gh-ost/doc/testing-on-replica.md

Testing on replica

gh-ost's design allows for trusted and reliable tests of the migration without compromising production data integrity.

Test on replica if you:

• Are unsure of gh-ost, have not gained confidence into its workings
• Just want to experiment with a real migration without affecting production (maybe measure migration time?)
• Wish to observe data change impact

What testing on replica means

Apply --test-on-replica --host=<a.replica>.

• gh-ost would connect to the indicated server
• Will verify this is indeed a replica and not a master
• Will perform everything on this replica. Other then checking who the master is, it will otherwise not touch it.
  • All INFORMATION_SCHEMA and SELECT queries run on the replica
  • Ghost table is created on the replica
  • Rows are copied onto the ghost table on the replica
  • Binlog events are read from the replica and applied to ghost table on the replica
  • So... everything

gh-ost will sync the ghost table with the original table.

• When it is satisfied, it will issue a STOP SLAVE IO_THREAD, effectively stopping replication
• Will finalize last few statements
• Will terminate. No table swap takes place. No table is dropped.

You are now left with the original table and the ghost table. They should be identical.

You now have the time to verify the tool works correctly. You may checksum the entire table data if you like.

• e.g. mysql -e 'select * from mydb.mytable' | md5sum mysql -e 'select * from mydb._mytable_gst' | md5sum

Cleanup

It's your job to:

• Drop the ghost table (at your leisure, you should be aware that a DROP can be a lengthy operation)
• Start replication back (via START SLAVE)