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Document problem LP 1389041: pt-table-checksum has high likelyhood to skip a table when row count is around chunk-size * chunk-size-limit.

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Daniel Nichter
2015-11-02 14:00:01 -08:00
parent f9468fd2cd
commit c2dff1d227
1 changed files with 16 additions and 5 deletions
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bin/pt-table-checksum
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@@ -12703,11 +12703,22 @@ exists on all replicas, printing L<"--progress"> messages while it waits.
=item 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
approximately the same. 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. This check cannot be disabled.
pt-table-checksum will check that the table size on all replicas is less than
L<"--chunk-size"> * L<"--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
L<"--chunk-size"> * L<"--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
L<"--chunk-size"> * L<"--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
L<"--chunk-size-limit"> helps avoid this problem.
This check cannot be disabled.
=item 4. Lag