Support --checksum-data flag, on-the-fly checksum verification #4
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This PR introduces
--checksum-data, an opt-in checksum verification that runs throughout the migration.With
--checksum-dataenabled, each rowcopy (a range of rows copied from the original table to the ghost table) is followed by a checksum on the two tables for that range.Checksums are executed concurrently to rowcopy and are the exception to the single thread model for
gh-ost.A checksum may well fail while the migration is running: since
gh-ostworks in async design, where binlog entries are applied at some point in time after they're generated, it's quite possible that ongoing traffic will make some checksums fail.A failed range's checksum is retried and retried until successful.
When
--checksum-datais enabled, cut-over does not complete if failed checksums are found. While tables are locked in preparation for cut-over, a grace period is given so that the checksum evaluation can run to completion.This is experimental.