Pros and cons of iSCSI storage networking in virtual infrastructures

ISCSI provides simplicity for virtualization storage networking, but iSCSI performance, network design, security and cost can present challenges.

For many admins, the decisions surrounding virtualization storage networking come down to network-attached storage vs. storage area network architectures such as Fibre Channel, Internet SCSI or Fibre Channel over Ethernet.

ISCSI storage networking in particular represents a dramatic shift in storage area network (SAN) deployment. Rather than separate, specialized storage networks dedicated to handling storage traffic alone, that same SCSI traffic uses TCP/IP to pass data and commands across an Ethernet LAN along with ordinary user traffic.

This creates a whole new set of pros and cons for storage networking decision-makers to consider.

The most notable advantage with this virtualization storage networking strategy is simplicity. Ethernet is ubiquitous— it’s easy to understand. ISCSI storage networking eliminates the additional knowledge base required to build, expand and support Fibre Channel SAN deployments.

ISCSI performance and other challenges

But there are also potential disadvantages to virtualization storage networking with iSCSI. The biggest traditional roadblock to this type of storage networking has been iSCSI performance issues. A big part of the iSCSI simplicity argument has relied on software to support initiators—iSCSI clients—which can reduce iSCSI performance. TCP Offload Engine (TOE) cards and specialized iSCSI host bus adapters (HBAs) can alleviate some of the processing overhead involved with iSCSI and enhance iSCSI performance.

Network design also plays a huge role in the success of virtualization storage networking with iSCSI. LAN issues such as bottlenecks, latency and dropped packets are common in Ethernet LANs. Most everyday applications can easily function with some network latency or wait for dropped packets to be resent, but these common Ethernet problems can cause severe problems for storage traffic and storage subsystems. Again, hardware solutions like TOE cards and the availability of faster 10 GbE can help address some of these problems.

Security is also a serious concern for iSCSI storage networking. Because storage traffic is passed along the LAN, the traffic can potentially be compromised by malware and passed to the Internet.

In many cases, iSCSI storage traffic is protected through a VPN, contained through network segmentation or restricted to its own separate physical Ethernet LAN.

The cost of iSCSI for virtualization storage networking is a matter of some debate. Although iSCSI has a reputation as an inexpensive SAN alternative, experts are quick to point out that it’s only cheap with simple deployments—for example, using readily available 1 Gigabit Ethernet and software initiators. The introduction of TOE cards or dedicated iSCSI HBAs can increase iSCSI costs.

In addition, the move to 10 GbE network interface cards, switching hardware and corresponding cabling can drive up the cost of iSCSI storage networking dramatically. “All of a sudden, you can have iSCSI priced more than Fibre Channel,” said Greg Schulz, founder and senior analyst at StorageIO in Stillwater, Minn.

ISCSI management considerations

Because iSCSI runs on an Ethernet network, there are few major deployment or management issues for administrators to consider with this virtualization storage networking method. If you know Ethernet, you can generally handle iSCSI. But it’s important to evaluate the management requirements of the infrastructure, including the switch and storage subsystems.

Schulz recommends that potential adopters look beyond the initial installation and consider the difficulty and complexity of ongoing tasks such as provisioning, replication, LUN management and troubleshooting. This is particularly important for organizations contemplating a move from Fibre Channel to iSCSI—or vice versa.

Another possible wrinkle with a move to iSCSI storage networking may be the assignment of management responsibilities. Remember that larger organizations often separate SAN management and LAN management. If an iSCSI SAN is deployed over an existing LAN, those management responsibilities need to be considered.

Bob Laliberte, senior analyst with the Enterprise Strategy Group in Milford, Mass., asks who’s going to manage iSCSI storage networking. “Is it the storage team or the network team? This gets into cultural and technology domain issues that usually need to be addressed by an executive,” he said.

Although Fibre Channel technology should remain viable for virtualization storage networking for many years ahead, some experts say that Ethernet-based SANs are quickly evolving to provide congestion-free infrastructure with less loss.

“We see the [Ethernet] roadmap becoming more attractive for continued use in a virtualized environment,” said Laliberte, citing enhancements that extend far beyond the added bandwidth of 10 GbE. These improvements may include advancements such as Cisco System’s Data Center Bridging that will improve Ethernet for virtual data centers.

More on iSCSI storage networking

Stephen J. Bigelow, a senior technology editor in the Data Center and Virtualization Media Group at TechTarget Inc., has more than 20 years of technical writing experience in the PC/technology industry. He holds a bachelor of science in electrical engineering, along with CompTIA A+, Network+, Security+ and Server+ certifications and has written hundreds of articles and more than 15 feature books on computer troubleshooting, including Bigelow’s PC Hardware Desk Reference and Bigelow’s PC Hardware Annoyances. Contact him at sbigelow@techtarget.com.

This was first published in May 2011

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