Fujitsu D2860 Technical Manual page 22

Main memory
Spare channel mode
In this mode, the channel C is a spare of the two active channels A and B. The
channel C is not available as system memory. The equipping is identical with
the performance channel mode.
All three channels must be populated identically with regards to size and
organization.
Memory slot populations within a channel do not have to be identical but the
same memory slot location across three channels must be populated the
same way.
Can run with differently rated memory modules and use the settings of the
slowest memory module within a channel.
Timings in the active channels (A and B) and the spare channel (C) can be
set to different values.
Only 2/3 of the maximum memory bandwidth is achievable.
The total physical memory available to the system is 2/3 of what is
populated.
22 Technical Manual
Features
D2860 (BX920)