NonStop S-Series Server Description Manual (G06.27+)

Input/Output Operations
HP NonStop S-Series Server Description Manual—520331-004
10-2
Storage and Communications I/O Compared
Storage and Communications I/O Compared
Figure 10-1 illustrates the basic similarities and differences between storage I/O and
communications I/O. Both modes of operation transfer information through the
ServerNet hardware whenever data needs to be exchanged between a processor and
a controller. However, workstations on an external network conceivably could
communicate among themselves for long periods of time without needing ServerNet
transfers, whereas, in the case of storage I/O, all data transfers are through the
ServerNet hardware.
In the case of storage I/O, there is generally an application of some kind that originates
the need for an I/O transfer. It sets up a buffer for receiving information from a storage
device, or sets up and fills a buffer for transmission to a storage device. Then the
application, through other software that typically includes the file system, sends its I/O
transfer request to an I/O process. The I/O process converts the request to commands
appropriate for the I/O device (such as SCSI commands) and sends these commands
through the ServerNet hardware to the controller associated with the target device.
At that point, still in the case of storage I/O, the controller assumes control of the I/O
operation, issuing commands to the I/O device and interacting with the ServerNet
hardware to transfer the specified data to or from the client’s data buffer. Transfers on
both sides of the ServerNet hardware, to and from the controller and to and from the
processor, are achieved with direct memory access (DMA), requiring no involvement of
the I/O process.
In the case of communications I/O, there is no I/O process. Instead, the communication
is between a NonStop host application in the server and an application in one of
several workstations. Transfers that are inbound to the host server (A) originate
externally by some request from a workstation or other device that is part of an
external network. Transfers that are outbound from the host server (B) typically provide
information or control from some service in the NonStop processor.
Queue services in the processor maintains an orderly list of such requests in circular
queues, so that sudden bursts of requests involving some particular resource can be
accommodated fairly. When a particular request rises to the top of its queue, queue
services and a controller interact through ServerNet services to transfer the data using
DMA on both sides of the ServerNet hardware.
Supported communications protocols include local area networks (LANs) and
Asynchronous Transfer Mode (ATM). As shown, wide area networks (WANs) and ATM
circuits interface to NonStop servers through a ServerNet wide area network (SWAN)
concentrator. A Common Communications ServerNet adapter (CCSA) is available to
which support the SS7 protocol.