adding documentation

doc/testing-on-replica.md

@@ -6,3 +6,33 @@ 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`)