postgresql – Is there any possible way to get a progress estimate to ALTER TYPE OF COLUMN in Postgres?

I know there is currently no officially supported way, but I have almost two 100% CPU hours on a 15,000,000 row table changing an INTEGER to SMALLINT and I have no idea how long this will continue. Is there a file I can see grow on the file system? Do you rewrite the table instead or create a new file? Is there a way to pause this so that I can run a benchmark on the same machine with fewer rows, so I can get a row per second estimate for this operation?