Provided by: percona-toolkit_3.2.1-1_all 
NAME
pt-table-checksum - Verify MySQL replication integrity.
SYNOPSIS
Usage: pt-table-checksum [OPTIONS] [DSN]
pt-table-checksum performs an online replication consistency check by executing checksum
queries on the master, which produces different results on replicas that are inconsistent
with the master. The optional DSN specifies the master host. The tool's "EXIT STATUS" is
non-zero if any differences are found, or if any warnings or errors occur.
The following command will connect to the replication master on localhost, checksum every
table, and report the results on every detected replica:
pt-table-checksum
This tool is focused on finding data differences efficiently. If any data is different,
you can resolve the problem with pt-table-sync.
RISKS
Percona Toolkit is mature, proven in the real world, and well tested, but all database
tools can pose a risk to the system and the database server. Before using this tool,
please:
• Read the tool's documentation
• Review the tool's known "BUGS"
• Test the tool on a non-production server
• Backup your production server and verify the backups
See also "LIMITATIONS".
DESCRIPTION
pt-table-checksum is designed to do the right thing by default in almost every case. When
in doubt, use "--explain" to see how the tool will checksum a table. The following is a
high-level overview of how the tool functions.
In contrast to older versions of pt-table-checksum, this tool is focused on a single
purpose, and does not have a lot of complexity or support many different checksumming
techniques. It executes checksum queries on only one server, and these flow through
replication to re-execute on replicas. If you need the older behavior, you can use
Percona Toolkit version 1.0.
pt-table-checksum connects to the server you specify, and finds databases and tables that
match the filters you specify (if any). It works one table at a time, so it does not
accumulate large amounts of memory or do a lot of work before beginning to checksum. This
makes it usable on very large servers. We have used it on servers with hundreds of
thousands of databases and tables, and trillions of rows. No matter how large the server
is, pt-table-checksum works equally well.
One reason it can work on very large tables is that it divides each table into chunks of
rows, and checksums each chunk with a single REPLACE..SELECT query. It varies the chunk
size to make the checksum queries run in the desired amount of time. The goal of chunking
the tables, instead of doing each table with a single big query, is to ensure that
checksums are unintrusive and don't cause too much replication lag or load on the server.
That's why the target time for each chunk is 0.5 seconds by default.
The tool keeps track of how quickly the server is able to execute the queries, and adjusts
the chunks as it learns more about the server's performance. It uses an exponentially
decaying weighted average to keep the chunk size stable, yet remain responsive if the
server's performance changes during checksumming for any reason. This means that the tool
will quickly throttle itself if your server becomes heavily loaded during a traffic spike
or a background task, for example.
Chunking is accomplished by a technique that we used to call "nibbling" in other tools in
Percona Toolkit. It is the same technique used for pt-archiver, for example. The legacy
chunking algorithms used in older versions of pt-table-checksum are removed, because they
did not result in predictably sized chunks, and didn't work well on many tables. All that
is required to divide a table into chunks is an index of some sort (preferably a primary
key or unique index). If there is no index, and the table contains a suitably small
number of rows, the tool will checksum the table in a single chunk.
pt-table-checksum has many other safeguards to ensure that it does not interfere with any
server's operation, including replicas. To accomplish this, pt-table-checksum detects
replicas and connects to them automatically. (If this fails, you can give it a hint with
the "--recursion-method" option.)
The tool monitors replicas continually. If any replica falls too far behind in
replication, pt-table-checksum pauses to allow it to catch up. If any replica has an
error, or replication stops, pt-table-checksum pauses and waits. In addition, pt-table-
checksum looks for common causes of problems, such as replication filters, and refuses to
operate unless you force it to. Replication filters are dangerous, because the queries
that pt-table-checksum executes could potentially conflict with them and cause replication
to fail.
pt-table-checksum verifies that chunks are not too large to checksum safely. It performs
an EXPLAIN query on each chunk, and skips chunks that might be larger than the desired
number of rows. You can configure the sensitivity of this safeguard with the
"--chunk-size-limit" option. If a table will be checksummed in a single chunk because it
has a small number of rows, then pt-table-checksum additionally verifies that the table
isn't oversized on replicas. This avoids the following scenario: a table is empty on the
master but is very large on a replica, and is checksummed in a single large query, which
causes a very long delay in replication.
There are several other safeguards. For example, pt-table-checksum sets its session-level
innodb_lock_wait_timeout to 1 second, so that if there is a lock wait, it will be the
victim instead of causing other queries to time out. Another safeguard checks the load on
the database server, and pauses if the load is too high. There is no single right answer
for how to do this, but by default pt-table-checksum will pause if there are more than 25
concurrently executing queries. You should probably set a sane value for your server with
the "--max-load" option.
Checksumming usually is a low-priority task that should yield to other work on the server.
However, a tool that must be restarted constantly is difficult to use. Thus, pt-table-
checksum is very resilient to errors. For example, if the database administrator needs to
kill pt-table-checksum's queries for any reason, that is not a fatal error. Users often
run pt-kill to kill any long-running checksum queries. The tool will retry a killed query
once, and if it fails again, it will move on to the next chunk of that table. The same
behavior applies if there is a lock wait timeout. The tool will print a warning if such
an error happens, but only once per table. If the connection to any server fails, pt-
table-checksum will attempt to reconnect and continue working.
If pt-table-checksum encounters a condition that causes it to stop completely, it is easy
to resume it with the "--resume" option. It will begin from the last chunk of the last
table that it processed. You can also safely stop the tool with CTRL-C. It will finish
the chunk it is currently processing, and then exit. You can resume it as usual
afterwards.
