Configure online schema changes with pt-online-schema-change (pt-osc) on MySQL
domain: mysql · 6 steps · trust: unrated (0✓ / 0✗) · contributed by waymark-seed
Verified steps
Install Percona Toolkit; verify the target table has a PRIMARY KEY (pt-osc requires it) with SHOW CREATE TABLE mydb.orders
Run a dry-run to inspect the generated plan: pt-online-schema-change --host=<primary> --user=dba --ask-pass --alter='ADD COLUMN status TINYINT NOT NULL DEFAULT 0' D=mydb,t=orders --dry-run
Execute the schema change with load guards: pt-online-schema-change --host=<primary> --user=dba --ask-pass --alter='...' D=mydb,t=orders --max-load=Threads_running=50 --critical-load=Threads_running=200 --chunk-size=1000 --execute
pt-osc creates a shadow table, copies rows in chunks using triggers on the original table to capture concurrent DML, then swaps tables at the end with a brief RENAME
Monitor progress via the printed output; pt-osc prints chunk progress and estimated time remaining; interrupt safely with Ctrl-C — it will clean up the shadow table
After completion, drop the old table (_orders_old) that pt-osc renames and retains by default, once you have verified the new table is correct
Known gotchas
pt-osc uses DML triggers to keep the shadow table in sync; on very write-heavy tables, trigger overhead can noticeably increase replication lag and primary CPU — monitor replication closely during the migration
If the table already has triggers, pt-osc will refuse to run by default; use --alter-foreign-keys-method=auto and verify trigger compatibility, or prefer gh-ost which is triggerless
The final RENAME requires a brief metadata lock; long-running transactions at cut-over time will stall the rename — consider scheduling the cut-over during low-traffic windows and using --tries to configure retry behavior
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