JSA supports high availability (HA) clusters, which are two JSA hosts that act as a single system. The primary host is the active host and the secondary host is the standby host. If the primary host fails, the secondary host takes over and becomes the active host1 To configure an HA cluster, you must ensure that the software versions on both primary and secondary hosts are the same, and that the cluster virtual IP address is an unused IP address on the same subnet as the primary and secondary hosts1 The secondary host does not backup multiple JSA primary hosts, as each HA cluster consists of only one primary and one secondary host1 The primary and secondary hosts do not need to be configured with the same storage devices, as long as they have enough disk space to store the same amount of data2 References:

1: HA Clusters | Junos OS | Juniper Networks

2: JSA Virtual Appliance | Juniper Networks US