After pt-table-checksum finishes checksumming all of the chunks in a table, it pauses and
waits for all detected replicas to finish executing the checksum queries. Once that is
finished, it checks all of the replicas to see if they have the same data as the master,
and then prints a line of output with the results. You can see a sample of its output
later in this documentation.
The tool prints progress indicators during time-consuming operations. It prints a
progress indicator as each table is checksummed. The progress is computed by the
estimated number of rows in the table. It will also print a progress report when it pauses
to wait for replication to catch up, and when it is waiting to check replicas for
differences from the master. You can make the output less verbose with the "--quiet"
option.
If you wish, you can query the checksum tables manually to get a report of which tables
and chunks have differences from the master. The following query will report every
database and table with differences, along with a summary of the number of chunks and rows
possibly affected:
SELECT db, tbl, SUM(this_cnt) AS total_rows, COUNT(*) AS chunks
FROM percona.checksums
WHERE (
master_cnt <> this_cnt
OR master_crc <> this_crc
OR ISNULL(master_crc) <> ISNULL(this_crc))
GROUP BY db, tbl;
The table referenced in that query is the checksum table, where the checksums are stored.
Each row in the table contains the checksum of one chunk of data from some table in the
server.
Version 2.0 of pt-table-checksum is not backwards compatible with pt-table-sync version
1.0. In some cases this is not a serious problem. Adding a "boundaries" column to the
table, and then updating it with a manually generated WHERE clause, may suffice to let pt-
table-sync version 1.0 interoperate with pt-table-checksum version 2.0. Assuming an
integer primary key named 'id', You can try something like the following:
ALTER TABLE checksums ADD boundaries VARCHAR(500);
UPDATE checksums
SET boundaries = COALESCE(CONCAT('id BETWEEN ', lower_boundary,
' AND ', upper_boundary), '1=1');
Take into consideration that by default, pt-table-checksum use "CRC32" checksums. "CRC32"
is not a cryptographic algorithm and for that reason it is prone to have collisions. On
the other hand, "CRC32" algorithm is faster and less CPU-intensive than "MD5" and "SHA1".
Related reading material: Percona Toolkit UDFs:
<https://www.percona.com/doc/percona-server/LATEST/management/udf_percona_toolkit.html>
How to avoid hash collisions when using MySQL’s CRC32 function:
<https://www.percona.com/blog/2014/10/13/how-to-avoid-hash-collisions-when-using-mysqls-crc32-function/>
LIMITATIONS
Replicas using row-based replication
pt-table-checksum requires statement-based replication, and it sets
"binlog_format=STATEMENT" on the master, but due to a MySQL limitation replicas do not
honor this change. Therefore, checksums will not replicate past any replicas using
row-based replication that are masters for further replicas.
The tool automatically checks the "binlog_format" on all servers. See
"--[no]check-binlog-format" .
(Bug 899415 <https://bugs.launchpad.net/percona-toolkit/+bug/899415>)
Schema and table differences
The tool presumes that schemas and tables are identical on the master and all
replicas. Replication will break if, for example, a replica does not have a schema
that exists on the master (and that schema is checksummed), or if the structure of a
table on a replica is different than on the master.
Percona XtraDB Cluster
pt-table-checksum works with Percona XtraDB Cluster (PXC) 5.5.28-23.7 and newer. The
number of possible Percona XtraDB Cluster setups is large given that it can be used with
regular replication as well. Therefore, only the setups listed below are supported and
known to work. Other setups, like cluster to cluster, are not support and probably don't
work.
Except where noted, all of the following supported setups require that you use the "dsn"
method for "--recursion-method" to specify cluster nodes. Also, the lag check (see
"REPLICA CHECKS") is not performed for cluster nodes.
Single cluster
The simplest PXC setup is a single cluster: all servers are cluster nodes, and there
are no regular replicas. If all nodes are specified in the DSN table (see
"--recursion-method"), then you can run the tool on any node and any diffs on any
other nodes will be detected.
All nodes must be in the same cluster (have the same "wsrep_cluster_name" value), else
the tool exits with an error. Although it's possible to have different clusters with
the same name, this should not be done and is not supported. This applies to all
supported setups.
Single cluster with replicas
Cluster nodes can also be regular masters and replicate to regular replicas. However,
the tool can only detect diffs on a replica if ran on the replica's "master node".
For example, if the cluster setup is,
node1 <-> node2 <-> node3
| |
| +-> replica3
+-> replica2
you can detect diffs on replica3 by running the tool on node3, but to detect diffs on
replica2 you must run the tool again on node2. If you run the tool on node1, it will
not detect diffs on either replica.
Currently, the tool does not detect this setup or warn about replicas that cannot be
checked (e.g. replica2 when running on node3).
Replicas in this setup are still subject to "--[no]check-binlog-format".
Master to single cluster
It is possible for a regular master to replicate to a cluster, as if the cluster were
one logical slave, like:
master -> node1 <-> node2 <-> node3
The tool supports this setup but only if ran on the master and if all nodes in the
cluster are consistent with the "direct replica" (node1 in this example) of the
master. For example, if all nodes have value "foo" for row 1 but the master has value
"bar" for the same row, this diff will be detected. Or if only node1 has this diff,
it will also be detected. But if only node2 or node3 has this diff, it will not be
detected. Therefore, this setup is used to check that the master and the cluster as a
whole are consistent.
In this setup, the tool can automatically detect the "direct replica" (node1) when ran
on the master, so you do not have to use the "dsn" method for "--recursion-method"
because node1 will represent the entire cluster, which is why all other nodes must be
consistent with it.
The tool warns when it detects this setup to remind you that it only works when used
as described above. These warnings do not affect the exit status of the tool; they're
only reminders to help avoid false-positive results.
