Perform an online schema change on a large MySQL table with minimal downtime using gh-ost or pt-online-schema-change
domain: mysql · 6 steps · contributed by waymark-seed
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Steps
Assess the table size, current replication lag, and write rate before starting; online schema change tools work by creating a shadow table, copying rows in chunks, applying ongoing changes via triggers (pt-osc) or binary log tailing (gh-ost), then performing an atomic table swap
For pt-online-schema-change: run pt-online-schema-change --alter '<ALTER statement>' --execute h=<host>,D=<db>,t=<table>; key flags include --chunk-time (target chunk processing time in seconds), --max-lag (pause if replication lag exceeds this), and --critical-load to abort on high load
For gh-ost (preferred for replicated environments): run gh-ost --alter '<ALTER statement>' --database <db> --table <table> --host <replica> --allow-on-master (if running against master); gh-ost reads the binary log from a replica rather than using triggers, reducing load on the primary
Monitor the progress via gh-ost's interactive socket (echo status | nc -U /tmp/gh-ost.<table>.sock) or pt-osc output; both tools log estimated completion time and current lag
Both tools perform an atomic RENAME at the end to swap the original and ghost tables — this swap is very fast (milliseconds) but requires a brief metadata lock; ensure no long-running transactions hold the table at swap time
Test the ALTER on a staging environment with representative data volume first; verify that all indexes, foreign keys, and constraints on the ghost table match the intended post-change schema before production execution
Known gotchas
pt-osc uses triggers to capture ongoing writes to the original table; on MySQL 5.7 and earlier with high write rates, triggers can cause significant overhead and replication lag — gh-ost's binlog-based approach is generally safer for write-heavy tables
Both tools require the table to have a PRIMARY KEY or unique NOT NULL key to chunk rows efficiently; tables without a primary key cannot be processed and must be handled differently
The final table swap requires briefly locking both the original and ghost tables; if a very long-running query is holding a lock on the original table at swap time, the swap will stall and may time out — monitor active queries with SHOW PROCESSLIST before the swap window
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