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Chapter 4. RAS 73
Figure 4-6 Dual pathed host
SAN/FICON/ESCON switches
Because a large number of hosts may be connected to the DS8000, each using multiple
paths, the number of host adapter ports that are available in the DS8000 may not be sufficient
to accommodate all the connections. The solution to this problem is the use of SAN switches
or directors to switch logical connections from multiple hosts. In a zSeries environment you
will need to select a SAN switch or director that also supports FICON. ESCON-attached hosts
may need an ESCON director.
A logic or power failure in a switch or director can interrupt communication between hosts and
the DS8000. We recommend that more than one switch or director be provided to ensure
continued availability. Ports from two different host adapters in two different I/O enclosures
should be configured to go through each of two directors. The complete failure of either
director leaves half the paths still operating.
Multi-pathing software
Each attached host operating system now requires a mechanism to allow it to manage
multiple paths to the same device, and to preferably load balance these requests. Also, when
a failure occurs on one redundant path, then the attached host must have a mechanism to
allow it to detect that one path is gone and route all I/O requests for those logical devices to
an alternative path. Finally, it should be able to detect when the path has been restored so
that the I/O can again be load balanced. The mechanism that will be used varies by attached
host operating system and environment as detailed in the next two sections.
Server
owning all
even LSS
logical
volumes
Server
owning all
odd LSS
logical
volumes
I/O enclosure 2
HPHP
HPHP
HPHP
HPHP
HPHP HPHP
Slot 1
Slot 2
Slot 4
Slot 5
Fibre
channel
Fibre
channel
ESCON
ESCON
I/O enclosure 3
HP HP
HP HP
HP HP
HP HP
HP HPHP HP
Slot 2
Slot
1
Slot 5
Slot 4
Fibre
channel
Fibre
channel
ESCON
ESCON
RIO-G
RIO-G
RIO-G RIO-G
RIO-G
RIO-G
RIO-G
RIO-G
Dual pathed host
HBA
HBA