gh-ost is a triggerless online schema migration solution for MySQL.
It is testable and provides pausability, dynamic control/reconfiguration, auditing, and many operational perks.
gh-ost produces a light workload on the master throughout the migration, decoupled from the existing workload on the migrated table.
gh-ost uses the binary log stream to capture table changes, and asynchronously applies them onto the ghost table. gh-ost takes upon itself some tasks that other tools leave for the database to perform. As result, gh-ost has greater control over the migration process; can truly suspend it; can truly decouple the migration’s write load from the master’s workload.
- Build your trust in gh-ost by testing it on replicas. gh-ost will issue same flow as it would have on the master, to migrate a table on a replica, without actually replacing the original table, leaving the replica with two tables you can then compare and satisfy yourself that the tool operates correctly. This is how we continuously test gh-ost in production.
- True pause: when gh-ost throttles, it truly ceases writes on master: no row copies and no ongoing events processing. By throttling, you return your master to its original workload.
- Dynamic control: you can interactively reconfigure gh-ost, even as migration still runs. You may forcibly initiate throttling.
- Auditing: you may query gh-ost for status. gh-ost listens on unix socket or TCP.
- Control over cut-over phase: gh-ost can be instructed to postpone what is probably the most critical step: the swap of tables, until such time that you’re comfortably available. No need to worry about ETA being outside office hours.
- External hooks can couple gh-ost with your particular environment.
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