The Service Processor (SP) allows for control and operation of various components of the rack over the management network. The SP is always used in conjunction with a Root of Trust (RoT).
There are three types of boards that contain an SP: the server sled (Gimlet), the rack switch (Sidecar), and the power shelf controller (PSC). Each SP runs a software image built specifically for that board type.
Server Sled
Each server sled (“Gimlet”) includes both a host CPU and SP. While the host CPU operates customer (and control plane) workloads, the service processor is responsible for certain chassis management functions (e.g. thermal control) and provides essential interfaces to the control plane for functions like host system power control, access to the system’s serial console, and upgrade of some device software.
Switches
Each rack contains two switches, known as “Sidecars”. Each Sidecar contains a Tofino switch ASIC that’s responsible for switching both control plane traffic and data plane (user) traffic within the rack and routing traffic outside the rack. Every sled in the rack has one physical network connection to each of the two Tofino switches (via the rack backplane).
Each Sidecar includes an SP — just like server sleds — but there is no host system within a Sidecar. Rather, the Tofino switch is connected via an external PCIe cable to one of the adjacent server sleds. That sled (a "Sidecar-adjacent Gimlet", sometimes shortened to "Scrimlet") has the unique ability to control, monitor, and exchange network traffic with the Tofino switch.
Each Sidecar also has a separate management network switch used exclusively for providing connectivity to the different SPs in the rack.
Power shelf and PSC
The power shelf itself is largely a passive backplane that houses six rectifiers and one Power Shelf Controller (PSC). Without a PSC, the power shelf remains largely functional, handling loss and restoration of external power as well as failures of individual rectifiers. The PSC gives us:
PMBUS-based monitoring of power draw, health and status of rectifiers, etc. as well as rectifier firmware upgrade
presence indication for each rectifier via dedicated pins
on/off control for each rectifier via dedicated pins
The PSC, while a much simpler system that Gimlet or Sidecar, also contains an SP (in the same Gemini complex of SP and RoT). The PSC’s SP has dedicated pins for presence and on/off control for each rectifier. It differs from the SP in Gimlet or Sidecar in that it is the main component of the PSC whereas those other SPs control the main component.