Troubleshoot TiDB Lightning

This document summarizes the common problems you might encounter when you use TiDB Lightning and their solutions.

Import speed is too slow

Normally it takes 2 minutes per thread for TiDB Lightning to import a 256 MB data file. If the speed is much slower than this, there is an error. You can check the time taken for each data file from the log mentioning restore chunk … takes. This can also be observed from metrics on Grafana.

There are several reasons why TiDB Lightning becomes slow:

Cause 1: region-concurrency is set too high, which causes thread contention and reduces performance.

  1. The setting can be found from the start of the log by searching region-concurrency.
  2. If TiDB Lightning shares the same machine with other services (for example, TiKV Importer), region-concurrency must be manually set to 75% of the total number of CPU cores.
  3. If there is a quota on CPU (for example, limited by Kubernetes settings), TiDB Lightning may not be able to read this out. In this case, region-concurrency must also be manually reduced.

Cause 2: The table schema is too complex.

Every additional index introduces a new KV pair for each row. If there are N indices, the actual size to be imported would be approximately (N+1) times the size of the Dumpling output. If the indices are negligible, you may first remove them from the schema, and add them back using CREATE INDEX after the import is complete.

Cause 3: Each file is too large.

TiDB Lightning works the best when the data source is broken down into multiple files of size around 256 MB so that the data can be processed in parallel. If each file is too large, TiDB Lightning might not respond.

If the data source is CSV, and all CSV files have no fields containing newline control characters (U+000A and U+000D), you can turn on "strict format" to let TiDB Lightning automatically split the large files.

[mydumper]
strict-format = true

Cause 4: TiDB Lightning is too old.

Try the latest version. Maybe there is new speed improvement.

The tidb-lightning process suddenly quits while running in background

It is potentially caused by starting tidb-lightning incorrectly, which causes the system to send a SIGHUP signal to stop the tidb-lightning process. In this situation, tidb-lightning.log usually outputs the following log:

[2018/08/10 07:29:08.310 +08:00] [INFO] [main.go:41] ["got signal to exit"] [signal=hangup]

It is not recommended to directly use nohup in the command line to start tidb-lightning. You can start tidb-lightning by executing a script.

In addition, if the last log of TiDB Lightning shows that the error is "Context canceled", you need to search for the first "ERROR" level log. This "ERROR" level log is usually followed by "got signal to exit", which indicates that TiDB Lightning received an interrupt signal and then exited.

The TiDB cluster uses lots of CPU resources and runs very slowly after using TiDB Lightning

If tidb-lightning abnormally exited, the cluster might be stuck in the "import mode", which is not suitable for production. The current mode can be retrieved using the following command:

tidb-lightning-ctl --config tidb-lightning.toml --fetch-mode

You can force the cluster back to "normal mode" using the following command:

tidb-lightning-ctl --config tidb-lightning.toml --fetch-mode

TiDB Lightning reports an error

could not find first pair, this shouldn't happen

This error occurs possibly because the number of files opened by TiDB Lightning exceeds the system limit when TiDB Lightning reads the sorted local files. In the Linux system, you can use the ulimit -n command to confirm whether the value of this system limit is too small. It is recommended that you adjust this value to 1000000 (ulimit -n 1000000) during the import.

checksum failed: checksum mismatched remote vs local

Cause: The checksum of a table in the local data source and the remote imported database differ. This error has several deeper reasons. You can further locate the reason by checking the log that contains checksum mismatched.

The lines that contain checksum mismatched provide the information total_kvs: x vs y, where x indicates the number of key-value pairs (KV pairs) calculated by the target cluster after the import is completed, and y indicates the number of key-value pairs generated by the local data source.

  • If x is greater, it means that there are more KV pairs in the target cluster.
    • It is possible that this table is not empty before the import and therefore affects the data checksum. It is also possible that TiDB Lightning has previously failed and shut down, but did not restart correctly.
  • If y is greater, it means that there are more KV pairs in the local data source.
    • If the checksum of the target database is all 0, it means that no import has occurred. It is possible that the cluster is too busy to receive any data.
    • It is possible that the exported data contains duplicate data, such as the UNIQUE and PRIMARY KEYs with duplicate values, or that the downstream table structure is case-insensitive while the data is case-sensitive.
  • Other possible reasons
    • If the data source is machine-generated and not backed up by Dumpling, make sure the data conforms to the table limits. For example, the AUTO_INCREMENT column needs to be positive and not 0.

Solutions:

  1. Delete the corrupted data using tidb-lightning-ctl, check the table structure and the data, and restart TiDB Lightning to import the affected tables again.

    tidb-lightning-ctl --config conf/tidb-lightning.toml --checkpoint-error-destroy=all
    
  2. Consider using an external database to store the checkpoints (change [checkpoint] dsn) to reduce the target database's load.

