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Network-based replication
If array-based replication is expensive and restrictive, and host-based replication puts a load on your servers and is difficult to administer, what's left? For many startups, and even a few well-established players, the network is the answer.

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The CDP connection
Several replication vendors now offer optional continuous data protection (CDP). For example, last fall, Kashya Inc. announced a CDP module for its KBX5000 Data Protection Appliance that integrates with Oracle and SQL Server through Microsoft VSS APIs. Similarly, InMage Systems Inc.'s DR-Scout comes with CDP built in, and XOsoft offers Enterprise Rewinder for its WANSync and WANSyncHA products.

At some point in the past couple of years, these replication vendors had an "Aha" moment, says Arun Taneja, founder and president of the Taneja Group, Hopkinton, MA. Like CDP products from vendors such as Mendocino Software and Revivio Inc., they were also trapping every write. "Since they have the foundation, they might as well play in the CDP space," says Taneja.

Coupled with long-distance replication, CDP promises to help users get their systems up—and in a consistent state—faster. "It's all about faster RTO [recovery time objective]," says Taneja.

But to be truly effective, CDP must be tightly integrated with the application. "The lion's share of the engineering work done by the Mendocinos and Revivios is about rolling back, making sure that the application is consistent," says Taneja. Whether or not replication players have engineered in the required intelligence to make sure the rollback brings the application to a consistent state remains to be seen.
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During the past couple of years a number of replication products have been released that rely in whole or in part on network-resident resources like dedicated appliances and, in some cases, an intelligent switch. Examples include Kashya Inc.'s KBX5000 Data Protection Appliance (see "The CDP connection"), which can be configured either as an appliance or as a blade in a Cisco Systems Inc. MDS switch; InMage's DR-Scout; and mirroring and replication add-ons for the entire coterie of virtualization platforms, including DataCore Software Corp.'s SANsymphony, FalconStor Software Inc.'s IPStor, Sanrad Inc.'s V-Switch and StoreAge Networking Technologies' Storage Virtualization Manager (SVM). In addition, Hitachi offers a sort of array/network hybrid in its TagmaStore Universal Storage Platform and IBM offers replication running on its SAN Volume Controller (SVC).

In larger heterogeneous environments, network-based replication offers the freedom to use any kind of storage, but the replication process itself runs on a separate network resource away from the application servers. "If you've got hundreds of servers, [managing replication on the server] gets pretty dicey," says Zophar Sante, vice president of marketing development at Sanrad. "Plus, everyone hates agents—they're a pain in the tuchus."

The question of agents is one of the reasons that Kashya, in addition to an appliance-based product, has architected KBX to run agentless on the Cisco MDS 9000 switch and, eventually, on Brocade's SilkWorm Fabric Application Platform. Running replication on the switch simplifies support, says Rick Walsworth, Kashya's vice president of marketing. "If you have hundreds of servers, you don't want to have to install a splitter driver on each one," he says. Furthermore, a lot of companies have strict no-agent policies for their production servers. "The driver does make it a more difficult sell upfront," says Walsworth.

But not always. TradeCard's Ercolino thought briefly about using the Kashya replication product on the Cisco MDS, "but that's an expensive switch," he says. He hasn't seen any performance impact on the system from loading the splitter driver, but he'd consider upgrading to the Cisco switch version if he sees a lag.

This was first published in May 2006

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