Maintaining quality of service for WAN storage over IP

Computer Technology Review, Feb, 2004 by Brian Larsen

As wide area networking of storage proliferates in today's demanding business continuity environment, quality of service (QoS) for Fibre Channel over IP (Internet Protocol) is a topic that many IT organizations are addressing--or soon will be.

IP is a mature, widely used network protocol that's emerging as the network of choice for storage-over-WAN applications. It wasn't too many years ago when IP was used only for file sharing and messaging between users. As more open systems platforms were deployed and IP enhancements were implemented, IP began to become the network of choice for data transfer to remote peripheral applications (even mainframe-based). Now, some of the largest banks in the U.S., as just one example, run their entire banking operations via IP networks. Until recently, the exception to this has been business continuity (disk mirroring) and disaster recovery (tape backup/restore) applications. These large, block-oriented, synchronous/semi-synchronous applications traditionally had to be tightly coupled directly to the processor owning the data. But improved QoS is now enabling widespread use of IP networks for wide area storage applications.

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This article should give you a better understanding of the issues involved in Fibre Channel over IP QoS for storage applications, and introduce you to solutions that address these issues.

If your organization uses a dedicated, private network, there are distinct steps you and your consultants can take to design your network and select the right products that will enable you to manage these QoS issues. You should take care to design your network down to the application level--which is a big topic all by itself (and thus beyond the scope of this article). In particular, you should consider how you can design your network to use the least amount of bandwidth--the most expensive part.

If you use a public network, the QoS issues we discuss here are largely out of your control. You're guaranteed a certain QoS in the service level agreement (SLA) you have with your provider. However, by understanding the issues presented here, you'll be in a much better position to negotiate that QoS in the first place. (In other words, forewarned is forearmed!) For example, storage applications require less than 1% packet loss, and you will find that many providers simply cannot provide that level of service.

Storage Traffic Is Demanding

Applications such as disk replication and remote tape backup are high-speed streaming applications that can generate data streams exceeding 100 megabytes per second (MBps). When translated into networking terms, that's approximately 800 megabits per second (Mbps). Network speeds can be virtually unlimited, but the costs can be very prohibitive to end users, who typically have 3-5 year contracts for bandwidth that require monthly payments. Costs vary depending on the parameters the application requires. Some of these parameters include network speed (in Mbps), length of network route, packet loss, and jitter to name a few. All of these parameters are considered when defining a service level agreement (SLA), to which the provider must adhere or the end user can impose penalties.

QoS Issues for Storage Over IP Networks

There are several issues any enterprise IT organization should be aware of to sustain high-speed storage over an IP network. In large part these issues are manageable, or are becoming more so, as new advances in storage networking are developed.

Networks that people believe are error-free do, in fact, have errors. In the world of IP, lots of things can cause network impairment or stress when it comes to wide area storage networks, including packet loss, jitter, and latency. Packet loss, for example, is common in any discussion of IP networks--in fact, it's a long-known IP fact-of-life. But what some might consider "normal" levels of packet loss are simply not acceptable when it comes to storage applications over wide area networks.

The existence of packet loss does not negate the viability for storage traffic over IP. There are technologies to reduce the impact of packet loss on storage applications by performing error recovery and retransmission at the IP packet level, resulting in less retransmitted data and quicker recovery. Taking advantage of such technology is key to successful storage application performance, especially when bridging the wide area network.

Data Integrity Is Critical

One of the most important issues regarding using IP networks for storage over wide area networks is reliable data transfer, or data integrity. Making sure the data is received and delivered intact creates a strong component of QoS, even though it is not associated with "standards" on IP networks. (One must remember that there is a QoS that has to transfer from the server HBA, through the fabric and the WAN network, and back again.)

IP networks are known for operating in a "send and forget" mode. That is, the sending device assumes the data will arrive at its destination, so it moves on to the next task without checking on the success or failure of what was sent previously. Error recovery in this case is usually an email from the intended recipient asking that the data be resent.