Specifications

SCSI to SATA RAID Subsystem | Administrator’s Manual Host-side and Drive-side Parameters | en 131
Bosch Security Systems F.01U.027.802 | V1 | 2006.11
8 Host-side and Drive-side Parameters
This chapter discusses the advanced options for configuring and maintaining a RAID system.
Each function is given a brief explanation as well as a configuration sample. Terminal screens
are used in the configuration samples. Some of the operations require basic knowledge of
RAID technology and are only recommended for an experienced user.
8.1 Host-side Parameters
Foreword: SCSI Channel, SCSI ID, and LUN
A SCSI channel (SCSI bus) can connect up to 15 devices (not including the 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 contending for the same SCSI ID are not allowed.
Fig. 8.1 SCSI ID/LUNs
Figure 8.1 shows this idea. If you file a 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 draw-
ers are the LUNs (Logical units). Each SCSI ID enables up to 32 LUNs. Data can be stored into
one of the LUNs of the SCSI ID. Most SCSI host adapters treat a LUN like another SCSI device.
8.1.1 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 four 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), the controller will return "busy.” Note 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.
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NOTICE!
All figures in this chapter are showing examples using the management hyper terminal screen.