「pt-table-sync」

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pt-table-sync,有效地同步MySQL表数据
在大量的使用案例证明,该工具已经成熟了(作者是这么承诺的),而且经过了良好的测试。但使用不当依旧会有问题。

相关文章:

博客: http://www.drdobbs.com/
PostgreSQL表内容对照工具: https://www.cri.ensmp.fr/people/coelho/pg_comparator/
远程MySQL表对照: http://www.perlmonks.org/?node_id=381053
远程MySQL表对照技术解释: http://cri.ensmp.fr/classement/doc/A-375.pdf
MySQL数据比较算法: http://tinyurl.com/mysql-data-diff-algorithm
webyog中的SQLyog Job Agent(SJA,表同步工具): https://www.webyog.com/
SJA和pt-table-sync的比较: http://tinyurl.com/maatkit-vs-sqlyog

使用前要注意的问题及风险。由于pt-table-sync会修改数据,所以使用的使用要小心。注意以下几点:

  • 完整阅读了工具手册,你要清楚自己在干嘛。
  • 熟知工具的BUGS,手册里都有BUG的描述。
  • 一定要在非生产环境进行了测试。
  • 备份生产数据,并且验证备份数据。
  • 一定要先运行”–dry-run” 和 “–print”来测试同步。

命令语法

pt-table-sync [OPTIONS] DSN [DSN]

This tool changes data, so for maximum safety, you should back up your data before using it. When

synchronizing a server that is a replication slave with the “–replicate” or “–sync-to-master”

methods, it always makes the changes on the replication master, never the replication slave directly.

This is in general the only safe way to bring a replica back in sync with its master; changes to the

replica are usually the source of the problems in the first place. However, the changes it makes on

the master should be no-op changes that set the data to their current values, and actually affect only

the replica.

Sync db.tbl on host1 to host2:

pt-table-sync –execute h=host1,D=db,t=tbl h=host2

Sync all tables on host1 to host2 and host3:

pt-table-sync –execute host1 host2 host3

Make slave1 have the same data as its replication master:

pt-table-sync –execute –sync-to-master slave1

Resolve differences that pt-table-checksum found on all slaves of master1:

pt-table-sync –execute –replicate test.checksum master1

Same as above but only resolve differences on slave1:

pt-table-sync –execute –replicate test.checksum \

–sync-to-master slave1

Sync master2 in a master-master replication configuration, where master2’s copy of db.tbl is known or

suspected to be incorrect:

pt-table-sync –execute –sync-to-master h=master2,D=db,t=tbl

Note that in the master-master configuration, the following will NOT do what you want, because it will

make changes directly on master2, which will then flow through replication and change master1’s data:

# Don’t do this in a master-master setup!

pt-table-sync –execute h=master1,D=db,t=tbl master2

DESCRIPTION

pt-table-sync does one-way and bidirectional synchronization of table data. It does not synchronize

table structures, indexes, or any other schema objects. The following describes one-way

synchronization. “BIDIRECTIONAL SYNCING” is described later.

This tool is complex and functions in several different ways. To use it safely and effectively, you

should understand three things: the purpose of “–replicate”, finding differences, and specifying

hosts. These three concepts are closely related and determine how the tool will run. The following is

the abbreviated logic:

if DSN has a t part, sync only that table:
if 1 DSN:
if –sync-to-master:

The DSN is a slave. Connect to its master and sync.
if more than 1 DSN:
The first DSN is the source. Sync each DSN in turn.
else if –replicate:
if –sync-to-master:
The DSN is a slave. Connect to its master, find records

of differences, and fix.
else:
The DSN is the master. Find slaves and connect to each,

find records of differences, and fix.
else:
if only 1 DSN and –sync-to-master:
The DSN is a slave. Connect to its master, find tables and

filter with –databases etc, and sync each table to the master.
else:
find tables, filtering with –databases etc, and sync each

DSN to the first.

pt-table-sync can run in one of two ways: with “–replicate” or without. The default is to run without

“–replicate” which causes pt-table-sync to automatically find differences efficiently with one of

several algorithms (see “ALGORITHMS”). Alternatively, the value of “–replicate”, if specified, causes

pt-table-sync to use the differences already found by having previously ran pt-table-checksum with its

own “–replicate” option. Strictly speaking, you don’t need to use “–replicate” because pt-table-sync

can find differences, but many people use “–replicate” if, for example, they checksum regularly using

pt-table-checksum then fix differences as needed with pt-table-sync. If you’re unsure, read each

tool’s documentation carefully and decide for yourself, or consult with an expert.

Regardless of whether “–replicate” is used or not, you need to specify which hosts to sync. There are

two ways: with “–sync-to-master” or without. Specifying “–sync-to-master” makes pt-table-sync expect

one and only slave DSN on the command line. The tool will automatically discover the slave’s master

and sync it so that its data is the same as its master. This is accomplished by making changes on the

master which then flow through replication and update the slave to resolve its differences. Be careful

though: although this option specifies and syncs a single slave, if there are other slaves on the same

master, they will receive via replication the changes intended for the slave that you’re trying to

sync.

