On SPARC T7 servers, the physical address space that is provided by DIMMs is interleaved for performance reasons. When a group of 16 DIMMs is present on a server, both 16-way and 15-way interleaving are supported. This means that if there is an unusable DIMM, the CPU node can still provide 15 DIMMs worth of physical address space. With DIMM sparing, the CPU node doesn't have to drop to an 8-way interface, which would only provide 8 DIMMs worth of physical address space.
If a DIMM is diagnosed to be faulty while the system is running, the memory will dynamically switch from 16-way to 15-way interleave by distributing the contents of the faulty DIMM into the other 15 DIMMS. To enable this redistribution, the platform firmware must initially reserve space for the contents of one DIMM. Consequently, only 15 DIMMs worth of physical address space will be made available to the system, even when 16 DIMMs are functioning.
Keep in mind that, due to the requirements of DIMM sparing, the amount of available memory reported might be less than the amount you had estimated based on the quantity and capacity of the server's DIMMs.
When DIMM sparing is enabled, if the server must unconfigure a DIMM at boot time or run time, the associated fault is treated as a nonserviceable fault, so a service notification is not issued. If one DIMM in a group of 16 is unconfigured, you do not need to replace that DIMM until another DIMM in that group is unconfigured.
DIMM sparing is enabled by default on CPU nodes that have 16 DIMMs installed. For example on a SPARC T7-1 server with only 8 DIMMs (and no memory risers), DIMM sparing is not available.