I have a large InnoDB table that I want to alter by adding a small column to. I'm prepared to wait a while, but I'm wondering how much disk space will the
ALTER TABLE command consume while it's running? Will I need enough room on my disk for two full copies of the table?
Yes. You will need enough space for two copies of the table. MySQL performs alterations of tables by creating a new table, copying data from the old, and then deleting the old table.