)
# 2011-10-14T03:20:30 Calling CopyRowsInsertSelect::copy()
Copying rows:   1% 24:52 remain
Copying rows:   3% 25:01 remain
...
Copying rows:  96% 00:43 remain
Copying rows:  98% 00:15 remain
# 2011-10-14T03:44:14 Calling OSCCaptureSync::sync()
# 2011-10-14T03:44:14 Renaming foreign key constraints in child table
# 2011-10-14T03:44:14 SET foreign_key_checks=0
# 2011-10-14T03:44:14 DROP TABLE `my_schema`.`my_table`
# 2011-10-14T03:44:49 RENAME TABLE `my_schema`.`__tmp_my_table` TO `my_schema`.`my_table`
# 2011-10-14T03:44:49 Calling CopyRowsInsertSelect::cleanup()
# 2011-10-14T03:44:49 Calling OSCCaptureSync::cleanup()
# 2011-10-14T03:44:49 DROP TRIGGER IF EXISTS `my_schema`.`mk_osc_del`
# 2011-10-14T03:44:49 DROP TRIGGER IF EXISTS `my_schema`.`mk_osc_ins`
# 2011-10-14T03:44:49 DROP TRIGGER IF EXISTS `my_schema`.`mk_osc_upd`
# 2011-10-14T03:44:49 ./pt-online-schema-change ended, exit status 0
 
real    24m20.777s
user    0m3.936s
sys     0m1.216s

Comments

Hi!

Yes, we also had similar issues with Percona Toolkit. But recently, we switched to Openark Kit and it works for our needs so far. Here is another issue with Percona Toolkit we faced: http://stackoverflow.com/questions/9187097/percona-toolkit-vs-openark-ki...

LngPHP
Tue, 02/07/2012 - 23:00

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