pt-table-checksum

pt-table-checksum是percona公司提供的一个用于在线比对主从数据一致性的工具。


 


实现原理


将一张大表分成多个chunk,每次针对一个chunk进行校验,同时将校验的结果通过REPLACE INTO语句写入到percona.checksums表中,然后该语句通过主从复制,在SLAVE中同样执行一次,校验的结果同样是写入到percona.checksums表中,最后,通过查询percona.checksums来获取主从不一致的信息。


 


常见用法


1. 基本用法


# pt-table-checksum -h192.168.244.10 -umonitor -pmonitor123


其中,monitor的最小权限如下(第二个权限是针对percona.checksums的):


GRANT SELECT, PROCESS, SUPER, REPLICATION SLAVE, REPLICATION CLIENT ON *.* TO 'monitor'@'192.168.244.10';


GRANT ALL PRIVILEGES ON `percona`.* TO 'monitor'@'192.168.244.10';


2. pt-table-checksum默认是运行在statement下,如果是其它日志格式,需加--no-check-binlog-format参数


# pt-table-checksum -h192.168.244.10 -umonitor -pmonitor123 --no-check-binlog-format


3. 如果主从复制中加了过滤条件,譬如binlog_ignore_db或replicate_do_db之类的参数,需加--no-check-replication-filters参数


# pt-table-checksum -h192.168.244.10 -umonitor -pmonitor123 --no-check-binlog-format --no-check-replication-filters


   如果在对被过滤表进行校验时,命令hang住了,可加--replicate-database参数。


4. 基于指定库的校验


# pt-table-checksum -h192.168.244.10 -umonitor -pmonitor123 --no-check-binlog-format --databases=test,test1


5. 基于指定表的校验


# pt-table-checksum -h192.168.244.10 -umonitor -pmonitor123 --no-check-binlog-format --tables=test2.test


其它具体用法,可参考另外一篇博客:
pt-table-checksum参数详解


 


通过打开general_log来看看其具体的执行过程,注意,测试表是test.test,共1000000条记录。



17 Query    SHOW GLOBAL STATUS LIKE 'Threads_running'
查看Threads_running变量是为了查看当前系统的负载情况
17 Query /*!40101 SET @OLD_SQL_MODE := @@SQL_MODE, @@SQL_MODE := '', @OLD_QUOTE := @@SQL_QUOTE_SHOW_CREATE, @@SQL_QUOTE_SHOW_CREATE := 1 */
设置会话变量
17 Query USE `test`
17 Query SHOW CREATE TABLE `test`.`test`
查看test表的表结构,选取分片键,一般为主键或唯一索引
17 Query /*!40101 SET @@SQL_MODE := @OLD_SQL_MODE, @@SQL_QUOTE_SHOW_CREATE := @OLD_QUOTE */
17 Query EXPLAIN SELECT * FROM `test`.`test` WHERE 1=1
查看test表的大概数量
17 Query SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`test` FORCE INDEX(`PRIMARY`) ORDER BY `id` LIMIT 1 /*first lower boundary*/
选择第一个chunk的下标,即id的最小值
17 Query SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`test` FORCE INDEX (`PRIMARY`) WHERE `id` IS NOT NULL ORDER BY `id` LIMIT 1 /*key_len*/
查看索引的长度
17 Query EXPLAIN SELECT /*!40001 SQL_NO_CACHE */ * FROM `test`.`test` FORCE INDEX (`PRIMARY`) WHERE `id` >= '1' /*key_len*/
查看实际使用的索引的长度,这个针对联合索引的场景。

17 Query USE `percona`
17 Query DELETE FROM `percona`.`checksums` WHERE db = 'test' AND tbl = 'test'
从percona.checksums表中删除之前的校验记录

