Doubt about pg_upgrade with link mode

Hi,

I'm trying to upgrade a PG 9.1 cluster to a 9.3 one.
I'm trying to use --link to do it in link mode since I don't have much
disk space for the migration.

I'm not totally sure that the --link parameter is being honored. I
can't see any mention about link mode in the migration logs and the
new cluster takes too much space.

I would like to understand how link mode really works. What does it
link and what does it copy. Unfortunately I didn't find information
about that. Any link that provides more info would be appreciated.

I've compare inodes from base files in the old data directory and the
new one and they are different.

copying "/9.1/base/18976/74003" to "/9.3/base/16416/73829"

$ stat /mnt/pg_data/base/18976/74001 --printf=%i
12454860

$ stat /mnt/pg_data-9.3/base/16416/73829 --printf=%i
10488427

It doesn't seem to be a hard link. Should it?