THE BASIC CONCEPTS behind iSCSI (Internet SCSI) aren’t new. Companies have been using data networking technology as a medium for storage data for years. What’s new is the approach, as well as the level of flexibility that iSCSI provides compared to earlier versions of NAS or SANs.
NAS devices, which include iSCSI devices, are simply storage devices that include NICs (network interface cards), which make the devices accessible via a standard network connection.
In the past, such devices have used proprietary protocols or have appeared on the network as servers, allowing network users to connect to them. But the devices did not appear as available storage on servers. Furthermore, because they were simply objects on the network, they suffered network traffic congestion along with everything else.
iSCSI devices are different in that they are accessed via an iSCSI HBA (host bus adapter). To the computer, the HBA looks just like any other SCSI HBA: It appears to be a storage device that you’d access via the server, just as you would with a directly attached storage device, such as an internal disk or a SAN. To the network, the iSCSI HBA appears to be a NIC. It has an IP address and communicates using standard IP network packets.
The difference is, when the server needs to move some data to storage, it transfers the data to its HBA, where it becomes standard SCSI-3 data. The data is then enclosed in an IP packet and is sent out via the Ethernet network. Once it gets to the iSCSI storage device, the IP packet information is stripped off, and the data is moved to the device’s internal SCSI controller, which in turn transfers it to disk.
One advantage of iSCSI is that it’s completely transparent. The server software sees only what appears to be a SCSI controller; the network only sees IP traffic. To the IT staff, it means that there’s little new to learn. iSCSI uses standard Ethernet infrastructure and standard SCSI provisions in the server software.
iSCSI’s simplicity makes this new storage protocol highly flexible. For example, your storage network can share infrastructure with your standard data network, just as you might with other NAS devices.
Because servers are talking to the network through their HBAs, implementing a separate storage network is easy and relatively inexpensive. Furthermore, because you’re talking to your storage through Ethernet rather than Fibre Channel, you gain even greater flexibility.
Consider distance, for example. A standard SAN using Fibre Channel is limited to speeds of 2Gbps and fiber lengths of approximately 30 kilometers. In testing at our lab at the University of Hawaii, we were able to extend Fibre Channel communications to about 40 kilometers under controlled conditions. This is handy if you want to have off-site mirroring, but suppose your MAN (metropolitan area network) extends farther than 40 kilometers?
Likewise, there’s the speed issue. Fibre Channel can currently handle speeds as fast as 2Gbps. Not bad for handling storage for an in-house SAN. But suppose you need to aggregate several SANs for that off-site mirroring?
In both cases, iSCSI provides a solution in 10GbE (10-Gigabit Ethernet). This new Ethernet specification is five times as fast as the fastest SAN, and it covers vastly more distance. Even better, the infrastructure for 10GbE is already available, so iSCSI at 10Gbps is also available.
The melding of SCSI and Ethernet allows companies to get essentially the best of both worlds. They can, for example, use iSCSI as a way to move storage traffic across long distances and use a remote SAN for storage. They can also back up their local storage at great distance, and they can use a single 10Gbps pipeline to consolidate all of their traffic, something they can’t do using more traditional storage technology.
Finally, there is the issue of management. Although virtually all NAS and SAN products have had some level of management, almost everything has been proprietary. If you had an IBM Corp. Shark handling your storage needs, for example, you couldn’t plan on using the same management software to handle your NAS traffic. With iSCSI, the network portion of the management issue is simplified. You can use the same network management tools you use for your other network management, just as you use the same switches and routers.
Overall, iSCSI offers a lot in terms of both simplicity and flexibility. That does not mean, however, that there are no downsides. In fact, there are two that should be mentioned up front.
The first is that encapsulating SCSI commands inside an IP packet adds overhead. After all, you have to pass all those IP and TCP headers, checksums, and the like along your network in addition to the SCSI commands that are the actual payload. There is less overhead on a Fibre Channel SAN, and none at all on a direct SCSI connection.
Second, not all management is created equal. Although your network management software may work with iSCSI as well, that doesn’t mean that you will be able to manage the iSCSI device itself without resorting to proprietary software. And you can assume that each company will develop its own device-level software that won’t work with products from other companies.
Finally, there’s the security issue. SCSI commands aren’t secure because they never had to be. The SCSI protocols were developed with the idea that they would be traveling on cables embedded in a single computer. Now that you’re transmitting your storage traffic across an external network, perhaps even the Internet, anyone with access can read your traffic. Better plan on encryption to protect your data.
THE BOTTOM LINE
Executive Summary: iSCSI is a significant enhancement to previous network storage technologies because it manages to simplify and expand the storage equation. On one hand, it uses familiar protocols; on the other hand, it extends the range of communication, eases management, and increases available bandwidth. iSCSI should also be easier and less expensive to implement than current storage technologies.
Test Center Perspective: The release of the final iSCSI standard is certainly significant. With iSCSI, your company’s network storage solution is suddenly easier to implement, has better manageability, costs less, and goes faster. That’s rare in any technology. The downsides to this storage protocol are few and minor, and the advantages are significant. For most companies, a move into iSCSI is a win-win situation.