Dual Channel PCI SCSI Adapters
ID: Q133492
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The information in this article applies to:
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Microsoft Windows NT operating system version 3.1
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Microsoft Windows NT Advanced Server version 3.1
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Microsoft Windows NT Workstation versions 3.5, 3.51
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Microsoft Windows NT Server versions 3.5, 3.51
SUMMARY
You can usually get better performance out of any SCSI device using a
multiplexed dual-channel PCI SCSI adapters. This article describes the SCSI
adapter and its advantages.
MORE INFORMATION
Multiplexed dual-channel PCI SCSI adapters are two SCSI controllers on one
card. The controllers often share a single interrupt, but each has its own
I/O address and interface. Although typical SCSI adapters use a single RISC
processor, a dual-channel SCSI adapter has two RISC processors (with each
processor controlling its own SCSI bus).
There are several reasons for using dual-channel PCI SCSI adapters, but the
main benefit is a dual-channel controller saves one slot over two single
controllers. By using one controller that switches between channels, dual-
channel adapters can support up to 14 devices while theoretically providing
the same throughput as a single-channel SCSI-2 adapter (which can support
just seven devices). On a dual-channel adapter, the two SCSI buses operate
independently. Data is transferred across the PCI bus by one controller at
a time, but there is no throughput loss because the PCI bus operates much
faster than the two SCSI buses combined. This assumes an aggregate SCSI-2
transfer rate of 20 MB per second (2 channels x 10MB per channel), compared
with the 132 MB per second burst-mode speed of the 33 mHz PCI bus.
Software-based disk striping, disk duplexing, and RAID support also work
better with dual-channel SCSI adapters. Disk striping offers the best
performance if there are multiple disk drives without disk duplexing or
RAID enabled. Dual-channel SCSI adapters can assume two basic
configurations during operation. The first distributes devices equally
between the two controllers. The second one takes a hint from dual-channel
EIDE (Enhanced IDE) controllers. Slower devices such as scanners, printers,
CD-ROM drives and tape drives are placed on one channel, whereas faster
devices such as hard disks and optical disks are placed on the other
channel. Separating slow and fast devices works better when not using
striping, duplexing, or RAID because each channel can run at the most
efficient speed for the devices attached to it. Microsoft Windows 95 and
Windows NT take better advantage of the dual-channel adapters because they
can issue multiple disk requests simultaneously.
Additional query words:
prodnt 3.10 dual-scsi
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