Q

How SCSI works

All disk-subsystem vendors support multiple LUNs behind each FC-port. Multiple hosts use (over an SAN-switch) the same FC-port on the disk-subsystem and each host can access only the disk that belongs to him. How does SCSI work in this case? Does it mean that there are multiple, virtual SCSI-buses over the same FC-channel? How does a SCSI-command from one host (example: SCSI-reset) affect all other hosts?

This is one of the reasons all the storage vendors are moving to Fibre on the back end. SCSI can support 15 devices on the bus, usually minus IDs 6 and 7, which are the usual IDs set aside for the controllers or adapters. Fibre FC-AL supports 127 devices on each loop, so vendors can use many more disks on the back end.

Connecting more than two hosts to a single SCSI bus usually requires a SCSI switch (like the one from BlackBox). SCSI LUN security is then performed in array firmware by assigning the host adapter ID to the LUN ID in the array. For those subsystems that have more than one SCSI bus, then a host can have access to ID's on each bus using separate target IDs set aside by the array.

If a particular subsystem vendor is using SCSI on the backend and Fibre on the host interfaces, then it's up to the subsystem firmware to map a WWN for each LUN created in the array. We can then assign a Fibre WWN from a Fibre connected host to the assigned WWN of the SCSI based LUN.

Chris


This was first published in July 2001

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