RocksDB support
Due to the limitations in the RocksDB engine like not supporting
binlog_format=STATEMENT or they way RocksDB handles Gap locks, pt-table-cheksum will
skip tables using RocksDB engine. More Information:
(<https://www.percona.com/doc/percona-server/LATEST/myrocks/limitations.html>)
OUTPUT
The tool prints tabular results, one line per table:
TS ERRORS DIFFS ROWS DIFF_ROWS CHUNKS SKIPPED TIME TABLE
10-20T08:36:50 0 0 200 0 1 0 0.005 db1.tbl1
10-20T08:36:50 0 0 603 3 7 0 0.035 db1.tbl2
10-20T08:36:50 0 0 16 0 1 0 0.003 db2.tbl3
10-20T08:36:50 0 0 600 0 6 0 0.024 db2.tbl4
Errors, warnings, and progress reports are printed to standard error. See also "--quiet".
Each table's results are printed when the tool finishes checksumming the table. The
columns are as follows:
TS The timestamp (without the year) when the tool finished checksumming the table.
ERRORS
The number of errors and warnings that occurred while checksumming the table. Errors
and warnings are printed to standard error while the table is in progress.
DIFFS
The number of chunks that differ from the master on one or more replicas. If
"--no-replicate-check" is specified, this column will always have zeros. If
"--replicate-check-only" is specified, then only tables with differences are printed.
ROWS
The number of rows selected and checksummed from the table. It might be different
from the number of rows in the table if you use the --where option.
DIFF_ROWS
The maximum number of differences per chunk. If a chunk has 2 different rows and
another chunk has 3 different rows, this value will be 3.
CHUNKS
The number of chunks into which the table was divided.
SKIPPED
The number of chunks that were skipped due one or more of these problems:
* 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)
As of pt-table-checksum 2.2.5, skipped chunks cause a non-zero "EXIT STATUS".
TIME
The time elapsed while checksumming the table.
TABLE
The database and table that was checksummed.
If "--replicate-check-only" is specified, only checksum differences on detected replicas
are printed. The output is different: one paragraph per replica, one checksum difference
per line, and values are separated by spaces:
Differences on h=127.0.0.1,P=12346
TABLE CHUNK CNT_DIFF CRC_DIFF CHUNK_INDEX LOWER_BOUNDARY UPPER_BOUNDARY
db1.tbl1 1 0 1 PRIMARY 1 100
db1.tbl1 6 0 1 PRIMARY 501 600
Differences on h=127.0.0.1,P=12347
TABLE CHUNK CNT_DIFF CRC_DIFF CHUNK_INDEX LOWER_BOUNDARY UPPER_BOUNDARY
db1.tbl1 1 0 1 PRIMARY 1 100
db2.tbl2 9 5 0 PRIMARY 101 200
The first line of a paragraph indicates the replica with differences. In this example
there are two: h=127.0.0.1,P=12346 and h=127.0.0.1,P=12347. The columns are as follows:
TABLE
The database and table that differs from the master.
CHUNK
The chunk number of the table that differs from the master.
CNT_DIFF
The number of chunk rows on the replica minus the number of chunk rows on the master.
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.
CHUNK_INDEX
The index used to chunk the table.
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.
EXIT STATUS
pt-table-checksum has three possible exit statuses: zero, 255, and any other value is a
bitmask with flags for different problems.
A zero exit status indicates no errors, warnings, or checksum differences, or skipped
chunks or tables.
A 255 exit status indicates a fatal error. In other words: the tool died or crashed. The
error is printed to "STDERR".
If the exit status is not zero or 255, then its value functions as a bitmask with these
flags:
FLAG BIT VALUE MEANING
================ ========= ==========================================
ERROR 1 A non-fatal error occurred
ALREADY_RUNNING 2 --pid file exists and the PID is running
CAUGHT_SIGNAL 4 Caught SIGHUP, SIGINT, SIGPIPE, or SIGTERM
NO_SLAVES_FOUND 8 No replicas or cluster nodes were found
TABLE_DIFF 16 At least one diff was found
SKIP_CHUNK 32 At least one chunk was skipped
SKIP_TABLE 64 At least one table was skipped
If any flag is set, the exit status will be non-zero. Use the bitwise "AND" operation to
check for a particular flag. For example, if "$exit_status & 16" is true, then at least
one diff was found.
As of pt-table-checksum 2.2.5, skipped chunks cause a non-zero exit status. An exit
status of zero or 32 is equivalent to a zero exit status with skipped chunks in previous
versions of the tool.
OPTIONS
This tool accepts additional command-line arguments. Refer to the "SYNOPSIS" and usage
information for details.
--ask-pass
group: Connection
Prompt for a password when connecting to MySQL.
--channel
type: string
Channel name used when connected to a server using replication channels. Suppose you
have two masters, master_a at port 12345, master_b at port 1236 and a slave connected
to both masters using channels chan_master_a and chan_master_b. If you want to run
pt-table-sync to synchronize the slave against master_a, pt-table-sync won't be able
to determine what's the correct master since SHOW SLAVE STATUS will return 2 rows. In
this case, you can use --channel=chan_master_a to specify the channel name to use in
the SHOW SLAVE STATUS command.
--[no]check-binlog-format
default: yes
Check that the "binlog_format" is the same on all servers.
See "Replicas using row-based replication" under "LIMITATIONS".
--binary-index
This option modifies the behavior of "--create-replicate-table" such that the
replicate table's upper and lower boundary columns are created with the BLOB data
type. This is useful in cases where you have trouble checksumming tables with keys
that include a binary data type or that have non-standard character sets. See
"--replicate".
--check-interval
type: time; default: 1; group: Throttle
Sleep time between checks for "--max-lag".
