NonStop Systems Introduction

NonStop Server Architecture
NonStop Systems Introduction—527825-001
7-6
Dual Access Paths for I/O
In the ServerNet architecture, each device controller is a separate node on the
ServerNet fabric, just like processors, and device controllers are therefore properly
identified as ServerNet addressable controllers, or SACs.
Each SAC manages one or more I/O devices—either directly or through some specific
protocol such as SCSI (small computer system interface), LAN (local area network),
ATM (asynchronous transmission mode), and so on. When a SAC receives an I/O
command from the I/O process, the controller activates the specified device and
prepares it to send input or receive output. Once activated, the device sends a stream
of data through the ServerNet hardware to memory, or receives a stream of data
through the ServerNet hardware from memory.
Looking at Figure 7-5, you can see that there are two ways of achieving fault tolerance
for the SACs and their related I/O devices. For the first way, notice the pair of SACs
connected to routers 1 and 4. A SCSI bus between the two SACs controls some
number of I/O devices. Any processor can access the devices on the SCSI bus
through either the ServerNet X fabric (router 4) or the ServerNet Y fabric (router 1). If
Figure 7-5. Dual Data Paths to I/O Hardware
Processor 0 Processor 1
ServerNet
interface
32
65
XY YX
ServerNet Y fabric
ServerNet X fabric
1
4
ServerNet addressable
controller (SAC)
SCSI
bus
SAC
SAC
SAC
SAC
ServerNet
interface
ServerNet addressable
controller (SAC)
VST094.vsd