In our case, the web servers containing the production version of the application, which are also present on a development server, as well as earlier versions are preserved by versioning system. The cache servers in turn contain only volatile data: their role is to store the result of the most common queries to avoid unnecessary requests to the databases which is not helpful. Ultimately, only the file, data base and the system versioning servers contain critical data.
The file server contains the documents stored and shared by students across the social network. The data contained in the database on the VM cannot be lost, otherwise the service could not be restored to the state it was in prior to an incident.
The data on the development server represents many months of work. This data must also be stored outside of the infrastructure. Add to this the VM which is used as the office VPN server, admin node and configuration management server (under Puppet), of which its backup is indispensable for quickly restarting the infrastructure if required.