  3. If TiDB Lightning was improperly restarted, see also the "How to properly restart TiDB Lightning" section in the FAQ.

Checkpoint for … has invalid status: (error code)

Cause: Checkpoint is enabled, and TiDB Lightning or TiKV Importer has previously abnormally exited. To prevent accidental data corruption, TiDB Lightning will not start until the error is addressed.

The error code is an integer smaller than 25, with possible values of 0, 3, 6, 9, 12, 14, 15, 17, 18, 20, and 21. The integer indicates the step where the unexpected exit occurs in the import process. The larger the integer is, the later step the exit occurs at.

Solutions:

If the error was caused by invalid data source, delete the imported data using tidb-lightning-ctl and start Lightning again.

tidb-lightning-ctl --config conf/tidb-lightning.toml --checkpoint-error-destroy=all

See the Checkpoints control section for other options.

ResourceTemporarilyUnavailable("Too many open engines …: …")

Cause: The number of concurrent engine files exceeds the limit specified by tikv-importer. This could be caused by misconfiguration. Additionally, if tidb-lightning exited abnormally, an engine file might be left at a dangling open state, which could cause this error as well.

Solutions:

  1. Increase the value of max-open-engines setting in tikv-importer.toml. This value is typically dictated by the available memory. This could be calculated by using:

    Max Memory Usage ≈ max-open-engines × write-buffer-size × max-write-buffer-number

  2. Decrease the value of table-concurrency + index-concurrency so it is less than max-open-engines.

  3. Restart tikv-importer to forcefully remove all engine files (default to ./data.import/). This also removes all partially imported tables, which requires TiDB Lightning to clear the outdated checkpoints.

    tidb-lightning-ctl --config conf/tidb-lightning.toml --checkpoint-error-destroy=all
    

cannot guess encoding for input file, please convert to UTF-8 manually

Cause: TiDB Lightning only recognizes the UTF-8 and GB-18030 encodings for the table schemas. This error is emitted if the file isn't in any of these encodings. It is also possible that the file has mixed encoding, such as containing a string in UTF-8 and another string in GB-18030, due to historical ALTER TABLE executions.

Solutions:

  1. Fix the schema so that the file is entirely in either UTF-8 or GB-18030.

  2. Manually CREATE the affected tables in the target database.

  3. Set [mydumper] character-set = "binary" to skip the check. Note that this might introduce mojibake into the target database.

[sql2kv] sql encode error = [types:1292]invalid time format: '{1970 1 1 …}'

Cause: A table contains a column with the timestamp type, but the time value itself does not exist. This is either because of DST changes or the time value has exceeded the supported range (Jan 1, 1970 to Jan 19, 2038).

Solutions:

  1. Ensure TiDB Lightning and the source database are using the same time zone.

    When executing TiDB Lightning directly, the time zone can be forced using the $TZ environment variable.

    # Manual deployment, and force Asia/Shanghai.
    TZ='Asia/Shanghai' bin/tidb-lightning -config tidb-lightning.toml
    
  2. When exporting data using Mydumper, make sure to include the --skip-tz-utc flag.

  3. Ensure the entire cluster is using the same and latest version of tzdata (version 2018i or above).

    On CentOS, run yum info tzdata to check the installed version and whether there is an update. Run yum upgrade tzdata to upgrade the package.

[Error 8025: entry too large, the max entry size is 6291456]

Cause: A single row of key-value pairs generated by TiDB Lightning exceeds the limit set by TiDB.

Solution:

Currently, the limitation of TiDB cannot be bypassed. You can only ignore this table to ensure the successful import of other tables.

Encounter rpc error: code = Unimplemented ... when TiDB Lightning switches the mode

Cause: Some node(s) in the cluster does not support switch-mode. For example, if the TiFlash version is earlier than v4.0.0-rc.2, switch-mode is not supported.

Solutions:

  • If there are TiFlash nodes in the cluster, you can update the cluster to v4.0.0-rc.2 or higher versions.
  • Temporarily disable TiFlash if you do not want to upgrade the cluster.

tidb lightning encountered error: TiDB version too old, expected '>=4.0.0', found '3.0.18'

TiDB Lightning Local-backend only supports importing data to TiDB clusters of v4.0.0 and later versions. If you try to use Local-backend to import data to a v2.x or v3.x cluster, the above error is reported. At this time, you can modify the configuration to use Importer-backend or TiDB-backend for data import.

Some nightly versions might be similar to v4.0.0-beta.2. These nightly versions of TiDB Lightning actually support Local-backend. If you encounter this error when using a nightly version, you can skip the version check by setting the configuration check-requirements = false. Before setting this parameter, make sure that the configuration of TiDB Lightning supports the corresponding version; otherwise, the import might fail.

restore table test.district failed: unknown columns in header [...]

This error occurs usually because the CSV data file does not contain a header (the first row is not column names but data). Therefore, you need to add the following configuration to the TiDB Lightning configuration file:

[mydumper.csv]
header = false
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