Alternatively, if you do not specify “–sync-to-master”, the first DSN given on the command line is the

source host. There is only ever one source host. If you do not also specify “–replicate”, then you

must specify at least one other DSN as the destination host. There can be one or more destination

hosts. Source and destination hosts must be independent; they cannot be in the same replication

topology. pt-table-sync will die with an error if it detects that a destination host is a slave

because changes are written directly to destination hosts (and it’s not safe to write directly to

slaves). Or, if you specify “–replicate” (but not “–sync-to-master”) then pt-table-sync expects one

and only one master DSN on the command line. The tool will automatically discover all the master’s

slaves and sync them to the master. This is the only way to sync several (all) slaves at once (because

“–sync-to-master” only specifies one slave).

Each host on the command line is specified as a DSN. The first DSN (or only DSN for cases like

“–sync-to-master”) provides default values for other DSNs, whether those other DSNs are specified on

the command line or auto-discovered by the tool. So in this example,

pt-table-sync –execute h=host1,u=msandbox,p=msandbox h=host2

the host2 DSN inherits the “u” and “p” DSN parts from the host1 DSN. Use the “–explain-hosts” option

to see how pt-table-sync will interpret the DSNs given on the command line.

LIMITATIONS

Replicas using row-based replication
pt-table-sync requires statement-based replication when used with the “–sync-to-master” or

“–replicate” option. Therefore it will set “binlog_format=STATEMENT” on the master for its

session if required. To do this user must have “SUPER” privilege.

OUTPUT

If you specify the “–verbose” option, you’ll see information about the differences between the tables.

There is one row per table. Each server is printed separately. For example,

# Syncing h=host1,D=test,t=test1

# DELETE REPLACE INSERT UPDATE ALGORITHM START END EXIT DATABASE.TABLE

# 0 0 3 0 Chunk 13:00:00 13:00:17 2 test.test1

Table test.test1 on host1 required 3 “INSERT” statements to synchronize and it used the Chunk algorithm

(see “ALGORITHMS”). The sync operation for this table started at 13:00:00 and ended 17 seconds later

(times taken from “NOW()” on the source host). Because differences were found, its “EXIT STATUS” was

2.

If you specify the “–print” option, you’ll see the actual SQL statements that the script uses to

synchronize the table if “–execute” is also specified.

If you want to see the SQL statements that pt-table-sync is using to select chunks, nibbles, rows,

etc., then specify “–print” once and “–verbose” twice. Be careful though: this can print a lot of

SQL statements.

There are cases where no combination of “INSERT”, “UPDATE” or “DELETE” statements can resolve

differences without violating some unique key. For example, suppose there’s a primary key on column a

and a unique key on column b. Then there is no way to sync these two tables with straightforward

UPDATE statements:

+—+—+ +—+—+

| a | b | | a | b |

+—+—+ +—+—+

| 1 | 2 | | 1 | 1 |

| 2 | 1 | | 2 | 2 |

+—+—+ +—+—+

The tool rewrites queries to “DELETE” and “REPLACE” in this case. This is automatically handled after

the first index violation, so you don’t have to worry about it.

Be careful when using pt-table-sync in any master-master setup. Master-master replication is

inherently tricky, and it’s easy to make mistakes. You need to be sure you’re using the tool correctly

for master-master replication. See the “SYNOPSIS” for the overview of the correct usage.

Also be careful with tables that have foreign key constraints with “ON DELETE” or “ON UPDATE”

definitions because these might cause unintended changes on the child tables. See

“–[no]check-child-tables”.

In general, this tool is best suited when your tables have a primary key or unique index. Although it

can synchronize data in tables lacking a primary key or unique index, it might be best to synchronize

that data by another means.

REPLICATION SAFETY

Synchronizing a replication master and slave safely is a non-trivial problem, in general. There are

all sorts of issues to think about, such as other processes changing data, trying to change data on the

slave, whether the destination and source are a master-master pair, and much more.

In general, the safe way to do it is to change the data on the master, and let the changes flow through

replication to the slave like any other changes. However, this works only if it’s possible to REPLACE

into the table on the master. REPLACE works only if there’s a unique index on the table (otherwise it

just acts like an ordinary INSERT).

If your table has unique keys, you should use the “–sync-to-master” and/or “–replicate” options to

sync a slave to its master. This will generally do the right thing. When there is no unique key on

the table, there is no choice but to change the data on the slave, and pt-table-sync will detect that

you’re trying to do so. It will complain and die unless you specify “–no-check-slave” (see

“–[no]check-slave”).

If you’re syncing a table without a primary or unique key on a master-master pair, you must change the

data on the destination server. Therefore, you need to specify “–no-bin-log” for safety (see

“–[no]bin-log”). If you don’t, the changes you make on the destination server will replicate back to

the source server and change the data there!

