Storage Architectures and Devices 99
Then, when the seek is complete and data is ready to be transferred, the
device can arbitrate for the bus and then reconnect with the controller to
transfer the data.
If the device is very efficient, it will even begin reading the data and place it
into a buffer before it reconnects. This allows it to burst the data across the
SCSI bus, thereby minimizing the time it needs to use or own the bus.
These techniques result in a more efficient use of the available SCSI bus
bandwidth. If the controller held onto the bus while waiting for the device to
seek, then the other devices would be locked out.
Since, in effect, multiple operations can occur simultaneously, this is also
known as overlapped operations or multi-threaded I/O on the SCSI bus. This
feature is very important in multi-tasking environments.
If you only have one SCSI device, then disable this feature. You will gain a
small amount of performance since there is a slight overhead associated with
the disconnect/reconnect sequence.
4.2.3 SCSI-III
ANSI continues to develop the SCSI-II specification to address issues of
cable definition, termination, confusing SCSI-II commands, and electrical and
signal timing definitions. The SCSI-III architecture encompasses the following
commands, interconnects, and protocols:
• The SCSI-III command set consists of five command sets that are derived
from SCSI-II command sets and a new SCSI-III command set for RAID
controllers.
• The interconnect technologies for SCSI-III specifications are:
• Fibre Channel Physical and Signaling Interface (FC)
• IEEE 1394 High Performance Serial Bus (FireWire)
• SCSI-III Parallel Interface (SPI)
• Serial Storage Architecture Bus (SSA)
• The SCSI-III protocol standards are:
• SCSI-III Interlock Protocol (SIP)
• SCSI-III Serial Storage Protocol (SSP)
• SCSI-III Serial Bus Protocol (SBP)
• Fiber Channel Protocol for SCSI (FCP)