Hot Technologies in Storage

Continued from page: 2

Tuesday, July 03, 2007

iSCSI vis-à-vis FC Where it fits?

Another hot storage trend relates to the use of iSCSI in SAN deployments. But before you leap on to the bandwagon you need to ensure that it meets your requirements. Here's a technical insight.

Small Computer Systems Interface (SCSI) commands are not new to the storage world, but SCSI over IP (iSCSI) is still a nascent phenomenon and is evolving with every passing day. Until recently, it was not even seen as a viable alternative to high performing and feature-rich Fibre Channel option for storage networking. Both vendors and users were skeptical about its performance under mission-critical mode. There was lack of support and availability of products was also scarce. But things are fast changing for iSCSI, and to date it's become the most exploited form of IP Storage.
What's motivating users to adopt iSCSI? Lesser capital and operational costs are the main motivators. Ease of management is right behind, which includes easier and faster deployment, leveraging existing IP skills. Let's delve deeper into the use of iSCSI protocol in storage networking and see where it fits the bill.

SCSI over IP
The iSCSI protocol is the means to enable block storage applications over TCP/IP networks. It is similar to the client/server model. The client being the initiator which actively issues commands and the target is the server which passively serves the requests generated by the initiator. A target has one or more logical units, with unique numbers (LUNs) to process these commands. After processing, these commands are stored in a Command Descriptor Block, which is encapsulated and reliably sent back to the initiator. The status of delivery is also reported. Basically, the iSCSI layer interfaces to the operating system's standard SCSI set. In practical applications, an initiator may have multiple targets resulting in multiple concurrent TCP connections active.

iSCSI adapters
Unlike FC Host Bus Adapters (HBAs), iSCSI network interface cards do not require FC at the host end and the network, which reduces the ongoing support and management costs. In addition, it offers several architectural options that vary based on the price, bandwidth, CPU usage, etc. The most cost effective option is to use a software-only solution, wherein a server can do block I/O over IP through an ordinary Gigabit Ethernet card.

Unfortunately, this approach suffers in terms of performance and CPU overhead, making it best suited for low-performance applications such as tape backup. But as CPUs are becoming more powerful, this is becoming an acceptable trade-off for the convenience of iSCSI storage access.

The other approach to having iSCSI involves using TCP off-load engines (TOEs). These are incorporated in network adapters to remove the burden of TCP overhead processing from the host CPU, and making it more efficient. The third approach lies on the other extreme, in which high performance iSCSI adapters off-load both TCP as well as iSCSI. Though it adds up to the cost, it provides both high-speed iSCSI transport and
minimal CPU usage.

iSCSI or FC
High performance iSCSI adapters and IP storage switches are proliferating in the market and dramatically changing the SAN landscape. It's difficult to say which type of SAN will meet your unique requirements, but consider FC over iSCSI where availability, high-performance and reliability are paramount, for example, to store transactions in banks. In other cases, you can choose iSCSI. For example, an iSCSI SAN will be appropriate for an organization which does not want to invest in acquiring or training staff to support FC.

IP networks often have higher latency than localized SANs. That means even if you have an 8 Gbps FC, let's say by 2008 and a 10 Gbps Ethernet, which in all probability would be a possibility by that time, it is less likely that people would fall for such small speed advantage. Moreover, for companies that have already stabilized on FC it would be even less likely.

Some OSs still don't have native iSCSI support and iSCSI management tools (SRMs) are also comparatively less developed. A lots of trust building is still needed in context of iSCSI.

Nonetheless, people will find iSCSI enticing for the other major reason-cost. Although prices of FC components are coming down fast, they're still much higher than their IP counterparts.Moreover, this price difference is likely to remain there at least in the foreseeable future. Even if you add in the cost of a TOE, it does not exceed the FC cost.

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