The generally safe thing to do on a master-master pair is to use the “–sync-to-master” option so you

don’t change the data on the destination server. You will also need to specify “–no-check-slave” to

keep pt-table-sync from complaining that it is changing data on a slave.

ALGORITHMS

pt-table-sync has a generic data-syncing framework which uses different algorithms to find differences.

The tool automatically chooses the best algorithm for each table based on indexes, column types, and

the algorithm preferences specified by “–algorithms”. The following algorithms are available, listed

in their default order of preference:

Chunk
Finds an index whose first column is numeric (including date and time types), and divides the

column’s range of values into chunks of approximately “–chunk-size” rows. Syncs a chunk at a time

by checksumming the entire chunk. If the chunk differs on the source and destination, checksums

each chunk’s rows individually to find the rows that differ.

It is efficient when the column has sufficient cardinality to make the chunks end up about the

right size.

The initial per-chunk checksum is quite small and results in minimal network traffic and memory

consumption. If a chunk’s rows must be examined, only the primary key columns and a checksum are

sent over the network, not the entire row. If a row is found to be different, the entire row will

be fetched, but not before.

Note that this algorithm will not work if chunking a char column where all the values start with

the same character. In that case, the tool will exit and suggest picking a different algorithm.

Nibble
Finds an index and ascends the index in fixed-size nibbles of “–chunk-size” rows, using a non-

backtracking algorithm (see pt-archiver for more on this algorithm). It is very similar to

“Chunk”, but instead of pre-calculating the boundaries of each piece of the table based on index

cardinality, it uses “LIMIT” to define each nibble’s upper limit, and the previous nibble’s upper

limit to define the lower limit.

It works in steps: one query finds the row that will define the next nibble’s upper boundary, and

the next query checksums the entire nibble. If the nibble differs between the source and

destination, it examines the nibble row-by-row, just as “Chunk” does.

GroupBy
Selects the entire table grouped by all columns, with a COUNT(*) column added. Compares all

columns, and if they’re the same, compares the COUNT(*) column’s value to determine how many rows

to insert or delete into the destination. Works on tables with no primary key or unique index.

Stream
Selects the entire table in one big stream and compares all columns. Selects all columns. Much

less efficient than the other algorithms, but works when there is no suitable index for them to

use.

Future Plans
Possibilities for future algorithms are TempTable (what I originally called bottom-up in earlier

versions of this tool), DrillDown (what I originally called top-down), and GroupByPrefix (similar

to how SqlYOG Job Agent works). Each algorithm has strengths and weaknesses. If you’d like to

implement your favorite technique for finding differences between two sources of data on possibly

different servers, I’m willing to help. The algorithms adhere to a simple interface that makes it

pretty easy to write your own.

BIDIRECTIONAL SYNCING

Bidirectional syncing is a new, experimental feature. To make it work reliably there are a number of

strict limitations:

* only works when syncing one server to other independent servers

* does not work in any way with replication

* requires that the table(s) are chunkable with the Chunk algorithm

* is not N-way, only bidirectional between two servers at a time

* does not handle DELETE changes

For example, suppose we have three servers: c1, r1, r2. c1 is the central server, a pseudo-master to

the other servers (viz. r1 and r2 are not slaves to c1). r1 and r2 are remote servers. Rows in table

foo are updated and inserted on all three servers and we want to synchronize all the changes between

all the servers. Table foo has columns:

id int PRIMARY KEY

ts timestamp auto updated

name varchar

Auto-increment offsets are used so that new rows from any server do not create conflicting primary key

(id) values. In general, newer rows, as determined by the ts column, take precedence when a same but

differing row is found during the bidirectional sync. “Same but differing” means that two rows have

the same primary key (id) value but different values for some other column, like the name column in

this example. Same but differing conflicts are resolved by a “conflict”. A conflict compares some

column of the competing rows to determine a “winner”. The winning row becomes the source and its

values are used to update the other row.

There are subtle differences between three columns used to achieve bidirectional syncing that you

should be familiar with: chunk column (“–chunk-column”), comparison column(s) (“–columns”), and

conflict column (“–conflict-column”). The chunk column is only used to chunk the table; e.g. “WHERE

id >= 5 AND id < 10″. Chunks are checksummed and when chunk checksums reveal a difference, the tool

selects the rows in that chunk and checksums the “–columns” for each row. If a column checksum

differs, the rows have one or more conflicting column values. In a traditional unidirectional sync,

the conflict is a moot point because it can be resolved simply by updating the entire destination row

with the source row’s values. In a bidirectional sync, however, the “–conflict-column” (in accordance

with other “–conflict-*” options list below) is compared to determine which row is “correct” or

“authoritative”; this row becomes the “source”.