--[no]check-plan
default: yes
Check query execution plans for safety. By default, this option causes pt-table-
checksum to run EXPLAIN before running queries that are meant to access a small amount
of data, but which could access many rows if MySQL chooses a bad execution plan. These
include the queries to determine chunk boundaries and the chunk queries themselves. If
it appears that MySQL will use a bad query execution plan, the tool will skip the
chunk of the table.
The tool uses several heuristics to determine whether an execution plan is bad. The
first is whether EXPLAIN reports that MySQL intends to use the desired index to access
the rows. If MySQL chooses a different index, the tool considers the query unsafe.
The tool also checks how much of the index MySQL reports that it will use for the
query. The EXPLAIN output shows this in the key_len column. The tool remembers the
largest key_len seen, and skips chunks where MySQL reports that it will use a smaller
prefix of the index. This heuristic can be understood as skipping chunks that have a
worse execution plan than other chunks.
The tool prints a warning the first time a chunk is skipped due to a bad execution
plan in each table. Subsequent chunks are skipped silently, although you can see the
count of skipped chunks in the SKIPPED column in the tool's output.
This option adds some setup work to each table and chunk. Although the work is not
intrusive for MySQL, it results in more round-trips to the server, which consumes
time. Making chunks too small will cause the overhead to become relatively larger. It
is therefore recommended that you not make chunks too small, because the tool may take
a very long time to complete if you do.
--[no]check-replication-filters
default: yes; group: Safety
Do not checksum if any replication filters are set on any replicas. The tool looks
for server options that filter replication, such as binlog_ignore_db and
replicate_do_db. If it finds any such filters, it aborts with an error.
If the replicas are configured with any filtering options, you should be careful not
to checksum any databases or tables that exist on the master and not the replicas.
Changes to such tables might normally be skipped on the replicas because of the
filtering options, but the checksum queries modify the contents of the table that
stores the checksums, not the tables whose data you are checksumming. Therefore,
these queries will be executed on the replica, and if the table or database you're
checksumming does not exist, the queries will cause replication to fail. For more
information on replication rules, see
<http://dev.mysql.com/doc/en/replication-rules.html>.
Replication filtering makes it impossible to be sure that the checksum queries won't
break replication (or simply fail to replicate). If you are sure that it's OK to run
the checksum queries, you can negate this option to disable the checks. See also
"--replicate-database".
See also "REPLICA CHECKS".
--check-slave-lag
type: string; group: Throttle
Pause checksumming until this replica's lag is less than "--max-lag". The value is a
DSN that inherits properties from the master host and the connection options
("--port", "--user", etc.). By default, pt-table-checksum monitors lag on all
connected replicas, but this option limits lag monitoring to the specified replica.
This is useful if certain replicas are intentionally lagged (with pt-slave-delay for
example), in which case you can specify a normal replica to monitor.
See also "REPLICA CHECKS".
--[no]check-slave-tables
default: yes; group: Safety
Checks that tables on slaves exist and have all the checksum "--columns". Tables
missing on slaves or not having all the checksum "--columns" can cause the tool to
break replication when it tries to check for differences. Only disable this check if
you are aware of the risks and are sure that all tables on all slaves exist and are
identical to the master.
--chunk-index
type: string
Prefer this index for chunking tables. By default, pt-table-checksum chooses the most
appropriate index for chunking. This option lets you specify the index that you
prefer. If the index doesn't exist, then pt-table-checksum will fall back to its
default behavior of choosing an index. pt-table-checksum adds the index to the
checksum SQL statements in a "FORCE INDEX" clause. Be careful when using this option;
a poor choice of index could cause bad performance. This is probably best to use when
you are checksumming only a single table, not an entire server.
--chunk-index-columns
type: int
Use only this many left-most columns of a "--chunk-index". This works only for
compound indexes, and is useful in cases where a bug in the MySQL query optimizer
(planner) causes it to scan a large range of rows instead of using the index to locate
starting and ending points precisely. This problem sometimes occurs on indexes with
many columns, such as 4 or more. If this happens, the tool might print a warning
related to the "--[no]check-plan" option. Instructing the tool to use only the first
N columns of the index is a workaround for the bug in some cases.
--chunk-size
type: size; default: 1000
Number of rows to select for each checksum query. Allowable suffixes are k, M, G.
You should not use this option in most cases; prefer "--chunk-time" instead.
This option can override the default behavior, which is to adjust chunk size
dynamically to try to make chunks run in exactly "--chunk-time" seconds. When this
option isn't set explicitly, its default value is used as a starting point, but after
that, the tool ignores this option's value. If you set this option explicitly,
however, then it disables the dynamic adjustment behavior and tries to make all chunks
exactly the specified number of rows.
There is a subtlety: if the chunk index is not unique, then it's possible that chunks
will be larger than desired. For example, if a table is chunked by an index that
contains 10,000 of a given value, there is no way to write a WHERE clause that matches
only 1,000 of the values, and that chunk will be at least 10,000 rows large. Such a
chunk will probably be skipped because of "--chunk-size-limit".
Selecting a small chunk size will cause the tool to become much slower, in part
because of the setup work required for "--[no]check-plan".
--chunk-size-limit
type: float; default: 2.0; group: Safety
Do not checksum chunks this much larger than the desired chunk size.
When a table has no unique indexes, chunk sizes can be inaccurate. This option
specifies a maximum tolerable limit to the inaccuracy. The tool uses <EXPLAIN> to
estimate how many rows are in the chunk. If that estimate exceeds the desired chunk
size times the limit (twice as large, by default), then the tool skips the chunk.
The minimum value for this option is 1, which means that no chunk can be larger than
"--chunk-size". You probably don't want to specify 1, because rows reported by
EXPLAIN are estimates, which can be different from the real number of rows in the
chunk. If the tool skips too many chunks because they are oversized, you might want
to specify a value larger than the default of 2.
You can disable oversized chunk checking by specifying a value of 0.
