Infortrend RAID Controller Manual

A SCSI channel (SCSI bus) can connect up to 15 devices (not
including the SCSI controller itself) when the Wide
Figure 10 - 4 SCSI ID/LUNs 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.
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10.3 Host-side and Drive-side SCSI
Parameters
Foreword: SCSI Channel, SCSI ID and LUN
The figure on the left is a very 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 a LUN like another SCSI device.
10.3.1 Host-side SCSI Parameters
Maximum concurrent host LUN connection (“nexus” in SCSI):
It is the arrangement of the controller internal resources for use with a
number of the current host nexus. If there are four hosts (A, B, C, and
D) and four host IDs/LUNs (ID 0, 1, 2 and 3) in this configuration,
host A accesses ID 0 (one nexus), host B accesses ID 1 (one nexus),
host C accesses ID 2 (one nexus) and host D accesses ID 3 (one nexus)
- all queued in the cache - that is called 4 nexus. If there are I/Os in
the cache with 4 different nexus, and another host I/O comes 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.
From the main menu, select “View and Edit Configuration
Parameters,” “Host-side SCSI Parameters,” then press [ENTER].
Choose “Max Number of Concurrent Host-LUN Connection,” then
press [ENTER]. A list of available selections will appear. Move the
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