To sync all three servers completely, two runs of pt-table-sync are required. The first run syncs c1

and r1, then syncs c1 and r2 including any changes from r1. At this point c1 and r2 are completely in

sync, but r1 is missing any changes from r2 because c1 didn’t have these changes when it and r1 were

synced. So a second run is needed which syncs the servers in the same order, but this time when c1 and

r1 are synced r1 gets r2’s changes.

The tool does not sync N-ways, only bidirectionally between the first DSN given on the command line and

each subsequent DSN in turn. So the tool in this example would be ran twice like:

pt-table-sync –bidirectional h=c1 h=r1 h=r2

The “–bidirectional” option enables this feature and causes various sanity checks to be performed.

You must specify other options that tell pt-table-sync how to resolve conflicts for same but differing

rows. These options are:

* –conflict-column

* –conflict-comparison

* –conflict-value

* –conflict-threshold

* –conflict-error”> (optional)

Use “–print” to test this option before “–execute”. The printed SQL statements will have comments

saying on which host the statement would be executed if you used “–execute”.

Technical side note: the first DSN is always the “left” server and the other DSNs are always the

“right” server. Since either server can become the source or destination it’s confusing to think of

them as “src” and “dst”. Therefore, they’re generically referred to as left and right. It’s easy to

remember this because the first DSN is always to the left of the other server DSNs on the command line.

EXIT STATUS

The following are the exit statuses (also called return values, or return codes) when pt-table-sync

finishes and exits.

STATUS MEANING

====== =======================================================

0 Success.

1 Internal error.

2 At least one table differed on the destination.

3 Combination of 1 and 2.

OPTIONS

Specify at least one of “–print”, “–execute”, or “–dry-run”.

“–where” and “–replicate” are mutually exclusive.

This tool accepts additional command-line arguments. Refer to the “SYNOPSIS” and usage information for

details.

–algorithms
type: string; default: Chunk,Nibble,GroupBy,Stream

Algorithm to use when comparing the tables, in order of preference.

For each table, pt-table-sync will check if the table can be synced with the given algorithms in

the order that they’re given. The first algorithm that can sync the table is used. See

“ALGORITHMS”.

–ask-pass
Prompt for a password when connecting to MySQL.

–bidirectional
Enable bidirectional sync between first and subsequent hosts.

See “BIDIRECTIONAL SYNCING” for more information.

–[no]bin-log
default: yes

Log to the binary log (“SET SQL_LOG_BIN=1”).

Specifying “–no-bin-log” will “SET SQL_LOG_BIN=0”.

–buffer-in-mysql
Instruct MySQL to buffer queries in its memory.

This option adds the “SQL_BUFFER_RESULT” option to the comparison queries. This causes MySQL to

execute the queries and place them in a temporary table internally before sending the results back

to pt-table-sync. The advantage of this strategy is that pt-table-sync can fetch rows as desired

without using a lot of memory inside the Perl process, while releasing locks on the MySQL table (to

reduce contention with other queries). The disadvantage is that it uses more memory on the MySQL

server instead.

You probably want to leave “–[no]buffer-to-client” enabled too, because buffering into a temp

table and then fetching it all into Perl’s memory is probably a silly thing to do. This option is

most useful for the GroupBy and Stream algorithms, which may fetch a lot of data from the server.

–[no]buffer-to-client
default: yes

Fetch rows one-by-one from MySQL while comparing.

This option enables “mysql_use_result” which causes MySQL to hold the selected rows on the server

until the tool fetches them. This allows the tool to use less memory but may keep the rows locked

on the server longer.

If this option is disabled by specifying “–no-buffer-to-client” then “mysql_store_result” is used

which causes MySQL to send all selected rows to the tool at once. This may result in the results

“cursor” being held open for a shorter time on the server, but if the tables are large, it could

take a long time anyway, and use all your memory.

For most non-trivial data sizes, you want to leave this option enabled.

This option is disabled when “–bidirectional” is used.

–charset
short form: -A; type: string

Default character set. If the value is utf8, sets Perl’s binmode on STDOUT to utf8, passes the

mysql_enable_utf8 option to DBD::mysql, and runs SET NAMES UTF8 after connecting to MySQL. Any

other value sets binmode on STDOUT without the utf8 layer, and runs SET NAMES after connecting to

MySQL.

–[no]check-child-tables
default: yes

Check if “–execute” will adversely affect child tables. When “–replace”, “–replicate”, or

“–sync-to-master” is specified, the tool may sync tables using “REPLACE” statements. If a table

being synced has child tables with “ON DELETE CASCADE”, “ON UPDATE CASCADE”, or “ON UPDATE SET

NULL”, the tool prints an error and skips the table because “REPLACE” becomes “DELETE” then

“INSERT”, so the “DELETE” will cascade to the child table and delete its rows. In the worst case,

this can delete all rows in child tables!

Specify “–no-check-child-tables” to disable this check. To completely avoid affecting child

tables, also specify “–no-foreign-key-checks” so MySQL will not cascade any operations from the

parent to child tables.

