NonStop S-Series Server Description Manual (G06.24+)

Input/Output Operations

HP NonStop S-Series Server Description Manual—520331-003

10-12

Packaging and Delivery of Storage Commands

A command entry (CE) in a request queue defines an I/O operation to be executed by

the controller firmware. Each such entry is 64 bytes in size, the maximum size that can

be conveyed in ServerNet packets. Figure 10-6 shows how these commands are

transmitted to a particular storage I/O device. The example shown assumes that the

device is a SCSI device.

When an I/O process needs to transfer data, it establishes one or more data buffers for

reception or transmission of data. The process then creates a command descriptor

block, which is a SCSI standard for command specification, then adds the client buffer

address (or addresses) and a header to format a complete 64-byte command entry.

The process then forwards the command entry to the module driver, which, by making

reference to information in the CE header, obtains the ServerNet ID and ServerNet

address of the destination from its work space and formats a ServerNet packet. Note

that the data part of the packet (D) consists of the entire CE. The CE is preceded by

the ServerNet packet header (H) and the ServerNet address (A) for writing the CE into

the destination queue, and it is followed by the cyclic redundancy check code (C).

When the module driver receives this packet, it forwards the packet to ServerNet

services for transmission through the ServerNet hardware to the ServerNet bus

interface (SBI) that is specified by the destination ServerNet ID in the packet header.

By reading the address field of the packet, the SBI is able to translate the ServerNet

address to a physical address and store the CE in the correct global queue.

The controller, after it polls its global request queues, eventually transfers the CE to

one of its own local queues, the one for the particular device being targeted. The

device is identified by a SCSI address contained within the CE, and not by a ServerNet

address or ServerNet ID. The controller uses command tags when identifying

commands issued to the storage I/O device in doing the external data transfer.