SAM-2 (SCSI Architecture Model) defines the relationship between initiator and target. Serial SCSI implementation such as Fibre Channel, serial-attached SCSI (SAS) and iSCSI are a component of the SAM-2 definition for SCSI-3 commands.
The client-server requests and responmses are exchanged across some form of physical transport governed by SCSI-3 service deliveryt protocol such as FC or iSCSI.
Read/Write of data are performed with a series of SCSI commands, delivery requests, delivery actions and responses. SCSI commands and parameters are specified in the Command Descriptor Block (CDB). CDB is encapsulated in the FCP IU (information unit).
The operating system views a WRTIE operation as a single operation but underneath, there are multiple SCSI exchanges:
(1) WRITE trigger the creation of a client in the initiator.
(2) Initiator in turn issues a SCSI command request to the target to prepare a buffer.
(3) The target device server issues a delivery action request when the buffer is ready.
(4) The initiator sends data block
(5) After the data is recieved, the target sends another delivery action request to ask for anotherdata block.
(6) When all data blocks have been recieved, the target sends a Response to mark the end of the WRITE operation
For READ operation, the request and response directions are reversed, with the host prepare buffer for data from the disk.
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