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Skipping chunks when using pt-online-schema-change

Description

Some of the documentation for pt-online-schema-change discusses skipping chunks but skipping any chunks would cause data loss.

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Sveta Smirnova January 30, 2023 at 1:11 PM

Lalit Choudhary October 23, 2018 at 1:45 PM

Hi Monty,

Thank you for the report.

    

pt-online-schema-change documentation has good example about why there is chunk-size-limit but it should also include the information about what will happen if --chunk-size-limit exceed.

In such case pt-online-schema-change  will exit with an error code, the data will remain untouched.

 

https://github.com/percona/percona-toolkit/blob/3.0/bin/pt-online-schema-change#L10360-L10377

die ts("Error copying rows at chunk " . $nibble_iter->nibble_number() . " of $tbl->{db}.$tbl->{tbl} because it is oversized. "

 

 

 

 

monty monty July 18, 2018 at 5:36 PM

Some excerpts from the documentation

--[no]check-plan

If it appears that MySQL will use a bad query execution plan, the tool will skip the chunk of the table.

The tool remembers the largest key_len seen, and skips chunks where MySQL reports that it will use a smaller prefix of the index.

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.

--chunk-size

Such a chunk will probably be skipped because of --chunk-size-limit.

--chunk-size-limit

If that estimate exceeds the desired chunk size times the limit, then the tool skips the chunk.

Done

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Created July 18, 2018 at 5:36 PM
Updated March 4, 2024 at 4:31 PM
Resolved April 17, 2023 at 2:36 PM