17 Query USE `test`
17 Query EXPLAIN SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1')) ORDER BY `id` LIMIT 2171, 2 /*next chunk boundary*/
17
Query SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1')) ORDER BY `id` LIMIT 2171, 2 /*next chunk boundary*/
确认本chunk的上限,以及下一个chunk的下限。
17 Query EXPLAIN SELECT COUNT(*) AS cnt, COALESCE(LOWER(CONV(BIT_XOR(CAST(CRC32(CONCAT_WS('#', `id`, `ip`, `no`, CONCAT(ISNULL(`ip`), ISNULL(`no`)))) AS UNSIGNED)), 10, 16)), 0) AS crc FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1')) AND ((`id` <= '2172')) /*explain checksum chunk*/
查看对本次chunk执行checksum操作的执行计划,确认读取的行数是否合理,选择的索引是否合适
17
Query REPLACE INTO `percona`.`checksums` (db, tbl, chunk, chunk_index, lower_boundary, upper_boundary, this_cnt, this_crc) SELECT 'test', 'test', '1', 'PRIMARY', '1', '2172', COUNT(*) AS cnt, COALESCE(LOWER(CONV(BIT_XOR(CAST(CRC32(CONCAT_WS('#', `id`, `ip`, `no`, CONCAT(ISNULL(`ip`), ISNULL(`no`)))) AS UNSIGNED)), 10, 16)), 0) AS crc FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1')) AND ((`id` <= '2172')) /*checksum chunk*/
关键操作,对本次chunk执行checksum操作,并将结果更新到percona.checksums表中。
17 Query SHOW WARNINGS
17 Query SELECT this_crc, this_cnt FROM `percona`.`checksums` WHERE db = 'test' AND tbl = 'test' AND chunk = '1'
查看本次操作校验的行数和校验和
17 Query UPDATE `percona`.`checksums` SET chunk_time = '0.059603', master_crc = '568c1ba3', master_cnt = '2172' WHERE db = 'test' AND tbl = 'test' AND chunk = '1'
将上面那个查询得到的行数和校验和更新到master_cnt和master_crc中。这样的话,主库的校验和在从库执行replace操作时被覆盖。

下面是针对第二个chunk执行的操作。
17 Query SHOW GLOBAL STATUS LIKE 'Threads_running'
17 Query EXPLAIN SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '2173')) ORDER BY `id` LIMIT 18219, 2 /*next chunk boundary*/
17 Query SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '2173')) ORDER BY `id` LIMIT 18219, 2 /*next chunk boundary*/
17 Query EXPLAIN SELECT COUNT(*) AS cnt, COALESCE(LOWER(CONV(BIT_XOR(CAST(CRC32(CONCAT_WS('#', `id`, `ip`, `no`, CONCAT(ISNULL(`ip`), ISNULL(`no`)))) AS UNSIGNED)), 10, 16)), 0) AS crc FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '2173')) AND ((`id` <= '20392')) /*explain checksum chunk*/
17 Query REPLACE INTO `percona`.`checksums` (db, tbl, chunk, chunk_index, lower_boundary, upper_boundary, this_cnt, this_crc) SELECT 'test', 'test', '2', 'PRIMARY', '2173', '20392', COUNT(*) AS cnt, COALESCE(LOWER(CONV(BIT_XOR(CAST(CRC32(CONCAT_WS('#', `id`, `ip`, `no`, CONCAT(ISNULL(`ip`), ISNULL(`no`)))) AS UNSIGNED)), 10, 16)), 0) AS crc FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '2173')) AND ((`id` <= '20392')) /*checksum chunk*/
17 Query SHOW WARNINGS
17 Query SELECT this_crc, this_cnt FROM `percona`.`checksums` WHERE db = 'test' AND tbl = 'test' AND chunk = '2'
17 Query UPDATE `percona`.`checksums` SET chunk_time = '0.022960', master_crc = '83371365', master_cnt = '18220' WHERE db = 'test' AND tbl = 'test' AND chunk = '2'

...
下面的校验和上面的并不相同,上述id值的范围是1~1000000,下面两个chunk的范围是<1和>1000000,为什么要这么做呢?
主要是考虑到从库有可能存在上述两个范围的数据。
17 Query SHOW GLOBAL STATUS LIKE 'Threads_running'
17 Query EXPLAIN SELECT COUNT(*), '0' FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` < '1')) ORDER BY `id` /*explain past lower chunk*/
17 Query REPLACE INTO `percona`.`checksums` (db, tbl, chunk, chunk_index, lower_boundary, upper_boundary, this_cnt, this_crc) SELECT 'test', 'test', '9', 'PRIMARY', NULL, '1', COUNT(*), '0' FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` < '1')) ORDER BY `id` /*past lower chunk*/
17 Query SHOW WARNINGS
17 Query SELECT this_crc, this_cnt FROM `percona`.`checksums` WHERE db = 'test' AND tbl = 'test' AND chunk = '9'
17 Query UPDATE `percona`.`checksums` SET chunk_time = '0.004492', master_crc = '0', master_cnt = '0' WHERE db = 'test' AND tbl = 'test' AND chunk = '9'

