Dell PowerVault 770N (Deskside NAS Appliance) - Manual (Page 23)

Mode Position of
Bus
Configuration
Switch
Function
Joined-
bus
mode
Up 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 enclosure are illuminated.
Split-
bus
mode
Center 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 enclosure
is illuminated while the system is in split-bus mode.
Cluster
mode
Down LVD termination is disabled and the buses are electrically joined. The cluster LED on the front of
the enclosure is illuminated while the system is in cluster mode.
NOTE: To change the SCSI bus mode, you must change the position of the bus configuration switch before turning on
the storage system. Using the bus configuration switch while the system is on does not affect system operation. If you
change the bus configuration switch while the system is running, the change will not take effect until you perform the
following sequence: shut down the nodes, reboot the storage system, and then power up the nodes.
The split-bus module has only one LED indicator (see Figure 2-2 for 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 length connections.
Management functions — Includes SES and SAF-TE reporting to the host initiator, control of all enclosure LED
indicators, and monitoring of all enclosure environmental elements such as temperature sensors, cooling modules, and
power supplies.
A system with redundant enclosure management features two EMMs that are designated as primary and secondary and can
be configured in either a cluster, joined-bus, or split-bus mode. A nonredundant configuration consists of one EMM and one
SCSI terminator card, and can be configured in a joined-bus mode only. In a redundant system, 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.
If a secondary EMM receives a message that the primary EMM has failed in joined-bus and cluster modes, the fault LED
indicator on the primary EMM is illuminated and the condition is reported back to the host initiator. The secondary EMM then
becomes active and holds the failed primary in a state of reset 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 back to the host
initiator.
NOTE: In split-bus mode, each EMM controls half of the enclosure. If one EMM fails in split-bus mode, the second EMM
reports the failure, but does not assume control of the entire SCSI bus.
The primary EMM is always plugged into the slot on the left (viewed from the back of the system). In a redundant joined-bus
configuration, the primary EMM assumes control of all the enclosure functionality. In addition, the primary EMM is the only
module that reports the status of the system to the host initiator through SES and SAF-TE 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.
Preparing the PERC Card for Clustering
The warning message shown in Figure 2-6 appears on your screen when you attempt to modify the configuration of the
shared storage system on your cluster by using the PERC BIOS configuration utility.