20.1.3 SCSI Adapters with RAID
A SCSI host controller driver may also support SCSI adapters with RAID capability. The following figure shows an example implementation with two physical SCSI channels and one logical channel. The two physical channels are implemented on the SCSI adapter. The RAID component then configures these two channels to produce a logical SCSI channel. The two physical channels each have Extended SCSI Pass Thru installed, but these are not be used except for diagnostic use. For the logical channel, the SCSI host controller driver produces another Extended SCSI Pass Thru Protocol (with physical bit turned off) instance based on the RAID configuration. Requests sent to the Extended SCSI Pass Thru protocol for the logical channel are processed by the SCSI host controller drivers and converted into requests on the physical SCSI channels. The platform firmware must only enumerate and boot from SCSI targets present on the logical SCSI channel.
The SCSI adapter hardware may not be able to expose the physical SCSI channel(s) to upper-level software when implementing RAID. If the physical SCSI channel cannot be exposed to upper software, then the SCSI host controller driver is only required to produce a single logical channel for the RAID.
Although the basic theory is the same as the one on a physical channel, it is different from a manufacturing and diagnostic perspective. If the physical SCSI channels are exposed, any SCSI command, including diagnostic ones, can be sent to an individual channel, which is very helpful on manufacturing lines. Furthermore, the diagnostic command can be sent simultaneously to all physical channels using the non-blocking mode that is supported by Extended SCSI Pass Thru Protocol. The diagnostic process may considerably benefit from the performance gain. In summary, it is suggested to expose physical SCSI channel whenever possible.
Of course, there are many possible designs for implementing SCSI RAID functionality. The point is that an SCSI host controller driver may be designed and implemented for a wide variety of SCSI adapters types, and those SCSI host controller drivers can produce the Extended SCSI Pass Thru Protocol for SCSI channels that contain SCSI targets that may be used as UEFI boot devices.