User`s guide

Chapter 4. Implementing the SAN Data Gateway 317
ESS considerations
As you configure and allow access from host systems to volumes that are in the
ESS through the Gateway, you must consider how the volumes were created
within the ESS.
Volumes in the ESS are assigned to a particular SCSI port in the ESS. As they
are assigned, you also specify the host type that will access these volumes.
Through the SAN Data Gateway it is possible to assign volumes to a specific
operating system in the ESS, but then have a host with a different operating
system access to these volumes.
As an example, look at Figure 4-48 on page 314. There we have a Windows NT
and an AIX host. The ESS was configured and the Gateway attached so that
SCSI 2 was assigned Windows NT volumes and SCSI 3 was assigned AIX
volumes. It would be possible in the SAN Data Gateway to allow the NT host to
access a LUN on SCSI 3 and the AIX host access to volumes on SCSI 2.
Here is where the device map would again be helpful in determining what
Gateway assigned LUNs should be assigned to each host.
Volumes within the ESS assigned to a particular SCSI port should be of the same
host type.
4.7 High availability considerations
Until now, our discussions of connections from the host to the SAN Data
Gateway and from the Gateway to the ESS, or other SCSI devices, have been
discussed with single host adapters or utilizing single SCSI connections.
However, special considerations must be taken into account in deciding to add
host adapters, or in providing redundant SCSI connections.
4.7.1 Single host with multiple Fibre Channel adapters
A host can have several adapters installed. If each adapter is assigned to
different LUNs using the SAN Data Gateway, there are no contention issues. As
each adapter is assigned specific LUNs, it cannot see or access any other LUNs
that may be present.
However, there may be situations where the adapters are used to perform load
balancing, and failover much like the SCSI connections from a host to the ESS
that are in place today. As the host in the SCSI situation requires the Subsystem
Device Driver (SDD) on each host, this is also true when using the Fibre Channel
adapters.