Here the web servers contain the live version of the application, which is also present on one of the development servers and the previous versions, stored by the version control system. The cache servers only contain volatile data. Their role is to store the results of the most common requests so that requests aren't sent to the database unnecessarily. Only the file server, database servers and the version control system contain critical data.
The file server contains the documents that are stored and shared by the students via the social network. Data on the VMs running on the database servers can't be lost; if it could, it wouldn't be possible to restore the service to the same state it was in before the incident.
Data on the development servers constitutes several months of work; it therefore needs to be backed up outside the infrastructure. To that we then add the VM that acts as the VPN server, admin node and configuration management server (on Puppet). It's vital to back up this VM so that the infrastructure can be rebooted quickly when required.