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Implementing SAN Internetworking Solutions
4
iSCSI Protocol
iSCSI is based on SCSI protocol that enables hosts to perform block
data I/O operations with a variety of target peripherals. Targets
include disk drives, tape devices, optical storage devices, printers,
and scanners. A standard host-to-peripheral SCSI connection is based
on a parallel transport mechanism with inherent distance and device
support limitations. For storage applications, these limitations have
caused development of high-speed serial transport technologies
based on networking architectures such as Fibre Channel and GbE. IP
storage networks based on serial gigabit transport layers overcome
the distance, performance, scalability, and availability restrictions of
parallel SCSI implementations.
By using SCSI protocols over network infrastructures, storage
networking enables flexible, high-speed block data transfers for
applications like tape backup, server clustering, storage
consolidation, and disaster recovery.
iSCSI protocol defines a means to enable block storage applications
over TCP/IP networks. An iSCSI initiator is typically a host (such as a
file server) that issues requests to read or write data. The target is a
passive resource (such as a disk array) that responds to initiator
requests. When a server application sends a request, the operating
system generates a packet with SCSI commands and a data request.
The packet is encapsulated and encrypted (if required). A packet
header is added and the resulting IP packet is transmitted over the
TCP/IP network. The target storage device decrypts and
disassembles the packet, then separates the SCSI commands and
request. SCSI commands are transmitted to the SCSI controller, then
to the SCSI storage device. Because iSCSI is bidirectional, the protocol
returns data in response to the original request.
Compared to the standard SCSI protocol, Fibre Channel provides
flexibility in terms of distance extension and switching capabilities.
Fibre Channel also preserves the common SCSI controller application
programming interface (API). Fibre Channel and iSCSI both preserve
the SCSI command set. These common features allow deployment of
storage solutions that rely on a combination of parallel SCSI and
serial Fibre Channel technologies.
Implementing SAN Internetworking Solutions
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