How does table locking affect table engine change from MyISAM to InnoDB?

Question

So I have been asked to change the engine of a few tables in a production database from MyISAM to InnoDB. I am trying to figure out how that will affect usage in production (as the server can afford no downtime).

I have read some conflicting information. Some information I have read state that the tables are locked and will not receive updates until after the conversion completes (IE, updates are not queued, just discarded until it completes).

In other places, I have read that while the table is locked, the inserts and updates will be queued until the operation is complete, and THEN the write actions are performed.

So what exactly is the story here?

Answer 1

This is directly from the manual :

In most cases, ALTER TABLE makes a temporary copy of the original table. MySQL waits for other operations that are modifying the table, then proceeds. It incorporates the alteration into the copy, deletes the original table, and renames the new one. While ALTER TABLE is executing, the original table is readable by other sessions. Updates and writes to the table that begin after the ALTER TABLE operation begins are stalled until the new table is ready, then are automatically redirected to the new table without any failed updates.

So, number two wins. They're not "failed", they're "stalled".

Answer 2

The latter is correct. All queries against a table that's being altered are blocked until the alter completes, and are processed once the alter finishes. Note that this includes read queries (SELECT) as well as write queries (INSERT, UPDATE, DELETE).