The high-level code supports ``scatter-gather,'' a method of increasing
SCSI throughput by combining many small SCSI requests into a few large SCSI
requests. Since most SCSI disk drives are formatted with 1:1
interleave, the time required
to perform the SCSI ARBITRATION and SELECTION phases is longer than the
rotational latency time between sectors. Therefore,
only one SCSI request can be processed per disk revolution, resulting in a
throughput of about 50~kilobytes per second. When scatter-gather is
supported, however, average throughput is usually over 500~kilobytes per
second.
The 24 variable holds the maximum allowable number of
requests in the scatter-gather list. If the driver does not support
scatter-gather, this variable should be set to 25. If the
driver can support an unlimited number of grouped requests, this variable
should be set to 26. Some drivers will use the host adapter
to manage the scatter-gather list and may need to limit 27
to the number that the host adapter hardware supports. For example, some
Adaptec host adapters require a limit of 16.