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What's the Big Deal with Windows and iSCSI?

The buzzword of the year in the storage industry seems to be Internet SCSI (iSCSI)--you can find legions of articles on the subject (just initiate a Google search for "iSCSI"). Since the Internet Engineering Task Force's (IETF's) official blessing of the iSCSI standard a year ago this month, Microsoft has been aggressively getting the word out about Windows support for iSCSI and has been lining up and certifying vendors. In June 2003, Microsoft announced its intentions to provide iSCSI support in Windows; in November, Microsoft delivered.

For me, the big deal about iSCSI is its ability to connect to storage resources--particularly expensive Storage Area Network (SAN)-based devices--using block-mode data transfer over "cheapnet." Network Attached Storage (NAS) devices that use file-based transfer have been available in the Windows arena for quite some time, but Microsoft has been reluctant to support them for various reasons both legitimate and illegitimate. File-based data transfer relies on upper-layer protocols such as Server Message Block (SMB), Common Internet File System (CIFS), and NFS. Block-mode data transfer uses lower-level protocols and is the key to allowing I/O-hungry Windows Server applications such as Microsoft Exchange and Microsoft SQL Server to use network-based storage.

ISCSI makes block-mode data transfer to and from network-based storage devices a reality and enables vendors to support using such devices with applications that have high I/O requirements. In practical terms, the result is that a Windows administrator who has an expensive SAN that was isolated on its own private infrastructure (or SAN fabric) can slap an iSCSI-based "head" device/server onto the SAN and make it widely available as a network-attached block-mode storage device. More important, if the iSCSI device's vendor is Microsoft certified (e.g., HP, EMC, Network Appliance--NetApp), Microsoft will support the device with Windows and with applications such as Exchange and SQL Server. ISCSI brings network-based storage to the Windows mainstream and builds bridges to more sophisticated storage solutions that previously were isolated islands of storage.

Of course, iSCSI in a Windows world isn't without concerns. One key concern about iSCSI is data security. Because iSCSI is actually just an encapsulation of the SCSI protocol within TCP/IP, the data that flows between a target (a storage device that supports iSCSI) and an initiator (the data requestor) is transmitted in the clear. In addition, no built-in authentication mechanism ensures that sender and receiver are legitimate. The IETF standard addresses this concern by specifying IP Security (IPSec) to encrypt the transfer and requiring authentication of iSCSI end nodes through the Challenge Handshake Authentication Protocol (CHAP). These measures aren't necessary for iSCSI to work, but they are available for businesses that require additional security.

I've always been a skeptic of using network-based storage with I/O-dependent applications purely for performance reasons. Although this performance concern is more important for file-mode NAS devices than for block-mode iSCSI-based devices, iSCSI's use of block-mode transfers doesn't resolve the problem.

Many things contribute to this performance concern. First is the fact that TCP/IP has much more overhead than a channel-based protocol such as SCSI or SCSI over Fibre Channel. ISCSI NIC vendors have attempted to improve performance through hardware-based protocol engines. This approach helps but doesn't completely resolve the problem. In addition, TCP/IP networks are fraught with latencies, which I/O-sensitive applications don't handle very well. For some I/O applications that are under a heavy load, iSCSI-attached storage still doesn't cut the mustard.

Like life, iSCSI is a series of trade-offs. Microsoft has made huge investments in iSCSI to bring it to the Windows mainstream and to take advantage of the apparent wave of industry interest in the technology. If industry projections are correct (Gartner predicts more than 1.5 million iSCSI-connected SANs by 2006), Microsoft's efforts surrounding iSCSI are well spent. If you're looking for ways to leverage a high-end storage infrastructure without the high costs of a pure SAN investment, iSCSI could be for you, and the $20,000 1TB SAN could become a reality. Now that Microsoft will support almost any application that uses certified iSCSI-attached storage, you have some assurance that you won't be going it alone.

For currently qualified iSCSI vendors and devices, see the first link below. The remaining links provide other iSCSI-related resources and information.

iSCSI hardware devices qualified under the Designed for Windows Logo Program

Microsoft Windows Storage Information

Microsoft Windows iSCSI White Paper

Microsoft iSCSI Software Initiator Package

Microsoft Designed for Windows Logo Program for iSCSI devices

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