iSCSI is short for Internet Small Computer System Interface. It’s an Internet Protocol standard for connecting data storage devices via a network. It permits SCSI commands to be sent over IP networks across great distances to manage storage. iSCSI is significant because it makes data storage and transmission faster and more flexible. It’s a TCP/IP-based protocol that lets SCSI function on top of the TCP layers. Packet delivery with iSCSI is different from how it is with IP, which always happens in a particular order. It sends the same commands used by SCSI software, but it sends them over the network. This is equally true for local area network (LAN) and wide area network (WAN) applications. iSCSI links all kinds of storage devices across a network and lets you treat them as if they’re local.
The iSCSI initiator works as an iSCSI client. On a PC it functions just like a SCSI bus adapter, but rather than linking physically with the SCSI devices, the iSCSI initiator transmits data over the network. Initiators can be of the software type, which uses code to emulate iSCSI. This is usually done in a software driver. It uses the networking hardware that’s in place to load SCSI devices for a PC using the iSCSI protocol. There are software iSCSI initiators to suit operating systems (like an iSCSI Windows Initiator, for instance) and they’re the most widely used way of setting up iSCSI. A hardware iSCSI initiator uses dedicated hardware, usually with integrated software. There are also hardware initiators, which aren’t as slow as iSCSI and don’t come with the as much chance of network interruptions. That’s one reason why services using a hardware iSCSI initiator can see increased performance.
One of the downsides of iSCSI, particularly for resource-heavy applications is the extra latency. The problem with wrapping SCSI packets around TCP/IP protocols is that it slows things down a tad. It also makes it hard to ensure a high-quality service and decent performance on mixed networks. For instance, if VoIP, software iSCSI, email and Excel spreadsheets are using the same connection without some form of QoS for performance, the results may be disappointing. By contrast, Fiber Channel SANs will likely only have disk traffic on that network, so performance will be much better.