17 Query SHOW GLOBAL STATUS LIKE 'Threads_running'
17 Query EXPLAIN SELECT COUNT(*), '0' FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` > '1000000')) ORDER BY `id` /*explain past upper chunk*/
17 Query REPLACE INTO `percona`.`checksums` (db, tbl, chunk, chunk_index, lower_boundary, upper_boundary, this_cnt, this_crc) SELECT 'test', 'test', '10', 'PRIMARY', '1000000', NULL, COUNT(*), '0' FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` > '1000000')) ORDER BY `id` /*past upper chunk*/
17 Query SHOW WARNINGS
17 Query SELECT this_crc, this_cnt FROM `percona`.`checksums` WHERE db = 'test' AND tbl = 'test' AND chunk = '10'
17 Query UPDATE `percona`.`checksums` SET chunk_time = '0.058622', master_crc = '0', master_cnt = '0' WHERE db = 'test' AND tbl = 'test' AND chunk = '10'


 


输出结果说明


# pt-table-checksum -h 192.168.244.10 -umonitor -pmonitor123 --no-check-binlog-format



            TS ERRORS  DIFFS     ROWS  CHUNKS SKIPPED    TIME TABLE
11-09T21:19:01 0 0 0 1 0 2.768 h2.h3
11-09T21:19:06 0 0 0 1 0 3.903 hello.h1
11-09T21:19:06 0 0 0 1 0 0.620 hello.h2
11-09T21:19:10 0 0 0 1 0 1.725 mysql.columns_priv
11-09T21:19:10 0 1 4 1 0 0.457 mysql.db
11-09T21:19:11 0 0 0 1 0 0.306 mysql.event
11-09T21:19:12 0 0 0 1 0 0.721 mysql.func
11-09T21:19:13 0 0 40 1 0 0.649 mysql.help_category
11-09T21:19:14 0 0 611 6 0 1.313 mysql.help_keyword
11-09T21:19:18 0 0 1218 4 0 3.432 mysql.help_relation
11-09T21:19:20 0 0 583 1 0 1.632 mysql.help_topic
11-09T21:19:21 0 0 0 1 0 0.501 mysql.ndb_binlog_index
11-09T21:19:21 0 0 0 1 0 0.319 mysql.plugin
11-09T21:19:22 0 0 0 1 0 0.624 mysql.proc
11-09T21:19:22 0 0 0 1 0 0.626 mysql.procs_priv
11-09T21:19:23 0 0 2 1 0 0.375 mysql.proxies_priv
11-09T21:19:24 0 0 0 1 0 0.806 mysql.servers
11-09T21:19:24 0 1 3 1 0 0.292 mysql.tables_priv
11-09T21:19:24 0 0 0 1 0 0.382 mysql.time_zone
11-09T21:19:25 0 0 0 1 0 0.398 mysql.time_zone_leap_second
11-09T21:19:25 0 0 0 1 0 0.386 mysql.time_zone_name
11-09T21:19:25 0 0 0 1 0 0.393 mysql.time_zone_transition
11-09T21:19:26 0 0 0 1 0 0.313 mysql.time_zone_transition_type
11-09T21:19:26 0 1 18 1 0 0.393 mysql.user
11-09T21:19:27 0 0 1 1 0 0.642 percona.dsns
11-09T21:19:30 0 1 41 1 0 2.613 test.checksum
11-09T21:19:31 0 0 0 1 0 0.669 test.ta
11-09T21:20:13 0 1 1000000 7 0 42.009 test.test
11-09T21:20:15 0 0 5 1 0 1.818 test1.test
11-09T21:20:16 0 0 5 1 0 0.546 test1.test1
11-09T21:20:17 0 1 12 1 0 0.508 test2.test
11-09T21:20:17 0 1 0 1 0 0.387 test2.test2
11-09T21:20:18 0 1 1000 1 0 0.707 test3.test3


TS:校验完表后的时间戳


ERRORS:校验过程中出现的errors和warnings的次数。


DIFFS:所有SLAVE中checksum值不相同的chunk的数量,如一主两从中,SLAVE1的chunk1与MASTER的checksum不同,SLAVE2的chunk1和chunk2不相同,则DIFFS的值为2。如果SLAVE2的chunk2和chunk3不相同,则DIFFS为3。


