Compaq Infortrend Manuale Utente

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.

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.