This check is only preformed if “–execute” and one of “–replace”, “–replicate”, or

“–sync-to-master” is specified. “–print” does not check child tables.

The error message only prints the first child table found with an “ON DELETE CASCADE”, “ON UPDATE

CASCADE”, or “ON UPDATE SET NULL” foreign key constraint. There could be other affected child

tables.

–[no]check-master
default: yes

With “–sync-to-master”, try to verify that the detected master is the real master.

–[no]check-slave
default: yes

Check whether the destination server is a slave.

If the destination server is a slave, it’s generally unsafe to make changes on it. However,

sometimes you have to; “–replace” won’t work unless there’s a unique index, for example, so you

can’t make changes on the master in that scenario. By default pt-table-sync will complain if you

try to change data on a slave. Specify “–no-check-slave” to disable this check. Use it at your

own risk.

–[no]check-triggers
default: yes

Check that no triggers are defined on the destination table.

Triggers were introduced in MySQL v5.0.2, so for older versions this option has no effect because

triggers will not be checked.

–chunk-column
type: string

Chunk the table on this column.

–chunk-index
type: string

Chunk the table using this index.

–chunk-size
type: string; default: 1000

Number of rows or data size per chunk.

The size of each chunk of rows for the “Chunk” and “Nibble” algorithms. The size can be either a

number of rows, or a data size. Data sizes are specified with a suffix of k=kibibytes,

M=mebibytes, G=gibibytes. Data sizes are converted to a number of rows by dividing by the average

row length.

–columns
short form: -c; type: array

Compare this comma-separated list of columns.

–config
type: Array

Read this comma-separated list of config files; if specified, this must be the first option on the

command line.

–conflict-column
type: string

Compare this column when rows conflict during a “–bidirectional” sync.

When a same but differing row is found the value of this column from each row is compared according

to “–conflict-comparison”, “–conflict-value” and “–conflict-threshold” to determine which row

has the correct data and becomes the source. The column can be any type for which there is an

appropriate “–conflict-comparison” (this is almost all types except, for example, blobs).

This option only works with “–bidirectional”. See “BIDIRECTIONAL SYNCING” for more information.

–conflict-comparison
type: string

Choose the “–conflict-column” with this property as the source.

The option affects how the “–conflict-column” values from the conflicting rows are compared.

Possible comparisons are one of these MAGIC_comparisons:

newest|oldest|greatest|least|equals|matches

COMPARISON CHOOSES ROW WITH

========== =========================================================

newest Newest temporal –conflict-column value

oldest Oldest temporal –conflict-column value

greatest Greatest numerical “–conflict-column value

least Least numerical –conflict-column value

equals –conflict-column value equal to –conflict-value

matches –conflict-column value matching Perl regex pattern
–conflict-value

This option only works with “–bidirectional”. See “BIDIRECTIONAL SYNCING” for more information.

–conflict-error
type: string; default: warn

How to report unresolvable conflicts and conflict errors

This option changes how the user is notified when a conflict cannot be resolved or causes some kind

of error. Possible values are:

* warn: Print a warning to STDERR about the unresolvable conflict

* die: Die, stop syncing, and print a warning to STDERR

This option only works with “–bidirectional”. See “BIDIRECTIONAL SYNCING” for more information.

–conflict-threshold
type: string

Amount by which one “–conflict-column” must exceed the other.

The “–conflict-threshold” prevents a conflict from being resolved if the absolute difference

between the two “–conflict-column” values is less than this amount. For example, if two

“–conflict-column” have timestamp values “2009-12-01 12:00:00” and “2009-12-01 12:05:00” the

difference is 5 minutes. If “–conflict-threshold” is set to “5m” the conflict will be resolved,

but if “–conflict-threshold” is set to “6m” the conflict will fail to resolve because the

difference is not greater than or equal to 6 minutes. In this latter case, “–conflict-error” will

report the failure.

This option only works with “–bidirectional”. See “BIDIRECTIONAL SYNCING” for more information.

–conflict-value
type: string

Use this value for certain “–conflict-comparison”.

This option gives the value for “equals” and “matches” “–conflict-comparison”.

This option only works with “–bidirectional”. See “BIDIRECTIONAL SYNCING” for more information.

–databases
short form: -d; type: hash

Sync only this comma-separated list of databases.

A common request is to sync tables from one database with tables from another database on the same

or different server. This is not yet possible. “–databases” will not do it, and you can’t do it

with the D part of the DSN either because in the absence of a table name it assumes the whole

server should be synced and the D part controls only the connection’s default database.

–defaults-file
short form: -F; type: string

Only read mysql options from the given file. You must give an absolute pathname.

–dry-run
Analyze, decide the sync algorithm to use, print and exit.

