In SCISI computer storage , computers and storage devices use a client-server model of communication. The computer is a client which requests the storage device to perform a service, e.g., to read or write data. The SCSI command architecture was originally defined for parallel buses but has been carried forward with minimal change for use with Fiber Channel, iSCSI, Serial Attached iSCSI, and other transport layers.
In the SCSI protocol, the initiator sends a SCSI command information unit to the target device. Data information units may then be transferred between the computer and device. Finally, the device sends a response information unit to the computer.
SCSI commands are sent in a Command Descriptor Block (CDB), which consists of a one byte operation code (opcode) followed by five or more bytes containing command-specific parameters. Upon receiving and processing the CDB the device will return a status code byte and other information.
By carrying SCSIcommands over IP networks, iSCSI is used to facilitate data transfers over intranets and to manage storage over long distances. iSCSI can be used to transmit data over Local Area Networks(LANs), Wide Area Networks (WANs), or the Internet and can enable location-independent data storage and retrieval.
iSCSI works by transporting block-level data between an iSCSI initiator on a server and an iSCSI target on a storage device. The iSCSI protocol encapsulated SCSI commands and assembles the data in packets for the TCP/IP layer. Packets are sent over the network using a point-to-point connection. Upon arrival, the iSCSI protocol disassembles the packets, separating the SCSI commands so the operating system (OS) will see the storage as a local SCSI device that can be formatted as usual. Today, some of iSCSI’s popularity in small to midsize businesses (SMBs) has to do with the way server vitulization makes use of storage pools. In a vitalized environment, the storage pool is accessible to all the hosts within the cluster and the cluster nodes nodes communicate with the storage pool over the network through the use of the iSCSI protocol.