System Bus Sizing
The system bus has a fixed bandwidth. Too many devices on the bus
can create more traffic than the bus can handle, which results
in contention and packet loss.
prtdiag -v reports on many system bus configuration
issues. It is possible to see if the bus is overloaded by adding
up the reported capacities of the devices on a bus and seeing if
they exceed the capacity of the bus.
Where possible, similar cards can be placed on the same board
so that the interrupts are directed to the same CPU (and associated
This is a table of typical system bus capacities:
The two peripheral buses on Sun systems are Sbus and PCI bus.
Sbus runs at 20-25MHz and comes in 32 or 64 bit sizes. Peak
Sbus bandwidth is 200 MB/s. The venerable Sbus has been
retired in favor of the newer PCI bus as PCI bus performance
PCI buses runs at 33 or 66MHz and may be 32 or 64 bit. The peak
PCI bus bandwidth is 528 MB/s for 64-bit buses at 66MHz.
(For desktop PC hardware, 33MHz PCI buses are still common.
33 MHz buses have peak bandwidths of 264 MB/s for 64-bit and
132 MB/s for 32-bit.)
Newer PCI-x buses run at 133MHz and allow up to 1066 MB/s.
PCI-x 2.0 defines clock rates of 266MHz and 533MHz, with
peak bandwidths of 2.1 GB/s and 4.2 GB/s, respectively.
Low profile PCI buses are becoming more common, since their smaller
form factor fits well with the increasing miniaturization of
the system. Low profile PCI comes in MD1 and MD2 flavors, with
the primary difference being the shorter length of the MD1 cards.
Currently, they do not support 64-bit PCI extensions.
Mini PCI cards are also produced for use in portable and
sealed case computers. They are small in size, do not support
64-bit extensions and have a different connector layout, but
still otherwise follow the PCI standard.
SCSI buses can operate at one of these speeds:
SCSI buses and devices
negotiate speed between the controller and the devices on the
- 4 MB/s (asynchronous)
- 5 MB/s (synchronous)
- 10 MB/s (fast)
- 20 MB/s (ultra, fast/wide or fast-20)
- 40 MB/s (ultra/wide or narrow ultra-2)
- 80MB/s (wide ultra-2)
- 160 MB/s (ultra-3 or ultra-160)
- 320 MB/s (ultra-320)
prtconf can report information that can
be used to determine the speed of a particular device.
scsi_options parameter can be set in the
/etc/system file to limit bus speed or set other characteristics.
Check device documentation to determine if these settings need to
SCSI chains may be made of single-ended (SE) or differential connections.
Differential connections come in low voltage (LVD) and high
voltage (HVD) variants. SE, LVD, and HVD should not be mixed,
as this may damage the equipment.
Differential connections permit longer chains, but the hardware
is usually more expensive. Single-ended chains must be less than
6m in length; LVD chains must be less than 12m in length;
HVD chains must be less than 20m for synchronous connections
or 25m for asynchronous connections. (Remember that chain length
includes the length of the connectors and cabling in the devices,
not just the external cable.)
Starting with Ultra 2,
only differential connections are available. Only LVD is
available for Ultra160 or Ultra320.
The SCSI target numbers represent attachment points on the SCSI
chain. Each target number may include as many as 8 devices
(luns or logical unit numbers). Embedded SCSI devices only include
Higher target numbers receive better service. On a narrow bus,
the target priorities run 7 -> 0. On a wide bus, they run
7 -> 0, then 15 -> 8. The host adapter is usually 7. This can
cause problems where busy disks and tape devices share a SCSI
bus, since tape devices are usually assigned target 6.