## Storage Device Driver Reference

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### Devices Not Capable of Scatter/Gather

Although this interface is clearly targeted for a disk device controller capable of scatter/gather, even devices that are not capable of scatter/ gather still realize overall system performance gains as a result of supporting multi-request, asynchronous I/O, and interrupt-time execution. The driver is responsible for deciding how to emulate scatter/gather. In general, emulating scatter/gather to programmed I/O devices introduces no significant overhead.

However, if DMA devices, which do not support scatter/gather, attempt to emulate scatter/gather by mapping each scatter/gather descriptor in a single request to one DMA operation, performance will be poor. The driver is better off staging transfers through a pair of contiguous buffers. One buffer can be serviced by the controller while the other is being set up for subsequent operations. While there is overhead incurred in block- copying data through the staging buffer, this is no worse than the overhead the file system would incur in contiguous cache blocks before submission to a standard OS/2 device driver.