Explanation
https://docs.paloaltonetworks.com/panorama/11-0/panorama-admin/manage-log-collection/manage-collector-gro
"Log redundancy is available only if each Log Collector has the same number of logging disks." (Recommended) Enable log redundancy across collectors if you are adding multiple Log Collectors to a single Collector group. Redundancy ensures that no logs are lost if any one Log Collector becomes unavailable. Each log will have two copies and each copy will reside on a different Log Collector. For example, if you have two Log Collectors in the collector group the log is written to both Log Collectors. Enabling redundancy creates more logs and therefore requires more storage capacity, reducing storage capability in half. When a Collector Group runs out of space, it deletes older logs. Redundancy also doubles the log processing traffic in a Collector Group, which reduces its maximum logging rate by half, as each Log Collector must distribute a copy of each log it receives.