That depends on the circumstances. To quote the manpage:

> If you use link mode, the upgrade will be much faster (no file copying) and use less disk space, but you will not be able to access your old cluster once you start the new cluster after the upgrade. Link mode also requires that the old and new cluster data directories be in the same file system. (Tablespaces and pg_wal can be on different file systems.) Clone mode provides the same speed and disk space advantages but does not cause the old cluster to be unusable once the new cluster is started. Clone mode also requires that the old and new data directories be in the same file system. This mode is only available on certain operating systems and file systems.

I'm not familiar this much with MySQL then: if you upgrade your MySQL database, can you still roll back to the old version? If you can't then this is the same restriction when you use the `pg_upgrade` tool in link mode.

(Note that one approach if concerned about the potential for downtime if it goes wrong might be to `pg_upgrade` your secondary in a hot-standby replication setup, then failover and `pg_upgrade` the former primary. In practice, `pg_upgrade` runs very quickly — seconds even on multi-terabyte databases — afaik it doesn't touch the actual data but just the metadata, but read the man pages for more intricate details on that :)).