When you use Oracle VM Server for SPARC, you can assign most of the memory in a SPARC T7 server to logical domains. However, a small portion of the server's memory is preassigned to software components, the hypervisor, and certain I/O devices. Also, if DIMM sparing is active, a portion of memory is reserved to enable the server to operate normally after a DIMM has failed.
To determine which portions of memory are unavailable for logical domains, type:
# ldm ls-devices -a mem
In the output, look for rows that include _sys_ in the Bound column. Each of those portions of memory is not available for logical domains.
For more information about DIMM sparing, refer to DIMM Sparing Overview in SPARC T7 Series Servers Administration Guide.