Coda file and data servers store several kinds of data.
The data which appears on clients as files, directories, and so on is stored on the file server, of course, but not in this format. Each Coda file server has at least one physical partition, usually named '/vicepa', '/vicepb', and so forth, where some of the contents of the Coda file system are stored in a binary format. There is approximately one file per file, but they are arranged in a hash tree.
The file servers store information about the cell's volumes (what kind they are, where they are, and so on), the mapping between Coda users and groups and their ID numbers, the list of groups to which each user belongs, the list of members for each group, the shared secrets which allow servers to communicate with each other, information about backup (whether each volume has been or will be backed up, when, so that you can do incremental backups); information about the cell (which machine is the master SCM server, what other servers there are, and so on).
Data on its way to its final home in the vice partitions gets stored in 'recoverable virtual memory' on that file server, to protect its integrity. Normally RVM requires two raw partitions on each file server, in addition to the partition(s) used for /vicepa, (/vicepb,...).
That is, if you plan to make a machine a Coda file server, you should have at least three physical partitions to be devoted to Coda: two raw partitions to be used by RVM, and one file system mounted on /vicepa to contain Coda files.