NonStop Systems Introduction for H-Series RVUs

Integrity NonStop NS-Series Server Architecture
NonStop Systems Introduction for H-Series RVUs—540083-001
7-9
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.
In Figure 7-6, 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, a pair of SACs connects to
routers 1 and 4. and 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 a failure disables
one of the pair of SACs or one of the paths, continuous communication can continue
through the other path and SAC.
For the second way, four SACs connect to routers 3 and 6. These SACs are dual-
ported, meaning that they connect to both the X fabric and the Y fabric. Dual porting
Figure 7-6. Dual Data Paths to I/O Hardware
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ServerNet Y fabric
ServerNet X fabric
1
4
ServerNet addressable
controller (SAC)
SCSI
bus
SAC
SAC
SAC
SAC
ServerNet addressable
controller (SAC)
VST094.vsd
Processor 0
ServerNet
interface
Processor 1
ServerNet
interface