Compaq Infortrend manual Host-side and Drive-side SCSI Parameters, Host-side SCSI Parameters

Figure 9 - 1 SCSI ID/LUNs

9.2Host-side and Drive-side SCSI Parameters

Foreword: SCSI Channel, SCSI ID and LUN

A SCSI channel (SCSI bus) can connect up to 15 devices (not including the SCSI controller itself) when the Wide

function is enabled (16-bit SCSI). It can connect up to 7 devices (not including the controller itself) when the Wide function is disabled (8-bit SCSI). Each device has one unique SCSI ID. Two devices owning the same SCSI ID is not allowed.

The figure on the left is a good example. If you are to file document into a cabinet, you must put the document into one of the drawers. From a SCSI’s point of view, a SCSI ID is like a cabinet, and the drawers are the LUNs. Each SCSI ID can have up to 32 LUNs (Logical Unit). Data can be stored into one of the LUNs of the SCSI ID. Most SCSI host adapters treat an LUN like

another SCSI device.

The same holds true for a Fibre channel host interface. 32 LUN’s are supported with each host ID.

9.2.1 Host-side SCSI Parameters

Maximum Concurrent Host LUN Connection (“Nexus” in

SCSI):

The configuration option adjusts the internal resources for use with a number of current host nexus. If there are four host computers (A, B, C, and D) accessing the array through four host IDs/LUNs (ID 0, 1, 2 and 3), host A through ID 0 (one nexus), host B through ID 1 (one nexus), host C through ID 2 (one nexus) and host D through ID 3 (one nexus) - all queued in the cache - that is called 4 nexus. If there are I/Os in the cache through 4 different nexus, and another host I/O comes down with a nexus different than the four in the cache (for example, host A access ID 3), controller will return "busy.” Mind that it is "concurrent" nexus, if the cache is cleared up, it will accept four different nexus again. Many I/Os can be accessed via the same nexus.

9-11 Infortrend