SCSI Solutions White Paper - HP-UX

4 Configuration

Setting up parallel SCSI solutions requires careful attention to configuration issues and details.

The following basic rules and definitions apply. Additional rules need to be considered in

shared SCSI bus environments as discussed in Section 5.

Basic Configuration Rules/Definitions

Note: It is generally not recommended to mix disks with tapes or other non-disk devices on the

same SCSI bus. This is due to performance and starvation issues that can result in mixed

configurations.

1. Electrical Signaling (do not mix HVD devices with LVD/SE devices on the

same bus): As discussed above in Section 1.2, devices and HBAs come in three

signaling types: the older SE and HVD, and the newer LVD. When LVD was defined in

Ultra2 SCSI and beyond, a multi-mode option was also defined allowing devices to

support both LVD and SE, with the restriction that in SE mode the bus is limited to Ultra

speeds. HVD devices cannot coexist on the same bus with LVD/SE devices, unless the

HVD devices are connected through an HVD-to-LVD converter.

2. Cable length (must not exceed maximum limits for a given type of bus):

Exceeding the maximum total cable length for a given type of bus can result in signal

integrity issues. It is therefore very important to adhere to the following limits on the

total cable length from beginning to end of the bus, including any cable internal to SCSI

enclosures, HBAs, disk arrays, etc. The maximum length for different types of buses is

shown in Table 1, and the common cases are summarized below:

§ SE Ultra SCSI: 3 meters up to 4 devices; 1.5 meters >4 devices

§ HVD up to Ultra SCSI: 25 meters

§ LVD Ultra2, Ultra160, and Ultra320: 12 meters

CAUTION: When determining cable length, the internal length within devices, enclosures,

or HBAs must be estimated and included. It is often thought, for example, that one can

connect 12 meters of external cable on an LVD bus, when in fact the HBAs and enclosures

may use 1 to 3 meters of bus length internally.

3. SCSI Addressing Definitions and Priorities: narrow SCSI buses support 8 SCSI

addresses, 0..7, with 7 being the highest priority and 0 the lowest, while wide SCSI

buses support 16 SCSI addresses, 0..15, with the priority (highest to lowest) going from

7 to 0 followed by 15 down to 8. Ultra160 and Ultra320 buses are wide-only.

4. Unique Address assignment for each device: Each device on a SCSI bus

(including SCSI initiators) must have a unique SCSI address. Duplicate SCSI addresses

on a SCSI bus can prevent systems from booting or otherwise operating correctly. SCSI

enclosures typically guarantee unique addresses among the various internal drives, with

mode settings to support higher priority initiator addresses as discussed below.

5. SCSI Initiator Addressing (must be higher priority than any target): Each

SCSI initiator must have a unique SCSI address and it must be at a higher priority than

any SCSI target on the bus. A SCSI initiator is an HBA port, and a SCSI target is a

peripheral device such as a disk or tape drive. SCSI enclosures typically have mode

settings that reserve various high priority address assignments for initiators (not allowing

them to be assigned to the enclosure’s devices).