ROWS:表中校验的记录数。通常情况下为表的总行数。如果指定了--where选项,则为符合条件的记录数。


CHUNKS:表被分割为多个chunk后,chunk的个数。


SKIPPED:跳过的chunk的个数,通常因为如下原因:



* MySQL not using the --chunk-index
* MySQL not using the full chunk index (--[no]check-plan)
* Chunk size is greater than --chunk-size * --chunk-size-limit
* Lock wait timeout exceeded (--retries)
* Checksum query killed (--retries)


TIME:校验表所花费的时间。


TABLE:校验的表名


 


replicate-check-only参数下的输出结果说明 


如果指定了--replicate-check-only参数,则意味着不会校验任何表,直接获取上次校验的结果。


# pt-table-checksum -h 192.168.244.10 -umonitor -pmonitor123 --no-check-binlog-format --replicate-check-only



Differences on hbase
TABLE CHUNK CNT_DIFF CRC_DIFF CHUNK_INDEX LOWER_BOUNDARY UPPER_BOUNDARY
mysql.db
1 1 1
mysql.tables_priv
1 -3 1
mysql.user
1 -4 1
test.checksum
1 0 1

Differences on test
TABLE CHUNK CNT_DIFF CRC_DIFF CHUNK_INDEX LOWER_BOUNDARY UPPER_BOUNDARY
mysql.db
1 -1 1
mysql.tables_priv
1 -1 1
mysql.user
1 -2 1
test.checksum
1 0 1
test.test
7 1000000 0 PRIMARY 1000000
test2.test
1 -10 1
test2.test2
1 1 1
test3.test3
1 2000 1


可以看出,它分别输出了不同SLAVE中的差异部分。


TABLE:校验的表名。


CHUNK:checksum值不相同的chunk的数量。


CNT_DIFF:The number of chunk rows on the replica minus the number of chunk rows on the master.即SLAVE中被校验的记录数减去MASTER中的记录数。


CRC_DIFF:1 if the CRC of the chunk on the replica is different than the CRC of the chunk on the master, else 0.如果校验值相同,则CRC_DIFF为0,否则为1。


上述test.test中CRC_DIFF中crc为0的原因是SLAVE比MASTER多1000000条记录,且这1000000条记录正好又是在最后一个chunk中(如下所示)。注:master中id最大值为1000000。



319 Query     REPLACE INTO `percona`.`checksums` (db, tbl, chunk, chunk_index, lower_boundary, upper_boundary, this_cnt, this_crc) SELECT 'test', 'test', '7', 'PRIMARY', '1000000', NULL, COUNT(*), '0' FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` > '1000000')) ORDER BY `id` /*past upper chunk*/


CHUNK_INDEX:用于将table切割成chunk的索引。


LOWER_BOUNDARY:The index values that define the lower boundary of the chunk.


UPPER_BOUNDARY:The index values that define the upper boundary of the chunk.


上述两个参数可用来定位具有不同checksum值的chunk对应的索引的上限和下限。


 


总结


1. pt-table-checksum对表进行校验时,并不需要表上面有任何索引。这时候,整张表即是一个chunk。



REPLACE INTO `percona`.`checksums` (db, tbl, chunk, chunk_index, lower_boundary, upper_boundary, this_cnt, this_crc) SELECT 'test2', 't1', '1', NULL, NULL, NULL, COUNT(*) AS cnt, COALESCE(LOWER(CONV(BIT_XOR(CAST(CRC32(CONCAT_WS('#', `id`, `name`, CONCAT(ISNULL(`id`), ISNULL(`name`)))) AS UNSIGNED)), 10, 16)), 0) AS crc FROM `test2`.`t1` /*checksum table*/ 


   如果表的行过多的话,它会报如下错误:


# pt-table-checksum -h192.168.244.10 -umonitor -pmonitor123 --chunk-size-limit=100 --no-check-binlog-format --tables=test2.t1



11-17T11:41:53 Cannot checksum table test2.t1: There is no good index and the table is oversized. at /usr/local/bin/pt-table-checksum
line
6528.


解决方法:


调整chunk-size-limit的值


# pt-table-checksum -h192.168.244.10 -umonitor -pmonitor123 --chunk-size-limit=1000 --no-check-binlog-format --tables=test2.t1



            TS ERRORS  DIFFS     ROWS  CHUNKS SKIPPED    TIME TABLE
11-17T11:41:59 0 0 100004 1 0 0.600 test2.t1


 


 


  


   


 

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