Dell PowerVault 221S (SCSI) - Manual (Page 20)

A REFERENCE GUIDE FOR OPTIMIZING DELL™ SCSI SOLUTIONS VER A02
PAGE 20 11/17/2005
The primary EMM must always be present. Regardless of whether SES redundancy is
available, the secondary EMM slot must always have either a EMM or Terminator module
installed to properly terminate the SCSI bus. Leaving this slot unpopulated is not
supported and will have undesirable consequences.
Configurations with two EMMs and one cable connected provide a level of redundancy.
Even though there is no data path redundancy, there is enclosure management
redundancy. The redundancy, however, is provided in the form of the SES enclosure
management services to monitor and control critical functions such as temperature, fan
speeds, initiating warnings/alerts, and in the worst case scenarios to safely power down
the enclosure to prevent any potential damage under adverse thermal conditions. SES
functions by default are performed by primary EMM. In the event there is a catastrophic
failure of the primary EMM, the secondary EMM automatically will detect this event and
perform enclosure management functions.
Split Bus Topology
This topology splits the PV22xS into two halves, essentially providing two independent,
electrically isolated, SCSI busses capable of supporting up to seven (7) HDD on each bus.
Because the two busses are electrically isolated, this allows simultaneous SCSI
transactions on each bus. Two cables are required, one connected to each EMM. The
cables can attach to two host systems (Figure 4-2) running different applications and
operate completely independent from one another, or they can be connected to two SCSI
channels on a single host system (Figure 4-3) to increase performance by splitting the I/O
traffic between two different physical data paths In either case, it is highly recommended
that both halves of the PV22xS be connected to the same type of SCSI RAID controllers
(e.g. PERC4 and PERC4 or PERC3 and PERC3). The primary EMM monitors the state
of the seven HDD in slots labeled 0 thru 5, and 8. The secondary EMM monitors the
state of the seven HDD in slots labeled 9 thru 15. SES management is provided by both
EMM modules to their respective SCSI hosts. SCSI failures on one bus will not affect the
other bus. Power and enclosure cooling, however, are shared. Power disruptions (e.g.,
power short on a HDD) will affect both busses equally.
Figure 4-2 Split Bus Cabling to Two Host System