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。

 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
 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
 Query    USE `test`
 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
 Query    /*!40101 SET @@SQL_MODE := @OLD_SQL_MODE, @@SQL_QUOTE_SHOW_CREATE := @OLD_QUOTE */
 Query    EXPLAIN SELECT * FROM `test`.`test` WHERE =1
            查看test表的大概数量
 Query    SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`test` FORCE INDEX(`PRIMARY`) ORDER BY `id` LIMIT  /*first lower boundary*/
            选择第一个chunk的下标，即id的最小值
 Query    SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`test` FORCE INDEX (`PRIMARY`) WHERE `id` IS NOT NULL ORDER BY `id` LIMIT  /*key_len*/
            查看索引的长度
 Query    EXPLAIN SELECT /*!40001 SQL_NO_CACHE */ * FROM `test`.`test` FORCE INDEX (`PRIMARY`) WHERE `id` >= '' /*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。
 Query    USE `percona`
 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
 Query    USE `test`
 Query    EXPLAIN SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '')) ORDER BY `id` LIMIT ,  /*next  chunk boundary*/
17 Query    SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '')) ORDER BY `id` LIMIT ,  /*next chunk boundary*/
            确认本chunk的上限，以及下一个chunk的下限。
 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)), , )), ) AS crc FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '')) AND ((`id` <= '')) /*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', '', 'PRIMARY', '', '', COUNT(*) AS cnt, COALESCE(LOWER(CONV(BIT_XOR(CAST(CRC32(CONCAT_WS('#', `id`, `ip`, `no`, CONCAT(ISNULL(`ip`), ISNULL(`no`)))) AS UNSIGNED)), , )), ) AS crc FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '')) AND ((`id` <= '')) /*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。
 Query    SHOW WARNINGS
 Query    SELECT this_crc, this_cnt FROM `percona`.`checksums` WHERE db = 'test' AND tbl = 'test' AND chunk = ''
            查看本次操作校验的行数和校验和
 Query    UPDATE `percona`.`checksums` SET chunk_time = '0.059603', master_crc = '568c1ba3', master_cnt = '' WHERE db = 'test' AND tbl = 'test' AND chunk = ''
            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。
 Query    SHOW GLOBAL STATUS LIKE 'Threads_running'
 Query    EXPLAIN SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '')) ORDER BY `id` LIMIT ,  /*next chunk boundary*/
 Query    SELECT /*!40001 SQL_NO_CACHE */ `id` FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '')) ORDER BY `id` LIMIT ,  /*next chunk boundary*/
 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)), , )), ) AS crc FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '')) AND ((`id` <= '')) /*explain checksum chunk*/
 Query    REPLACE INTO `percona`.`checksums` (db, tbl, chunk, chunk_index, lower_boundary, upper_boundary, this_cnt, this_crc) SELECT 'test', 'test', '', 'PRIMARY', '', '', COUNT(*) AS cnt, COALESCE(LOWER(CONV(BIT_XOR(CAST(CRC32(CONCAT_WS('#', `id`, `ip`, `no`, CONCAT(ISNULL(`ip`), ISNULL(`no`)))) AS UNSIGNED)), , )), ) AS crc FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '')) AND ((`id` <= '')) /*checksum chunk*/
 Query    SHOW WARNINGS
 Query    SELECT this_crc, this_cnt FROM `percona`.`checksums` WHERE db = 'test' AND tbl = 'test' AND chunk = ''
 Query    UPDATE `percona`.`checksums` SET chunk_time = '0.022960', master_crc = '', master_cnt = '' WHERE db = 'test' AND tbl = 'test' AND chunk = ''

...
            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。
 Query    SHOW GLOBAL STATUS LIKE 'Threads_running'
 Query    EXPLAIN SELECT  COUNT(*), '' FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` < '')) ORDER BY `id` /*explain past lower chunk*/
 Query    REPLACE INTO `percona`.`checksums` (db, tbl, chunk, chunk_index, lower_boundary, upper_boundary, this_cnt, this_crc) SELECT 'test', 'test', '', 'PRIMARY', NULL, '', COUNT(*), '' FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` < '')) ORDER BY `id` /*past lower chunk*/
 Query    SHOW WARNINGS
 Query    SELECT this_crc, this_cnt FROM `percona`.`checksums` WHERE db = 'test' AND tbl = 'test' AND chunk = ''
 Query    UPDATE `percona`.`checksums` SET chunk_time = '0.004492', master_crc = '', master_cnt = '' WHERE db = 'test' AND tbl = 'test' AND chunk = ''

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

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
-09T21::                                     2.768 h2.h3
-09T21::                                     3.903 hello.h1
-09T21::                                     0.620 hello.h2
-09T21::                                     1.725 mysql.columns_priv
-09T21::                                     0.457 mysql.db
-09T21::                                     0.306 mysql.event
-09T21::                                     0.721 mysql.func
-09T21::                                    0.649 mysql.help_category
-09T21::                                   1.313 mysql.help_keyword
-09T21::                                  3.432 mysql.help_relation
-09T21::                                   1.632 mysql.help_topic
-09T21::                                     0.501 mysql.ndb_binlog_index
-09T21::                                     0.319 mysql.plugin
-09T21::                                     0.624 mysql.proc
-09T21::                                     0.626 mysql.procs_priv
-09T21::                                     0.375 mysql.proxies_priv
-09T21::                                     0.806 mysql.servers
-09T21::                                     0.292 mysql.tables_priv
-09T21::                                     0.382 mysql.time_zone
-09T21::                                     0.398 mysql.time_zone_leap_second
-09T21::                                     0.386 mysql.time_zone_name
-09T21::                                     0.393 mysql.time_zone_transition
-09T21::                                     0.313 mysql.time_zone_transition_type
-09T21::                                    0.393 mysql.user
-09T21::                                     0.642 percona.dsns
-09T21::                                    2.613 test.checksum
-09T21::                                     0.669 test.ta
-09T21::                              42.009 test.test
-09T21::                                     1.818 test1.test
-09T21::                                     0.546 test1.test1
-09T21::                                    0.508 test2.test
-09T21::                                     0.387 test2.test2
-09T21::                                  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
mysql.tables_priv  -
mysql.user  -
test.checksum      

Differences on test
TABLE CHUNK CNT_DIFF CRC_DIFF CHUNK_INDEX LOWER_BOUNDARY UPPER_BOUNDARY
mysql.db  -
mysql.tables_priv  -
mysql.user  -
test.checksum
test.test    PRIMARY
test2.test  -
test2.test2
test3.test3      

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。

 Query     REPLACE INTO `percona`.`checksums` (db, tbl, chunk, chunk_index, lower_boundary, upper_boundary, this_cnt, this_crc) SELECT 'test', 'test', '', 'PRIMARY', '', NULL, COUNT(*), '' FROM `test`.`test` FORCE INDEX(`PRIMARY`) WHERE ((`id` > '')) 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', '', 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