User`s guide

318 Implementing an Open IBM SAN
Software, similar to SDD, is required in the host for it to recognize that it may
have two or more paths to the same volume. If this was not loaded on the host,
the host would recognize the volumes as being different, and there will be
contention problems, and data corruption.
As SDD is currently only supported on SCSI adapters and not with Fibre Channel
adapters, it is also not supported when using the SAN Data Gateway to connect
to an ESS.
4.7.2 Multiple SCSI connections
The SAN Data Gateway can have all four SCSI channels connected to a single
ESS. A volume in the ESS can be assigned to more than one SCSI port in the
ESS. If these SCSI ports, that have the same volume assigned to it are
connected to a SAN Data Gateway, the Gateway will assign multiple LUN ID
numbers to the same volume. This is because the Gateway, upon discovery, or
scanning of the SCSI buses, will view the volumes on each SCSI channel as
separate volumes. For further explanation on this refer to 4.4.2, “Device
discovery” on page 294.
If each LUN ID was then masked and zoned to different host systems, it is vital
that the hosts have some access sharing software loaded to control access and
avoid data corruption.
If the LUN IDs were assigned to the same host, then again software similar to
SDD is required for the host to recognize that it has multiple paths to the same
volume.
As stated earlier, SDD is not supported on the SAN Data Gateway connection to
an ESS.
4.7.3 Adding Fibre Channel switches
As switches are added to allow for more host attachments, there are
considerations about how many paths the host has to the SAN Data Gateway.
If we refer back to Figure 4-48 on page 314, there is a single path from the switch
to the Gateway.
If another Fibre Channel path from the switch to the Gateway was added, each
host now has two paths to access the same LUNs. Each host will
see the
volumes twice. Once again, to prevent the host from recognizing the same
volume twice, software similar to SDD is required.