Implies “–verbose” so you can see the results. The results are in the same output format that

you’ll see from actually running the tool, but there will be zeros for rows affected. This is

because the tool actually executes, but stops before it compares any data and just returns zeros.

The zeros do not mean there are no changes to be made.

–engines
short form: -e; type: hash

Sync only this comma-separated list of storage engines.

–execute
Execute queries to make the tables have identical data.

This option makes pt-table-sync actually sync table data by executing all the queries that it

created to resolve table differences. Therefore, the tables will be changed! And unless you also

specify “–verbose”, the changes will be made silently. If this is not what you want, see

“–print” or “–dry-run”.

–explain-hosts
Print connection information and exit.

Print out a list of hosts to which pt-table-sync will connect, with all the various connection

options, and exit.

–float-precision
type: int

Precision for “FLOAT” and “DOUBLE” number-to-string conversion. Causes FLOAT and DOUBLE values to

be rounded to the specified number of digits after the decimal point, with the ROUND() function in

MySQL. This can help avoid checksum mismatches due to different floating-point representations of

the same values on different MySQL versions and hardware. The default is no rounding; the values

are converted to strings by the CONCAT() function, and MySQL chooses the string representation. If

you specify a value of 2, for example, then the values 1.008 and 1.009 will be rounded to 1.01, and

will checksum as equal.

–[no]foreign-key-checks
default: yes

Enable foreign key checks (“SET FOREIGN_KEY_CHECKS=1”).

Specifying “–no-foreign-key-checks” will “SET FOREIGN_KEY_CHECKS=0”.

–function
type: string

Which hash function you’d like to use for checksums.

The default is “CRC32”. Other good choices include “MD5” and “SHA1”. If you have installed the

“FNV_64” user-defined function, “pt-table-sync” will detect it and prefer to use it, because it is

much faster than the built-ins. You can also use MURMUR_HASH if you’ve installed that user-defined

function. Both of these are distributed with Maatkit. See pt-table-checksum for more information

and benchmarks.

–help
Show help and exit.

–[no]hex-blob
default: yes

“HEX()” “BLOB”, “TEXT” and “BINARY” columns.

When row data from the source is fetched to create queries to sync the data (i.e. the queries seen

with “–print” and executed by “–execute”), binary columns are wrapped in HEX() so the binary data

does not produce an invalid SQL statement. You can disable this option but you probably shouldn’t.

–host
short form: -h; type: string

Connect to host.

–ignore-columns
type: Hash

Ignore this comma-separated list of column names in comparisons.

This option causes columns not to be compared. However, if a row is determined to differ between

tables, all columns in that row will be synced, regardless. (It is not currently possible to

exclude columns from the sync process itself, only from the comparison.)

–ignore-databases
type: Hash

Ignore this comma-separated list of databases.

(system databases such as information_schema and performance_schema are ignored by default)

–ignore-engines
type: Hash; default: FEDERATED,MRG_MyISAM

Ignore this comma-separated list of storage engines.

–ignore-tables
type: Hash

Ignore this comma-separated list of tables.

Table names may be qualified with the database name.

–ignore-tables-regex
type: string; group: Filter

Ignore tables whose names match the Perl regex.

–[no]index-hint
default: yes

Add FORCE/USE INDEX hints to the chunk and row queries.

By default “pt-table-sync” adds a FORCE/USE INDEX hint to each SQL statement to coerce MySQL into

using the index chosen by the sync algorithm or specified by “–chunk-index”. This is usually a

good thing, but in rare cases the index may not be the best for the query so you can suppress the

index hint by specifying “–no-index-hint” and let MySQL choose the index.

This does not affect the queries printed by “–print”; it only affects the chunk and row queries

that “pt-table-sync” uses to select and compare rows.

–lock
type: int

Lock tables: 0=none, 1=per sync cycle, 2=per table, or 3=globally.

This uses “LOCK TABLES”. This can help prevent tables being changed while you’re examining them.

The possible values are as follows:

VALUE MEANING

===== =======================================================

0 Never lock tables.

1 Lock and unlock one time per sync cycle (as implemented
by the syncing algorithm). This is the most granular

level of locking available. For example, the Chunk

algorithm will lock each chunk of C<N> rows, and then

unlock them if they are the same on the source and the

destination, before moving on to the next chunk.
2 Lock and unlock before and after each table.

3 Lock and unlock once for every server (DSN) synced, with
C<FLUSH TABLES WITH READ LOCK>.

A replication slave is never locked if “–replicate” or “–sync-to-master” is specified, since in

theory locking the table on the master should prevent any changes from taking place. (You are not

changing data on your slave, right?) If “–wait” is given, the master (source) is locked and then

the tool waits for the slave to catch up to the master before continuing.

If “–transaction” is specified, “LOCK TABLES” is not used. Instead, lock and unlock are

implemented by beginning and committing transactions. The exception is if “–lock” is 3.

If “–no-transaction” is specified, then “LOCK TABLES” is used for any value of “–lock”. See

“–[no]transaction”.

