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

TL;DR gh-ost will make all changes on a replica and leave both original and ghost tables for you to compare.

Issuing a test drive

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, stopping replication
Will finalize last few statements
Will swap tables via normal cut-over, and immediately revert the swap.
Will terminate. No table is dropped.

You are now left with the original table and the ghost table. When using a trivial alter statement, such as engine-innodb, both tables 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 order by id' | md5sum mysql -e 'select * from mydb._mytable_gst order by id' | md5sum
or of course only select the shared columns before/after the migration
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.
When adding/dropping columns, you will want to use the explicit list of shared columns before/after migration. This list is printed by gh-ost at the beginning of the migration.

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)

Examples

Simple:

shell

$ gh-ost --host=myhost.com --conf=/etc/gh-ost.cnf --database=test --table=sample_table --alter="engine=innodb" --chunk-size=2000 --max-load=Threads_connected=20 --initially-drop-ghost-table --initially-drop-old-table --test-on-replica --verbose --execute

Elaborate:

shell

$ gh-ost --host=myhost.com --conf=/etc/gh-ost.cnf --database=test --table=sample_table --alter="engine=innodb" --chunk-size=2000 --max-load=Threads_connected=20 --switch-to-rbr --initially-drop-ghost-table --initially-drop-old-table --test-on-replica --postpone-cut-over-flag-file=/tmp/ghost-postpone.flag --exact-rowcount --concurrent-rowcount --allow-nullable-unique-key --verbose --execute

Count exact number of rows (makes ETA estimation very good). This goes at the expense of paying the time for issuing a SELECT COUNT(*) on your table. We use this lovingly.
Automatically switch to RBR if replica is configured as SBR. See also: migrating with SBR
allow iterating on a UNIQUE KEY that has NULLable columns (at your own risk)

Further notes

Do not confuse --test-on-replica with --migrate-on-replica; the latter performs the migration and keeps it that way (does not revert the table swap nor stops replication)

As part of testing on replica, gh-ost issues a STOP SLAVE. This requires the SUPER privilege. See related discussion on https://github.com/github/gh-ost/issues/162