の08 Project Web Environment Details にある
>Group Directories are Read-Only: The project web servers mount project group directories read-only. The following means are available for storing writable data (i.e. for project applications):
>Place the data in the /tmp/persistent directory, accessible from and shared between the project shell and web servers. Projects should create their own subdirectory in /tmp/persistent and make frequent backups of the data. We strongly discourage the use of file-writing applications with our services, due to the security implications of our shared web environment. Users of applications in the project web services that require file-writing are strongly encouraged to look for alternatives that can use a MySQL database for data storage instead.