The failfast mechanism panics a failed node, but it does not prevent the failed node from rebooting. After the panic, the node might reboot and attempt to rejoin the cluster.
If a node loses connectivity to other nodes in the cluster, and it is not part of a partition that can achieve quorum, it is forcibly removed from the cluster by another node. Any node that is part of the partition that can achieve quorum places reservations on the shared disks. The node that does not have quorum then panics as a result of the failfast mechanism.