--chunk-time
type: float; default: 0.5
Adjust the chunk size dynamically so each checksum query takes this long to execute.
The tool tracks the checksum rate (rows per second) for all tables and each table
individually. It uses these rates to adjust the chunk size after each checksum query,
so that the next checksum query takes this amount of time (in seconds) to execute.
The algorithm is as follows: at the beginning of each table, the chunk size is
initialized from the overall average rows per second since the tool began working, or
the value of "--chunk-size" if the tool hasn't started working yet. For each
subsequent chunk of a table, the tool adjusts the chunk size to try to make queries
run in the desired amount of time. It keeps an exponentially decaying moving average
of queries per second, so that if the server's performance changes due to changes in
server load, the tool adapts quickly. This allows the tool to achieve predictably
timed queries for each table, and for the server overall.
If this option is set to zero, the chunk size doesn't auto-adjust, so query checksum
times will vary, but query checksum sizes will not. Another way to do the same thing
is to specify a value for "--chunk-size" explicitly, instead of leaving it at the
default.
--columns
short form: -c; type: array; group: Filter
Checksum only this comma-separated list of columns. If a table doesn't have any of
the specified columns it will be skipped.
This option applies to all tables, so it really only makes sense when checksumming one
table unless the tables have a common set of columns.
--config
type: Array; group: Config
Read this comma-separated list of config files; if specified, this must be the first
option on the command line.
See the "--help" output for a list of default config files.
--create-replicate-table =item --no-create-replicate-table
default: yes
Create the "--replicate" database and table if they do not exist. The structure of
the replicate table is the same as the suggested table mentioned in "--replicate".
--databases
short form: -d; type: hash; group: Filter
Only checksum this comma-separated list of databases.
--databases-regex
type: string; group: Filter
Only checksum databases whose names match this Perl regex.
--defaults-file
short form: -F; type: string; group: Connection
Only read mysql options from the given file. You must give an absolute pathname.
--disable-qrt-plugin
Disable the QRT (Query Response Time) plugin if it is enabled.
--[no]empty-replicate-table
default: yes
Delete previous checksums for each table before checksumming the table. This option
does not truncate the entire table, it only deletes rows (checksums) for each table
just before checksumming the table. Therefore, if checksumming stops prematurely and
there was preexisting data, there will still be rows for tables that were not
checksummed before the tool was stopped.
If you're resuming from a previous checksum run, then the checksum records for the
table from which the tool resumes won't be emptied.
To empty the entire replicate table, you must manually execute "TRUNCATE TABLE" before
running the tool.
--engines
short form: -e; type: hash; group: Filter
Only checksum tables which use these storage engines.
--explain
cumulative: yes; default: 0; group: Output
Show, but do not execute, checksum queries (disables "--[no]empty-replicate-table").
If specified twice, the tool actually iterates through the chunking algorithm,
printing the upper and lower boundary values for each chunk, but not executing the
checksum queries.
--fail-on-stopped-replication
If replication is stopped, fail with an error (exit status 128) instead of waiting
until replication is restarted.
--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.
--function
type: string
Hash function for checksums (FNV1A_64, MURMUR_HASH, SHA1, MD5, CRC32, etc).
The default is to use CRC32(), but MD5() and SHA1() also work, and you can use your
own function, such as a compiled UDF, if you wish. The function you specify is run in
SQL, not in Perl, so it must be available to MySQL.
MySQL doesn't have good built-in hash functions that are fast. CRC32() is too prone
to hash collisions, and MD5() and SHA1() are very CPU-intensive. The FNV1A_64() UDF
that is distributed with Percona Server is a faster alternative. It is very simple to
compile and install; look at the header in the source code for instructions. If it is
installed, it is preferred over MD5(). You can also use the MURMUR_HASH() function if
you compile and install that as a UDF; the source is also distributed with Percona
Server, and it might be better than FNV1A_64().
--help
group: Help
Show help and exit.
--host
short form: -h; type: string; default: localhost; group: Connection
Host to connect to.
--ignore-columns
type: Hash; group: Filter
Ignore this comma-separated list of columns when calculating the checksum. If a table
has all of its columns filtered by --ignore-columns, it will be skipped.
--ignore-databases
type: Hash; group: Filter
Ignore this comma-separated list of databases.
--ignore-databases-regex
type: string; group: Filter
Ignore databases whose names match this Perl regex.
--ignore-engines
type: Hash; default: FEDERATED,MRG_MyISAM; group: Filter
Ignore this comma-separated list of storage engines.
--ignore-tables
type: Hash; group: Filter
Ignore this comma-separated list of tables. Table names may be qualified with the
database name. The "--replicate" table is always automatically ignored.
--ignore-tables-regex
type: string; group: Filter
Ignore tables whose names match the Perl regex.
--max-lag
type: time; default: 1s; group: Throttle
Pause checksumming until all replicas' lag is less than this value. After each
checksum query (each chunk), pt-table-checksum looks at the replication lag of all
replicas to which it connects, using Seconds_Behind_Master. If any replica is lagging
more than the value of this option, then pt-table-checksum will sleep for
"--check-interval" seconds, then check all replicas again. If you specify
"--check-slave-lag", then the tool only examines that server for lag, not all servers.
The tool waits forever for replicas to stop lagging. If any replica is stopped, the
tool waits forever until the replica is started. Checksumming continues once all
replicas are running and not lagging too much.
The tool prints progress reports while waiting. If a replica is stopped, it prints a
progress report immediately, then again at every progress report interval.
See also "REPLICA CHECKS".
