bionic (1) pt-table-checksum.1p.gz
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 suporting 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 CHUNKS SKIPPED TIME TABLE
10-20T08:36:50 0 0 200 1 0 0.005 db1.tbl1
10-20T08:36:50 0 0 603 7 0 0.035 db1.tbl2
10-20T08:36:50 0 0 16 1 0 0.003 db2.tbl3
10-20T08:36:50 0 0 600 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.
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 syncronize 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 checksuming 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.
--[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.
--[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.
--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, "--[no]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 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 <https://www.percona.com/version-check>.
--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://bugs.launchpad.net/percona-toolkit>. 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 2011-2017 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 3.0.6
POD ERRORS
Hey! The above document had some coding errors, which are explained below:
Around line 12567:
Non-ASCII character seen before =encoding in 'MySQL’s'. Assuming UTF-8