Actually, the answer is a little more straightforward when Radius is involved.   No package including Radius should be reading from a flat file (cached or not).  In the case of Radius, the users file can quickly become a problem after a few thousand users.    With SQL, proper indexing can allow lookups to be fairly fast but even then after a few hundred thousand users, SQL starts to ache.   LDAP used as a general purpose user/information store was designed to scale to literally millions of users so it does well as a back-end authentication source due to its scalability and speed (far faster than MySQL, Postgres, or Oracle for that matter).   SQL (MySQL for example) on the other hand is quite nice for storing the Radius accounting data.  
 
Read from LDAP and write to SQL.  Hmmmm.......   A nice blend of technologies that excel in their respective areas.  
 
Our servers have run in this configuration almost flawlessly (given a few DOS attacks) and auth users in a few seconds after PPP negotiations.
 
 
 

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