--max-load
type: Array; default: Threads_running=25; group: Throttle
Examine SHOW GLOBAL STATUS after every chunk, and pause if any status variables are
higher than the threshold. The option accepts a comma-separated list of MySQL status
variables to check for a threshold. An optional "=MAX_VALUE" (or ":MAX_VALUE") can
follow each variable. If not given, the tool determines a threshold by examining the
current value and increasing it by 20%.
For example, if you want the tool to pause when Threads_connected gets too high, you
can specify "Threads_connected", and the tool will check the current value when it
starts working and add 20% to that value. If the current value is 100, then the tool
will pause when Threads_connected exceeds 120, and resume working when it is below 120
again. If you want to specify an explicit threshold, such as 110, you can use either
"Threads_connected:110" or "Threads_connected=110".
The purpose of this option is to prevent the tool from adding too much load to the
server. If the checksum queries are intrusive, or if they cause lock waits, then other
queries on the server will tend to block and queue. This will typically cause
Threads_running to increase, and the tool can detect that by running SHOW GLOBAL
STATUS immediately after each checksum query finishes. If you specify a threshold for
this variable, then you can instruct the tool to wait until queries are running
normally again. This will not prevent queueing, however; it will only give the server
a chance to recover from the queueing. If you notice queueing, it is best to decrease
the chunk time.
--password
short form: -p; type: string; group: Connection
Password to use when connecting. If password contains commas they must be escaped
with a backslash: "exam\,ple"
--pause-file
type: string
Execution will be paused while the file specified by this param exists.
--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.
--plugin
type: string
Perl module file that defines a "pt_table_checksum_plugin" class. A plugin allows you
to write a Perl module that can hook into many parts of pt-table-checksum. This
requires a good knowledge of Perl and Percona Toolkit conventions, which are beyond
this scope of this documentation. Please contact Percona if you have questions or
need help.
See "PLUGIN" for more information.
--port
short form: -P; type: int; group: Connection
Port number to use for connection.
--progress
type: array; default: time,30
Print progress reports to STDERR.
The value is a comma-separated list with two parts. The first part can be percentage,
time, or iterations; the second part specifies how often an update should be printed,
in percentage, seconds, or number of iterations. The tool prints progress reports for
a variety of time-consuming operations, including waiting for replicas to catch up if
they become lagged.
--quiet
short form: -q; cumulative: yes; default: 0
Print only the most important information (disables "--progress"). Specifying this
option once causes the tool to print only errors, warnings, and tables that have
checksum differences.
Specifying this option twice causes the tool to print only errors. In this case, you
can use the tool's exit status to determine if there were any warnings or checksum
differences.
--recurse
type: int
Number of levels to recurse in the hierarchy when discovering replicas. Default is
infinite. See also "--recursion-method" and "REPLICA CHECKS".
--recursion-method
type: array; default: processlist,hosts
Preferred recursion method for discovering replicas. pt-table-checksum performs
several "REPLICA CHECKS" before and while running.
Although replicas are not required to run pt-table-checksum, the tool cannot detect
diffs on slaves that it cannot discover. Therefore, a warning is printed and the
"EXIT STATUS" is non-zero if no replicas are found and the method is not "none". If
this happens, try a different recursion method, or use the "dsn" method to specify the
replicas to check.
Possible methods are:
METHOD USES
=========== =============================================
processlist SHOW PROCESSLIST
hosts SHOW SLAVE HOSTS
cluster SHOW STATUS LIKE 'wsrep\_incoming\_addresses'
dsn=DSN DSNs from a table
none Do not find slaves
The "processlist" method is the default, because "SHOW SLAVE HOSTS" is not reliable.
However, if the server uses a non-standard port (not 3306), then the "hosts" method
becomes the default because it works better in this case.
The "hosts" method requires replicas to be configured with "report_host",
"report_port", etc.
The "cluster" method requires a cluster based on Galera 23.7.3 or newer, such as
Percona XtraDB Cluster versions 5.5.29 and above. This will auto-discover nodes in a
cluster using "SHOW STATUS LIKE 'wsrep\_incoming\_addresses'". You can combine
"cluster" with "processlist" and "hosts" to auto-discover cluster nodes and replicas,
but this functionality is experimental.
The "dsn" method is special: rather than automatically discovering replicas, this
method specifies a table with replica DSNs. The tool will only connect to these
replicas. This method works best when replicas do not use the same MySQL username or
password as the master, or when you want to prevent the tool from connecting to
certain replicas. The "dsn" method is specified like: "--recursion-method
dsn=h=host,D=percona,t=dsns". The specified DSN must have D and t parts, or just a
database-qualified t part, which specify the DSN table. The DSN table must have the
following structure:
CREATE TABLE `dsns` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`parent_id` int(11) DEFAULT NULL,
`dsn` varchar(255) NOT NULL,
PRIMARY KEY (`id`)
);
DSNs are ordered by "id", but "id" and "parent_id" are otherwise ignored. The "dsn"
column contains a replica DSN like it would be given on the command line, for example:
"h=replica_host,u=repl_user,p=repl_pass".
The "none" method makes the tool ignore all slaves and cluster nodes. This method is
not recommended because it effectively disables the "REPLICA CHECKS" and no
differences can be found. It is useful, however, if you only need to write checksums
on the master or a single cluster node. The safer alternative is
"--no-replicate-check": the tool finds replicas and cluster nodes, performs the
"REPLICA CHECKS", but does not check for differences. See "--[no]replicate-check".
--replicate
type: string; default: percona.checksums
Write checksum results to this table. The replicate table must have this structure
(MAGIC_create_replicate):
CREATE TABLE checksums (
db CHAR(64) NOT NULL,
tbl CHAR(64) NOT NULL,
chunk INT NOT NULL,
chunk_time FLOAT NULL,
chunk_index VARCHAR(200) NULL,
lower_boundary TEXT NULL,
upper_boundary TEXT NULL,
this_crc CHAR(40) NOT NULL,
this_cnt INT NOT NULL,
master_crc CHAR(40) NULL,
master_cnt INT NULL,
ts TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
PRIMARY KEY (db, tbl, chunk),
INDEX ts_db_tbl (ts, db, tbl)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
Note: lower_boundary and upper_boundary data type can be BLOB. See "--binary-index".
