iSCSI is short for Internet SCSI or Internet Small Computer System Interface. It is an Internet Protocol standard for data storage via a network. The technology allows SCSI commands to be transferred over IP networks at long distances for storage management purposes. iSCSI is an important technology because it increases the flexibility and speed of data storage and transmission. iSCSI is a TCP/IP-based protocol which enables SCSI to function on top of the TCP. Packet delivery with iSCSI is not the same as with IP which happens in a fixed order. It sends the same commands used by SCSI software, however, over the network. This is true with both local area network (LAN) and wide area network (WAN) applications. iSCSI devices are disks, tapes, CDs, and other storage devices on another networked computer that you can connect to.
One of the disadvantages of iSCSI, specifically for important or more resource-demanding applications, is the higher latency. The problem is that wrapping SCSI packets around TCP/IP protocols has some overhead. Guaranteeing the quality of service and ample performance on mixed networks is also problematic. For example, if your VoIP, software iSCSI, email and Microsoft Word documents are using the same connection without some form of QoS for performance, the results may be unsatisfactory. That’s one of the nice things about Fibre Channel SANs. It’s almost guaranteed there won’t be any other traffic except disk traffic on that network.
The iSCSI initiator works like an iSCSI client. It’s used with a PC just like a SCSI bus adapter, but instead of communicating via a physical cable with the SCSI devices, the iSCSI initiator relays data over the network. An initiator may be one of the following types: Software initiator: A software iSCSI initiator uses code to emulate iSCSI. Usually, this is done in a software driver that uses the networking hardware already in place in order to load SCSI devices for a PC by using the iSCSI protocol. There are software iSCSI initiators for the majority of operating systems (e.g. an iSCSI Windows Initiator). They’re also the most widely used method of setting up iSCSI. Hardware initiator: A hardware iSCSI initiator uses dedicated hardware, usually with integrated software. A hardware initiator eliminates the overhead of iSCSI and potential network interruptions. That’s one reason servers that use hardware iSCSI initiator may see a performance increase.