HP Tape Library Interface Manager Software - Manual (Page 30)

30 Choosing the Right Configuration Chapter 2
Host Device Configuration
The host system using a FC host bus adapter (HBA) will typically map FC
devices into the existing device mapping scheme used by that operating
system. (Refer to your HBA manual for the mapping table.) The FC usually
maps FC addresses to SCSI target addresses. In such a configuration, the IM
corresponds to a bus:target identifier, with the SCSI devices attached to the IM
appearing as logical units (LUNs). In addition, operating systems can extend
the available SCSI limit of 15 Target IDs per bus.
Although this is not an issue for the operating system or most applications,
there are cases where older applications or Windows NT can have
expectations about what are valid SCSI IDs, and not deal correctly with certain
mappings. In particular, applications have been seen to exhibit difficulties
addressing Target IDs greater than 15 (e.g. 16 and up). This problem can be
resolved by configuring the IM to use hard addressing, and setting the AL_PA
used by the IM to a value that the HBA will map to, with an ID having a value
less than 16.
SCSI Addressing
A FC interface card appears on each SCSI bus as a single initiator. The default
initiator ID is 7. No other device on the SCSI bus may use this address. Note
that the FC host itself is not connected to the bus. Their SCSI operations are
passed down to the individual SCSI target devices through the use of the IM’s
SCSI Initiator ID.
The IM negotiates for the maximum values for transfer rates and bandwidth on
a SCSI bus. If an attached SCSI device does not allow the full rates, the IM will
use the best rates it can negotiate for that device. Negotiation is on a device
specific basis, so the IM can support a mix of SCSI device types on the same
SCSI bus.
Buffered Tape Writes
Buffered Tape Writes is an option designed to enhance system performance.
By returning status on consecutive write commands prior to the tape device
receiving data, Buffered Tape Writes remove the latency of waiting for
responses from the tape device. In the event that data does not transfer
correctly for any reason, the interface card will return a check condition on a
subsequent command.