45 lines
2.2 KiB
Markdown
45 lines
2.2 KiB
Markdown
# Testing on replica
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`gh-ost`'s design allows for trusted and reliable tests of the migration without compromising production data integrity.
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Test on replica if you:
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- Are unsure of `gh-ost`, have not gained confidence into its workings
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- Just want to experiment with a real migration without affecting production (maybe measure migration time?)
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- Wish to observe data change impact
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## What testing on replica means
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TL;DR `gh-ost` will make all changes on a replica and leave both original and ghost tables for you to compare.
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## Issuing a test drive
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Apply `--test-on-replica --host=<a.replica>`.
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- `gh-ost` would connect to the indicated server
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- Will verify this is indeed a replica and not a master
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- Will perform _everything_ on this replica. Other then checking who the master is, it will otherwise not touch it.
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- All `INFORMATION_SCHEMA` and `SELECT` queries run on the replica
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- Ghost table is created on the replica
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- Rows are copied onto the ghost table on the replica
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- Binlog events are read from the replica and applied to ghost table on the replica
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- So... everything
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`gh-ost` will sync the ghost table with the original table.
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- When it is satisfied, it will issue a `STOP SLAVE IO_THREAD`, effectively stopping replication
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- Will finalize last few statements
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- Will terminate. No table swap takes place. No table is dropped.
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You are now left with the original table **and** the ghost table. They _should_ be identical.
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You now have the time to verify the tool works correctly. You may checksum the entire table data if you like.
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- e.g.
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`mysql -e 'select * from mydb.mytable order by id' | md5sum`
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`mysql -e 'select * from mydb._mytable_gst order by id' | md5sum`
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- or of course only select the shared columns before/after the migration
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- We use the trivial `engine=innodb` for `alter` when testing. This way the resulting ghost table is identical in structure to the original table (including indexes) and we expect data to be completely identical. We use `md5sum` on the entire dataset to confirm the test result.
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### Cleanup
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It's your job to:
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- Drop the ghost table (at your leisure, you should be aware that a `DROP` can be a lengthy operation)
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- Start replication back (via `START SLAVE`)
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