By default, "--create-replicate-table" is true, so the database and the table
specified by this option are created automatically if they do not exist.
Be sure to choose an appropriate storage engine for the replicate table. If you are
checksumming InnoDB tables, and you use MyISAM for this table, a deadlock will break
replication, because the mixture of transactional and non-transactional tables in the
checksum statements will cause it to be written to the binlog even though it had an
error. It will then replay without a deadlock on the replicas, and break replication
with "different error on master and slave." This is not a problem with pt-table-
checksum; it's a problem with MySQL replication, and you can read more about it in the
MySQL manual.
The replicate table is never checksummed (the tool automatically adds this table to
"--ignore-tables").
--[no]replicate-check
default: yes
Check replicas for data differences after finishing each table. The tool finds
differences by executing a simple SELECT statement on all detected replicas. The
query compares the replica's checksum results to the master's checksum results. It
reports differences in the DIFFS column of the output.
--replicate-check-only
Check replicas for consistency without executing checksum queries. This option is
used only with "--[no]replicate-check". If specified, pt-table-checksum doesn't
checksum any tables. It checks replicas for differences found by previous
checksumming, and then exits. It might be useful if you run pt-table-checksum quietly
in a cron job, for example, and later want a report on the results of the cron job,
perhaps to implement a Nagios check.
--replicate-check-retries
type: int; default: 1
Retry checksum comparison this many times when a difference is encountered. Only when
a difference persists after this number of checks is it considered valid. Using this
option with a value of 2 or more alleviates spurious differences that arise when using
the --resume option.
--replicate-database
type: string
USE only this database. By default, pt-table-checksum executes USE to select the
database that contains the table it's currently working on. This is is a best effort
to avoid problems with replication filters such as binlog_ignore_db and
replicate_ignore_db. However, replication filters can create a situation where there
simply is no one right way to do things. Some statements might not be replicated, and
others might cause replication to fail. In such cases, you can use this option to
specify a default database that pt-table-checksum selects with USE, and never changes.
See also "--[no]check-replication-filters".
--resume
Resume checksumming from the last completed chunk (disables
"--[no]empty-replicate-table"). If the tool stops before it checksums all tables,
this option makes checksumming resume from the last chunk of the last table that it
finished.
--retries
type: int; default: 2
Retry a chunk this many times when there is a nonfatal error. Nonfatal errors are
problems such as a lock wait timeout or the query being killed.
--run-time
type: time
How long to run. Default is to run until all tables have been checksummed. These
time value suffixes are allowed: s (seconds), m (minutes), h (hours), and d (days).
Combine this option with "--resume" to checksum as many tables within an allotted
time, resuming from where the tool left off next time it is ran.
--separator
type: string; default: #
The separator character used for CONCAT_WS(). This character is used to join the
values of columns when checksumming.
--skip-check-slave-lag
type: DSN; repeatable: yes
DSN to skip when checking slave lag. It can be used multiple times. Example:
--skip-check-slave-lag h=127.1,P=12345 --skip-check-slave-lag h=127.1,P=12346
--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; group: Connection
Set the MySQL variables in this comma-separated list of "variable=value" pairs.
By default, the tool sets:
wait_timeout=10000
innodb_lock_wait_timeout=1
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; group: Connection
Socket file to use for connection.
--slave-skip-tolerance
type: float; default: 1.0
When a master table is marked to be checksumed in only one chunk but a slave table
exceeds the maximum accepted size for this, the table is skipped. Since number of
rows are often rough estimates, many times tables are skipped needlessly for very
small differences. This option provides a max row excess tolerance to prevent this.
For example a value of 1.2 will tolerate slave tables with up to 20% excess rows.
--tables
short form: -t; type: hash; group: Filter
Checksum only this comma-separated list of tables. Table names may be qualified with
the database name.
--tables-regex
type: string; group: Filter
Checksum only tables whose names match this Perl regex.
--trim
Add TRIM() to VARCHAR columns (helps when comparing 4.1 to >= 5.0). This is useful
when you don't care about the trailing space differences between MySQL versions that
vary in their handling of trailing spaces. MySQL 5.0 and later all retain trailing
spaces in VARCHAR, while previous versions would remove them. These differences will
cause false checksum differences.
--truncate-replicate-table
Truncate the replicate table before starting the checksum. This parameter differs
from --empty-replicate-table which only deletes the rows for the table being
checksumed when starting the checksum for that table, while --truncate-replicate-table
will truncate the replicate table at the beginning of the process and thus, all
previous checksum information will be losti, even if the process stops due to an
error.
--user
short form: -u; type: string; group: Connection
User for login if not current user.
--version
group: Help
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 its own version and also the versions of the
following software: operating system, Percona Monitoring and Management (PMM), MySQL,
Perl, MySQL driver for Perl (DBD::mysql), and Percona Toolkit. 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.
A secure connection to Percona’s Version Check database server is done to perform
these checks. Each request is logged by the server, including software version numbers
and unique ID of the checked system. The ID is generated by the Percona Toolkit
installation script or when the Version Check database call is done for the first
time.
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
<https://www.percona.com/doc/percona-toolkit/LATEST/version-check.html>.
--where
type: string
Do only rows matching this WHERE clause. You can use this option to limit the
checksum to only part of the table. This is particularly useful if you have append-
only tables and don't want to constantly re-check all rows; you could run a daily job
to just check yesterday's rows, for instance.
