Optimization of wide area network (WAN) links between data centers is not trivial. In fact, WAN optimization for a data center interconnect involves one of the most critical and bandwidth-hungry applications to manage across a WAN link: storage replication.
Enterprises typically replicate vast amounts of data over a data center interconnect for business continuity and disaster recovery. They may also replicate this data across data centers in order to maintain multiple data centers in an active-active configuration. Other traffic types might also traverse the link, but when an enterprise engages with a WAN optimization vendor about a data center interconnect, storage replication will dominate the conversation.
Optimizing the data center interconnect: Deduplication is essential
Since storage replication can saturate a network link rapidly, WAN optimization vendors tend to focus on conserving bandwidth by reducing the amount of traffic passing back and forth. They achieve this mainly through deduplication and compression.
"Almost all of these [storage replication] protocols were developed on storage area networks [SANs] that initially ran on Fibre Channel inside a data center," said Rick Tinsley, CEO of WAN optimization vendor Silver Peak. "Now they're trying to stretch these over wider distances over wide area networks. A lot of them struggle with that."
The WAN optimization requirements for the delivery of applications and services to end users tend to differ from the requirements for a data center interconnect, Tinsley said. End-user applications are more sensitive to latency, which requires a great deal of protocol acceleration, particularly of the Transmission Control Protocol (TCP).
With replication, WAN optimization vendors try to master storage protocols in order to maximize the amount of data deduplication and compression they can do on a data center interconnect. The less data that must cross the wire, the less money an enterprise must spend on bandwidth.
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"On really high-latency, high-bandwidth links, protocol efficiency becomes an issue. You could have a gigabit link and it's only running at 50 megabits," said Joe Skorupa, research vice president with Gartner. "The poster child for that problem is SRDF [Symmetrix Remote Data Facility] from EMC."
WAN optimization vendors study storage replication protocols
WAN optimization vendors who compete in the storage replication market emphasize their abilities to optimize these replication protocols, particularly EMC Corp.'s SRDF. WAN optimization vendors, including Silver Peak, Riverbed Technology and F5 Networks, expend a tremendous amount of effort to update their products when storage vendors update these protocols. In fact, many enterprises consider it essential for their WAN optimization vendor to be certified by their storage vendor.
Several years ago Brian Crandall, global systems architect for the $1.7 billion anti-aging company Nu Skin, selected Silver Peak for WAN optimization between his corporate data center and his disaster recovery data center. He chose Silver Peak because it was the only vendor certified for SRDF by EMC at the time.
EMC won't guarantee support of storage replication problems if a customer introduces a noncertified vendor into an environment, Crandall said.
"They'll say, 'Sorry, that's another variable you added to the mix. It's not certified so we can't support that.' Because we are an enterprise company, supportability is paramount. We need to make sure that vendors that play in our environment are maintainable and supported by all our vendors. They have to play in the same sand box," he said.
Choosing WAN optimization for a data center interconnect
When selecting a WAN optimization technology for a data center interconnect, a network engineer should work closely with the storage team. In fact, the storage team often takes the lead on WAN optimization for replication. In many cases, storage vendors resell WAN optimization products.
"Sometimes the storage team makes one decision and the networking team makes a different decision, and those guys quite often don't get along," Skorupa said. "So forcing them to do a single [WAN optimization] vendor can be problematic. Sometimes the vendor they pick to solve specific problems out to the branch might not have what it takes to solve the problems of the storage team, so you might have two different vendors."
Still, if the network engineer is involved in optimizing the data center interconnect, he or she should get a sense of how much throughput is required and how much data will be moving across the link, according to Miles Kelly, senior director for product marketing at Riverbed. Network-monitoring software can provide that visibility. Riverbed, for example, will advise potential customers to use its Cascade network-monitoring software to evaluate the replication traffic.
With that information in hand, an enterprise should then evaluate how many simultaneous TCP connections a vendor can support, Kelly said. It should also consider how much data storage it will need on the appliance, since deduplication of replication traffic can require a large cache.
Engineers should also evaluate the quality of service (QoS) capabilities of the WAN optimization vendors, Skorupa said.
"You're likely to have multiple streams going down the wire. You need QoS that works at a very high bandwidth so you can protect critical traffic. There may be some replication traffic that is really business critical and those arrays can't ever be more than a few seconds out of sync. Other arrays may be OK if they get 15 or 20 seconds out of sync," he said. "You might also be piggybacking some other interactive traffic over that same network. It might be relatively low bandwidth, but very sensitive to disruption. You might have video-conferencing traffic links between your big data centers."
Data center interconnect WAN optimization appliances may do double duty
Some enterprises will use the same WAN optimization appliance on both the data center interconnect and the delivery of applications and services to end users. If this is the case, the network engineer will want to make sure the chosen vendor can support both types of optimization in his environment.
Crandall of Nu Skin originally installed Silver Peak for optimization of storage replication on his data center interconnect. However, as the data center delivered more applications and services to far-flung branch offices, Crandall had his Silver Peak box do double duty.
"That same appliance that manages connectivity [for replication] also manages our services to our remote sites and other markets," he said. "Shortly after we began to use these appliances for replication we started using them for our remote sites."
Let us know what you think about the story; email: Shamus McGillicuddy, News Director
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