Dell PowerVault 220S (SCSI) - Manual (Page 8)

System Overview : Dell PowerVault 220S and 221S Systems Service Manual
file:///C|/Users/rishi_sood/Desktop/220s/en/sm/intro.htm[3/7/2013 12:09:13 PM]
See Table 5 for a description of split-bus module modes and functions.
NOTE: To change the SCSI bus mode, you must change the position of the bus configuration switch before turning on the storage system. Changing the position of the bus
configuration switch while the system is on will not affect system operation. If you change the configuration, you must first reboot the host system and then the server storage
system for the changes to take effect. For more information on SCSI bus modes, see your Installation and Troubleshooting Guide.
Table 5. Split-Bus Module Modes
Mode LED Icon Position of Bus
Configuration Switch
Function
Joined-
bus
mode
Top for rack systems,
right for tower systems
LVD termination on the split-bus module is disabled, electrically joining the two SCSI buses to form one contiguous bus. In this mode,
neither the split-bus nor the cluster LED indicators on the front of the system (see Figure 2 for locations) are illuminated.
Split-
bus
mode
Center for all systems LVD termination on the split-bus module is enabled and the two buses are electrically isolated, resulting in two seven-drive SCSI buses.
The split-bus LED indicator on the front of the system (see Figure 2 for location) is illuminated while the system is in split-bus mode.
Cluster
mode
Bottom for rack
systems, left for tower
systems
LVD termination is disabled and the buses are electrically joined. The cluster LED on the front of the system (see Figure 2 for location) is
illuminated while the system is in cluster mode.
The split-bus module has only one LED indicator (see Figure 5 for LED location), which is illuminated when the module is receiving power.
Enclosure Management Module (EMM)
The EMM serves two primary functions in your storage system:
SCSI bus expansion — Acts as a buffer for the SCSI bus, electrically dividing the bus into two independent segments while logically allowing all SCSI bus traffic to pass through it
transparently. The buffer improves the quality of the SCSI signals and allows longer cable connections.
Management functions — Includes SES and SAFTE reporting to the host system, control of all system LED indicators, and monitoring of all environmental elements, such as
temperature sensors, cooling modules, and power supplies.
A system with redundant enclosure management features has two EMMs that are designated as primary and secondary that can be configured in either a cluster or joined-bus mode. A
system with nonredundant enclosure management has one EMM and one SCSI terminator card configured in a joined-bus mode or two EMMs configured in a split-bus mode. In
redundant EMM systems, only one EMM per SCSI bus is active at one time, so only one EMM per SCSI bus can respond to SCSI commands from an initiator.
In joined-bus and cluster modes, if a secondary EMM receives a message that the primary EMM has failed, the fault LED indicator on the primary EMM is illuminated and the condition is
reported to the host server. The secondary EMM becomes active and holds the failed primary in a reset state until it is replaced. If the primary EMM detects that the secondary has
failed, the secondary's fault LED indicator is illuminated and the failed status is reported to the host system.
NOTE: In split-bus mode, although each EMM monitors the entire storage system, it controls only half of the hard-drive slots. If one EMM fails in split-bus mode, the second EMM
will report the failure, but will not assume control of the hard-drive slots that were previously controlled by the failed EMM.
NOTE: The EMM is "warm-pluggable." This means it can be removed or inserted while the power is on. However, all I/O activity between the host and the storage system must be
stopped.
The primary EMM is always plugged into the slot on the left (viewed from the back of the system). In systems with redundant EMMs configured for joined-bus mode, the primary EMM
assumes control of all the system's functionality. In addition, the active EMM is the only module that reports status of the system to the host system through SES and SAFTE protocols.
Because the secondary EMM must assume the responsibilities of the primary in the event that the primary fails, both the primary and secondary EMMs are continuously monitoring the
status of the system's components.