  Driven by the rising popularity of cloud storage, the costs associated with
implementing reliable storage services from a collection of fault-prone servers
have recently become an actively studied question. The well-known ABD result
shows that an f-tolerant register can be emulated using a collection of 2f + 1
fault-prone servers each storing a single read-modify-write object type, which
is known to be optimal. In this paper we generalize this bound: we investigate
the inherent space complexity of emulating reliable multi-writer registers as a
fucntion of the type of the base objects exposed by the underlying servers, the
number of writers to the emulated register, the number of available servers,
and the failure threshold. We establish a sharp separation between registers,
and both max-registers (the base object types assumed by ABD) and CAS in terms
of the resources (i.e., the number of base objects of the respective types)
required to support the emulation; we show that no such separation exists
between max-registers and CAS. Our main technical contribution is lower and
upper bounds on the resources required in case the underlying base objects are
fault-prone read/write registers. We show that the number of required registers
is directly proportional to the number of writers and inversely proportional to
the number of servers.
