- ATTO FibreBridge Installation & Operations Manual

ATTO Technology, Inc. ATTO FibreBridge Installation & Operation Manual
- 36 -
ATTO Technology, Inc.
Appendix A: Fibre Channel to SCSI Address Mapping
The ATTO Technology FibreBridge allows parallel SCSI devices to participate in a Fibre Channel
arbitrated loop or on a fabric. Fibre Channel and parallel SCSI use different models to address devices.
The FibreBridge translates between these addressing models. This paper describes both addressing
models and address mapping, and provides guidelines for establishing addresses in typical
configurations.
The following chart provides an overview of data and control flow between the application and the SCSI
device through a number of stages. Many details of operation are hidden due to a very high level of
abstraction.
Application
Read & write data.
Operating System
SCSI LUN addressing in OS native form, e.g. dense LUN list starting at 0.
Host Bus Adapter
Maps OS native LUNs to Addressing Method (PDAM, VDAM, LUAM, none).
FCP (SCSI)
FCP_CMD: 4 levels of 16-bit LUNs plus CDB plus data.
FibreBridge
Parse LUN (per Addressing Method) into SCSI Bus or FibreBridge/SCSI address.
Executes SCSI commands directed to the FibreBridge.
SCSI bus
Transport SCSI address, CDB, and data to/from SCSI device.
SCSI device
SCSI device operation.
Addressing Devices Connected to the FibreBridge
SCSI devices connected to the FibreBridge also show up as Fibre Channel LUNs to the host computer.
This is intuitive because the SCSI devices must be on the same addressing level as the SCSI portion of
the FibreBridge. This implies that the FibreBridge must be set to a different ID than the devices on the
bus.
Even though the host computer will see the SCSI portion of the FibreBridge as a LUN, we should instead
refer to this address as a SCSI Target ID (to be consistent with the SCSI world). SCSI devices connected
directly to the FibreBridge will also be referred to as having Target IDs instead of LUNs. This is what
allows you to have a LUN of a LUN. If the FibreBridge is a Target ID, and a RAID Controller connected to
the FibreBridge is a Target ID, then SCSI drives connected to the RAID Controller can be addressed as
LUNs - or a LUN of a Fibre Channel LUN (or, as we just defined it – a LUN of a Target ID). One problem
now remains. How does the host computer identify these drives? The SCSI Target IDs (FibreBridge,
directly connected drives, & RAID controllers) are combined with LUNs (drives connected to the RAID
Controllers) into one level of addressing – LUNs, as seen by the host computer.
This "combining" of SCSI Target IDs and SCSI LUNs into one level of addressing is made possible by the
number of bits available in the first level addressing field (two bytes) within the FCP_ LUN field. The
FibreBridge designates a certain number of bits for the SCSI LUN, the Target ID, and the Bus. The Bus
Identifier is used to distinguish between the two SCSI busses on the FibreBridge and will be discussed in
more detail below. The methodology behind this follows the rules of SCSI Hierarchical Addressing.