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SCSI Hardware on CHALLENGE and Onyx Systems

Two SCSI-2 controllers are incorporated in each POWER Channel-2 board. Additional controllers can be added in two groups of 3 on HIO modules, for a maximum of 8 controllers per channel. The SCSI controllers are DMA devices that make near-optimum use of the POWER Channel-2 bus bandwidth.

Using 8-bit data transfers, a data rate of approximately 7 megabytes per second can be reached, per controller. Using 16-bit SCSI, data rates near 14 MB per second can be achieved, per controller.

Note: These data rates can be achieved by configuring the system to perform DMA directly into the requesting process's address space without buffering. This requires the use of Direct file access for disk files (see "Using Direct I/O"). The generic SCSI device driver also performs DMA directly in or out of the process's buffers (see "Generic SCSI Device Driver"). All Challenge/Onyx SCSI controllers are capable of differential output and so can be cabled longer distances from the system cabinet.


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