sharding

    A form of data partitioning in which a large
   database table is split over multiple servers in order
   to balance load (load balancing).  Some property of the data
   is used to select which server should handle a given row,
   e.g. the primary id modulo the number of servers.

   Sharding should be a last resort in database performance
   optimisation because of the difficulty of changing the
   allocation of data to servers, e.g. if the number of servers
   changes or the distribution is found to be uneven.

   <Sharding Your Database, Ovid, perl.org>.

   (2010-05-16)