Not just another iSCSI article: Microsoft Windows dynamic storage capability + iSCSI wall change direct attached storage forever - Connectivity

Computer Technology Review, Sept, 2003 by Ryo Koyama

By now, you've undoubtedly heard of the Internet Small Computer System Interface (iSCSI), the emerging protocol standard that allows SCSI devices to interconnect over TCP/IP And when you think of iSCSI, your first reaction is an evolutionary technology that provides Ethernet ubiquity to Storage Area Networks (SANs) mid block capability to Network Attached Storage (NAS), both of which are true. What's not much talked about, however, is the revolutionary impact that deploying iSCSI will have on Gigabit Ethernet and how this will forever change the Direct Attached Storage (DAS) landscape.

Dramatic improvement in storage technology, coupled with the explosive growth of the Internet, has fundamentally changed the way in which storage is used. Only a decade ago we grappled with files larger than 1.44 megabytes. Today we carry hundreds to thousands of megabytes on our key chains and in our portable music players. On the consumer side, graphics and audio files (.jpg, .mpg, .mp3) consume hundreds of gigabytes of our personal storage, while our office environments presentations and product documents (.ppt, .doc, .pdf) and the ever-dreaded e-mail files (.pst) do the same. Not only are these files bigger than ever before but also more daunting is the expectation that the data will always and forever be available at a moment's notice.

The best evidence of a shift in storage usage is the transition of storage's role, from tactical to strategic, in the minds of CIOs and IT managers everywhere. This is not to say that storage capacity wasn't an issue a decade ago, but compared to the complexity of storage management today, adding another disk to a server seems almost elementary. Quantitatively, since the peak of the Internet boom in January 2000, the storage networking market has grown from a $1.3 billion dollar industry to an estimated $4.7 billion dollar industry by January 2003. From 2000 through 2003, the total storage market is estimated at over $25 billion dollars.

Direct Attached Storage: What is it Good For?.

As the name implies, DAS is storage that is directly connected to a computer. Typically a hard disk or JBOD (just a bunch of disks) connected via a SCSI or IDE bus that moves data as blocks via low-level commands. In terms of quantity, DAS is still the most widely deployed means of adding storage to a network; however, fundamental limitations of the technology preclude its ability to address the ever-expanding user demands on storage.

* Physical limitations

* Lack of scalability

* Cost/complexity of management

On a practical level the major drawback of DAS is that, unlike NAS, DAS must be accessed through the server on which it is attached, not only using valuable resources of this "host" machine but also impeding direct assess to the storage, as you would get in a SAN environment.

Needless to say, in the minds of IT professionals, DAS is the ugly stepchild to NAS and SAN. As good fortune would have it, like Cinderella, DAS has a fairy godmother. The combination of iSCSI, transport offload, and Dynamic Storage (found in Microsoft Windows XP and 2000) will transform DAS into a robust storage networking solution.

Ethernet: Hold on, Not so Fast ...

In theory, Gigabit Ethernet provides throughput equivalent to that of a 32-bit 66MHz bus. Unfortunately, due to an unexpected computational limitation these levels are rarely reached and. if achieved at all, come at great cost to overall system performance. Ethernet relies on the Transmission Control Protocol and the Internet Protocol (TCP/IP) to ensure data integrity and to provide its robustness. TCP/IP's success in scaling with the ever-expanding Internet is a testament to its fortitude. Unfortunately, this capability comes at a cost: computational load. Industry experts agree that it requires roughly IMHz of modern processor operation for every 1 megabit of TCP/IP data that is being moved. Extrapolated for high-speed networking, this means that in order to achieve full-duplex Gigabit Ethernet a 2GHz processor would be fully consumed processing the TCP/IP protocol stack, leaving no headroom for applications or other functions (see Figure 1).

[FIGURE 1 OMITTED]

To address this limitation, an industry-wide initiative developed the transport offload computing model. It differs from legacy computing architectures in that it shifts responsibility for protocol processing from the host to the network interface. The network interface then becomes responsible for performing some or all of the functions corresponding to layers one through five of the OSI reference model (see Figure 2).

[FIGURE 2 OMITTED]

To support this new computing model, various vendors are producing transport offload engines (TOEs), which take the form of dedicated semiconductors that perform high-speed TCP/IP computation. Additionally, since some of these solutions are rendered in dedicated logic they provide the additional benefit of providing low latency throughput, effectively transforming Ethernet from a networking connection technology into a long haul high-speed interconnect bus.