This option is much like the -w option to mysqldump. Do not specify the WHERE
keyword. You might need to quote the value. Here is an example:
pt-table-checksum --where "ts > CURRENT_DATE - INTERVAL 1 DAY"
REPLICA CHECKS
By default, pt-table-checksum attempts to find and connect to all replicas connected to
the master host. This automated process is called "slave recursion" and is controlled by
the "--recursion-method" and "--recurse" options. The tool performs these checks on all
replicas:
1. "--[no]check-replication-filters"
pt-table-checksum checks for replication filters on all replicas because they can
complicate or break the checksum process. By default, the tool will exit if any
replication filters are found, but this check can be disabled by specifying
"--no-check-replication-filters".
2. "--replicate" table
pt-table-checksum checks that the "--replicate" table exists on all replicas, else
checksumming can break replication when updates to the table on the master replicate
to a replica that doesn't have the table. This check cannot be disabled, and the tool
waits forever until the table exists on all replicas, printing "--progress" messages
while it waits.
3. Single chunk size
If a table can be checksummed in a single chunk on the master, pt-table-checksum will
check that the table size on all replicas is less than "--chunk-size" *
"--chunk-size-limit". This prevents a rare problem where the table on the master is
empty or small, but on a replica it is much larger. In this case, the single chunk
checksum on the master would overload the replica.
Another rare problem occurs when the table size on a replica is close to
"--chunk-size" * "--chunk-size-limit". In such cases, the table is more likely to be
skipped even though it's safe to checksum in a single chunk. This happens because
table sizes are estimates. When those estimates and "--chunk-size" *
"--chunk-size-limit" are almost equal, this check becomes more sensitive to the
estimates' margin of error rather than actual significant differences in table sizes.
Specifying a larger value for "--chunk-size-limit" helps avoid this problem.
This check cannot be disabled.
4. Lag
After each chunk, pt-table-checksum checks the lag on all replicas, or only the
replica specified by "--check-slave-lag". This helps the tool not to overload the
replicas with checksum data. There is no way to disable this check, but you can
specify a single replica to check with "--check-slave-lag", and if that replica is the
fastest, it will help prevent the tool from waiting too long for replica lag to abate.
5. Checksum chunks
When pt-table-checksum finishes checksumming a table, it waits for the last checksum
chunk to replicate to all replicas so it can perform the "--[no]replicate-check".
Disabling that option by specifying --no-replicate-check disables this check, but it
also disables immediate reporting of checksum differences, thereby requiring a second
run of the tool with "--replicate-check-only" to find and print checksum differences.
PLUGIN
The file specified by "--plugin" must define a class (i.e. a package) called
"pt_table_checksum_plugin" with a "new()" subroutine. The tool will create an instance of
this class and call any hooks that it defines. No hooks are required, but a plugin isn't
very useful without them.
These hooks, in this order, are called if defined:
init
before_replicate_check
after_replicate_check
get_slave_lag
before_checksum_table
after_checksum_table
Each hook is passed different arguments. To see which arguments are passed to a hook,
search for the hook's name in the tool's source code, like:
# --plugin hook
if ( $plugin && $plugin->can('init') ) {
$plugin->init(
slaves => $slaves,
slave_lag_cxns => $slave_lag_cxns,
repl_table => $repl_table,
);
}
The comment "# --plugin hook" precedes every hook call.
Please contact Percona if you have questions or need help.
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
copy: no
DSN table database.
• F
dsn: mysql_read_default_file; copy: yes
Defaults file for connection values.
• 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: no
Socket file to use for connection.
• t
copy: no
DSN table table.
• 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-checksum ... > 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
For a list of known bugs, see <http://www.percona.com/bugs/pt-table-checksum>.
Please report bugs at <https://jira.percona.com/projects/PT>. Include the following
information in your bug report:
• Complete command-line used to run the tool
• 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".
DOWNLOADING
Visit <http://www.percona.com/software/percona-toolkit/> to download the latest release of
Percona Toolkit. Or, get the latest release from the command line:
wget percona.com/get/percona-toolkit.tar.gz
wget percona.com/get/percona-toolkit.rpm
wget percona.com/get/percona-toolkit.deb
You can also get individual tools from the latest release:
wget percona.com/get/TOOL
Replace "TOOL" with the name of any tool.
AUTHORS
Baron Schwartz and Daniel Nichter
ACKNOWLEDGMENTS
Claus Jeppesen, Francois Saint-Jacques, Giuseppe Maxia, Heikki Tuuri, James Briggs, Martin
Friebe, and Sergey Zhuravlev
ABOUT PERCONA TOOLKIT
This tool is part of Percona Toolkit, a collection of advanced command-line tools for
MySQL developed by Percona. Percona Toolkit was forked from two projects in June, 2011:
Maatkit and Aspersa. Those projects were created by Baron Schwartz and primarily
developed by him and Daniel Nichter. Visit <http://www.percona.com/software/> to learn
about other free, open-source software from Percona.
COPYRIGHT, LICENSE, AND WARRANTY
This program is copyright 2011-2018 Percona LLC and/or its affiliates, 2007-2011 Baron
Schwartz.
THIS PROGRAM IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING,
WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE.
This program is free software; you can redistribute it and/or modify it under the terms of
the GNU General Public License as published by the Free Software Foundation, version 2; OR
the Perl Artistic License. On UNIX and similar systems, you can issue `man perlgpl' or
`man perlartistic' to read these licenses.
You should have received a copy of the GNU General Public License along with this program;
if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
MA 02111-1307 USA.
VERSION
pt-table-checksum 3.2.1
POD ERRORS
Hey! The above document had some coding errors, which are explained below:
Around line 12821:
Non-ASCII character seen before =encoding in 'MySQL’s'. Assuming UTF-8