Let's assume there are no vulnerabilities in the application layer, one disk, and no physical firewall (just iptables rules in the server itself), hence in this case all the DDoS attack does is to overload processor, RAM, and the disk file system by eventually causing huge amounts of log data from the server firewall (I am not sure if this would happen in an ordinary scenario).
Hypothetical (in my eyes) examples of stored data inconveniences:
Disk overloaded by firewall log data caused by DDoS.
The need to move the disk to another system in order to clean the log files.
Disk overload causing damage or loss of stored data.
That's to illustrate a bit the range of inconveniences, known or hypothetical, that I am talking about (storage and data integrity and conservation specifically), when the conditions are as decribed priorly.
Coming back to the original question, is it mandatory to add a physical firewall to prevent such kind of inconveniences? Or are there other kind of alternative solutions while keeping the same physical setup?