IBM DS8000 manual SAN/FICON/ESCON switches, Multi-pathing software

Dual pathed host

HBA HBA

Server
owning all	RIO-G
even LSS
logical	RIO-G
volumes

HP HP HP HP		HP HP HP HP	HP HP	HP HP
Fibre		Fibre	ESCON	ESCON
channel		channel	Slot 4		Slot 5
Slot 1		Slot 2
RIO-G	I/O enclosure 2				RIO-G

RIO-G	I/O enclosure 3				RIO-G
Slot 1		Slot 2	Slot 4		Slot 5
Fibre		Fibre	ESCON	ESCON
channel		channel
HP HP HP HP		HP HP HP HP	HP HP		HP HP

RIO-G

RIO-G

Server owning all odd LSS logical volumes

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.

Chapter 4. RAS 73