–lock-and-rename
Lock the source and destination table, sync, then swap names. This is useful as a less-blocking

ALTER TABLE, once the tables are reasonably in sync with each other (which you may choose to

accomplish via any number of means, including dump and reload or even something like pt-archiver).

It requires exactly two DSNs and assumes they are on the same server, so it does no waiting for

replication or the like. Tables are locked with LOCK TABLES.

–password
short form: -p; type: string

Password to use when connecting. If password contains commas they must be escaped with a

backslash: “exam\,ple”

–pid
type: string

Create the given PID file. The tool won’t start if the PID file already exists and the PID it

contains is different than the current PID. However, if the PID file exists and the PID it

contains is no longer running, the tool will overwrite the PID file with the current PID. The PID

file is removed automatically when the tool exits.

–port
short form: -P; type: int

Port number to use for connection.

–print
Print queries that will resolve differences.

If you don’t trust “pt-table-sync”, or just want to see what it will do, this is a good way to be

safe. These queries are valid SQL and you can run them yourself if you want to sync the tables

manually.

–recursion-method
type: array; default: processlist,hosts

Preferred recursion method used to find slaves.

Possible methods are:

METHOD USES

=========== ==================

processlist SHOW PROCESSLIST

hosts SHOW SLAVE HOSTS

none Do not find slaves

The processlist method is preferred because SHOW SLAVE HOSTS is not reliable. However, the hosts

method is required if the server uses a non-standard port (not 3306). Usually pt-table-sync does

the right thing and finds the slaves, but you may give a preferred method and it will be used

first. If it doesn’t find any slaves, the other methods will be tried.

–replace
Write all “INSERT” and “UPDATE” statements as “REPLACE”.

This is automatically switched on as needed when there are unique index violations.

–replicate
type: string

Sync tables listed as different in this table.

Specifies that “pt-table-sync” should examine the specified table to find data that differs. The

table is exactly the same as the argument of the same name to pt-table-checksum. That is, it

contains records of which tables (and ranges of values) differ between the master and slave.

For each table and range of values that shows differences between the master and slave,

“pt-table-checksum” will sync that table, with the appropriate “WHERE” clause, to its master.

This automatically sets “–wait” to 60 and causes changes to be made on the master instead of the

slave.

If “–sync-to-master” is specified, the tool will assume the server you specified is the slave, and

connect to the master as usual to sync.

Otherwise, it will try to use “SHOW PROCESSLIST” to find slaves of the server you specified. If it

is unable to find any slaves via “SHOW PROCESSLIST”, it will inspect “SHOW SLAVE HOSTS” instead.

You must configure each slave’s “report-host”, “report-port” and other options for this to work

right. After finding slaves, it will inspect the specified table on each slave to find data that

needs to be synced, and sync it.

The tool examines the master’s copy of the table first, assuming that the master is potentially a

slave as well. Any table that shows differences there will NOT be synced on the slave(s). For

example, suppose your replication is set up as A->B, B->C, B->D. Suppose you use this argument and

specify server B. The tool will examine server B’s copy of the table. If it looks like server B’s

data in table “test.tbl1” is different from server A’s copy, the tool will not sync that table on

servers C and D.

–slave-user
type: string

Sets the user to be used to connect to the slaves. This parameter allows you to have a different

user with less privileges on the slaves but that user must exist on all slaves.

–slave-password
type: string

Sets the password to be used to connect to the slaves. It can be used with –slave-user and the

password for the user must be the same on all slaves.

–set-vars
type: Array

Set the MySQL variables in this comma-separated list of “variable=value” pairs.

By default, the tool sets:

wait_timeout=10000

Variables specified on the command line override these defaults. For example, specifying

“–set-vars wait_timeout=500” overrides the defaultvalue of 10000.

The tool prints a warning and continues if a variable cannot be set.

–socket
short form: -S; type: string

Socket file to use for connection.

–sync-to-master
Treat the DSN as a slave and sync it to its master.

Treat the server you specified as a slave. Inspect “SHOW SLAVE STATUS”, connect to the server’s

master, and treat the master as the source and the slave as the destination. Causes changes to be

made on the master. Sets “–wait” to 60 by default, sets “–lock” to 1 by default, and disables

“–[no]transaction” by default. See also “–replicate”, which changes this option’s behavior.

–tables
short form: -t; type: hash

Sync only this comma-separated list of tables.

Table names may be qualified with the database name.

–timeout-ok
Keep going if “–wait” fails.

If you specify “–wait” and the slave doesn’t catch up to the master’s position before the wait

times out, the default behavior is to abort. This option makes the tool keep going anyway.

Warning: if you are trying to get a consistent comparison between the two servers, you probably

don’t want to keep going after a timeout.

–[no]transaction
Use transactions instead of “LOCK TABLES”.

The granularity of beginning and committing transactions is controlled by “–lock”. This is

enabled by default, but since “–lock” is disabled by default, it has no effect.

