Because a diskless server does not use any disk storage space, you must ensure that the server can hold all the expected data in memory, see Maximum file size in diskless servers.
A diskless server can contain disk-based tables (D-tables), in-memory tables (M-tables), or both. However, since the whole database must be stored in memory, there is no benefit in using D-tables.
D-tables are stored in the database cache, so you must ensure that the cache is large enough to accommodate the expected size of the D-tables. Even if all the tables are M-tables, the database cache is still required to hold system tables, see Cache size in diskless servers.