SCSI Solutions White Paper - HP-UX

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6. Bus Termination: Each end of a SCSI bus, and only the ends, must be terminated.
If improperly terminated at the ends, bus integrity may be compromised. Termination in
the middle can result in some devices being inaccessible. See the discussion on
termination in Section 3 for additional details.
5 High Availability Configurations
High Availability (HA) refers to the ability to keep a system or component operational
(available) for a significant percentage of the time. We refer to HA configurations in pSCSI as
configurations which work together within larger HA solutions to allow for failure of a
component without bringing down the operation of the system or access to data. Shared bus
configurations
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are a key aspect of HA support in pSCSI. The following general rules and
definitions apply to shared pSCSI buses.
Shared Bus Configuration Rules/Definitions
Note: Shared SCSI buses require that both the HBA and the host drivers for the HBA support
multiple SCSI initiators. See Section 2.4 and the referenced support matrices for details of which
drivers/HBAs support multiple initiators.
1. Shared SCSI bus – termination: two of the initiators must be at each end of the
bus with termination enabled at the HBA port.
2. Shared SCSI bus – targets: The only target devices allowed on a shared SCSI bus
are disk devices, including disk arrays. No other devices, such as CD-ROMs, Magneto-
Optical disks, tape drives, or scanners, are allowed on a shared SCSI bus in HP-UX.
3. Shared SCSI bus – initiators: There can be no more than four SCSI initiators (HBA
ports) on any shared SCSI bus: one at each end of the bus, and up to two additional
initiators attached via V-cables in the middle of the bus. As noted in section 3.1.1,
attachment of HBAs in the middle of the bus requires that the HBA’s auto-termination be
disabled. Using more than two initiators on a bus will result in online repairability issues
as discussed in item 7 below.
4. Cables/Connectors: In-Line-Terminated (ILT) cables and V-cables are not supported
at Ultra2 speeds or higher (see section 3.1). The MSA30 MI and DS2300 enclosures
provide similar functionality to ILT cables in certain configurations, as discussed below.
5. Electrical Isolation of MSA30 MI and DS2300 SCSI ports: These enclosures
have four SCSI ports to which up to four HBA ports (initiators) can be attached
(DS2300 supports a maximum of two initiators). Each pair of SCSI ports in these
enclosures is electrically isolated from each other, allowing an HBA attached to one of
the port pairs to be disconnected without affecting the operation of HBAs attached to
the other, as discussed in more detail in Online Repairability below. In the MSA30 MI
the electrical isolation is a natural result of its split-bus operation: each pair of SCSI
ports is attached to a separate bus. In the DS2300 each pair of SCSI ports is
associated with a module called the Bus Control Card (BCC). The DS2300 can be
configured in either split-bus or full-bus mode. In split-bus mode, the separate buses
provide electrical isolation. In full-bus mode, the electrical isolation is provided with
respect to cable disconnection in that the DS2300 detects when a BCC is un-terminated
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A shared SCSI bus is a bus attached to multiple SCSI initiators.