Most options that enable locking also disable transactions by default, so if you want to use

transactional locking (via “LOCK IN SHARE MODE” and “FOR UPDATE”, you must specify “–transaction”

explicitly.

If you don’t specify “–transaction” explicitly “pt-table-sync” will decide on a per-table basis

whether to use transactions or table locks. It currently uses transactions on InnoDB tables, and

table locks on all others.

If “–no-transaction” is specified, then “pt-table-sync” will not use transactions at all (not even

for InnoDB tables) and locking is controlled by “–lock”.

When enabled, either explicitly or implicitly, the transaction isolation level is set “REPEATABLE

READ” and transactions are started “WITH CONSISTENT SNAPSHOT”.

–trim
“TRIM()” “VARCHAR” columns in “BIT_XOR” and “ACCUM” modes. Helps when comparing MySQL 4.1 to >=

5.0.

This is useful when you don’t care about the trailing space differences between MySQL versions

which vary in their handling of trailing spaces. MySQL 5.0 and later all retain trailing spaces in

“VARCHAR”, while previous versions would remove them.

–[no]unique-checks
default: yes

Enable unique key checks (“SET UNIQUE_CHECKS=1”).

Specifying “–no-unique-checks” will “SET UNIQUE_CHECKS=0”.

–user
short form: -u; type: string

User for login if not current user.

–verbose
short form: -v; cumulative: yes

Print results of sync operations.

See “OUTPUT” for more details about the output.

–version
Show version and exit.

–[no]version-check
default: yes

Check for the latest version of Percona Toolkit, MySQL, and other programs.

This is a standard “check for updates automatically” feature, with two additional features. First,

the tool checks the version of other programs on the local system in addition to its own version.

For example, it checks the version of every MySQL server it connects to, Perl, and the Perl module

DBD::mysql. Second, it checks for and warns about versions with known problems. For example,

MySQL 5.5.25 had a critical bug and was re-released as 5.5.25a.

Any updates or known problems are printed to STDOUT before the tool’s normal output. This feature

should never interfere with the normal operation of the tool.

For more information, visit <
–wait
short form: -w; type: time

How long to wait for slaves to catch up to their master.

Make the master wait for the slave to catch up in replication before comparing the tables. The

value is the number of seconds to wait before timing out (see also “–timeout-ok”). Sets “–lock”

to 1 and “–[no]transaction” to 0 by default. If you see an error such as the following,

MASTER_POS_WAIT returned -1

It means the timeout was exceeded and you need to increase it.

The default value of this option is influenced by other options. To see what value is in effect,

run with “–help”.

To disable waiting entirely (except for locks), specify “–wait” 0. This helps when the slave is

lagging on tables that are not being synced.

–where
type: string

“WHERE” clause to restrict syncing to part of the table.

–[no]zero-chunk
default: yes

Add a chunk for rows with zero or zero-equivalent values. The only has an effect when

“–chunk-size” is specified. The purpose of the zero chunk is to capture a potentially large

number of zero values that would imbalance the size of the first chunk. For example, if a lot of

negative numbers were inserted into an unsigned integer column causing them to be stored as zeros,

then these zero values are captured by the zero chunk instead of the first chunk and all its non-

zero values.

DSN OPTIONS

These DSN options are used to create a DSN. Each option is given like “option=value”. The options are

case-sensitive, so P and p are not the same option. There cannot be whitespace before or after the “=”

and if the value contains whitespace it must be quoted. DSN options are comma-separated. See the

percona-toolkit manpage for full details.

· A

dsn: charset; copy: yes

Default character set.

· D

dsn: database; copy: yes

Database containing the table to be synced.

· F

dsn: mysql_read_default_file; copy: yes

Only read default options from the given file

· h

dsn: host; copy: yes

Connect to host.

· p

dsn: password; copy: yes

Password to use when connecting. If password contains commas they must be escaped with a

backslash: “exam\,ple”

· P

dsn: port; copy: yes

Port number to use for connection.

· S

dsn: mysql_socket; copy: yes

Socket file to use for connection.

· t

copy: yes

Table to be synced.

· u

dsn: user; copy: yes

User for login if not current user.

ENVIRONMENT

The environment variable “PTDEBUG” enables verbose debugging output to STDERR. To enable debugging and

capture all output to a file, run the tool like:

PTDEBUG=1 pt-table-sync … > FILE 2>&1

Be careful: debugging output is voluminous and can generate several megabytes of output.

SYSTEM REQUIREMENTS

You need Perl, DBI, DBD::mysql, and some core packages that ought to be installed in any reasonably new

version of Perl.

BUGS

· Tool “–version”

· MySQL version of all servers involved

· Output from the tool including STDERR

· Input files (log/dump/config files, etc.)

If possible, include debugging output by running the tool with “PTDEBUG”; see “ENVIRONMENT”.