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Host-based replication
If storage agnosticism is your thing, there's always the other tried-and-true approach: host-based replication. Representatives of this approach include IBM's mainframe-based Extended Remote Copy (XRC), Softek Storage Solutions Corp.'s Replicator, Symantec Corp.'s Veritas Volume Replicator (VVR), XOsoft's WANSync and WANSyncHA, the open-source rsync and specialized database utilities.

Because host-based replication occurs at the operating system level, it's entirely indifferent to the underlying infrastructure. "We work on any OS, any storage and any network," proclaims Sean Derrington, senior group product manager at Symantec.

That open stance allows for some pretty nifty replication configurations, including one:one, one:many and many:many configurations—for up to 32 different locations, says Derrington. In May, Symantec will announce what it calls a "bunker mirror." Similar to array-based multisite replication like EMC's SRDF/Star, the Symantec product enables replication to any distance—even out of synchronous range. Like array-based multisite replication products, Symantec VVR's new capability works by simultaneously replicating data synchronously and asynchronously to two separate sites. But unlike array-based multisite replication, it won't replicate the entire data set to the synchronous site—only the storage replication log, which comprises approximately 15% of the total replicated capacity. If the

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primary site goes down, the replication logs from the synchronous site can then be quickly synchronized to the DR site and business can resume. For a more cost-effective (and some would argue simpler) solution, there are many host-based replication products that function at the file level, such as EMC's RepliStor, NSI Software Inc.'s Double-Take and Symantec's Veritas Replication Exec.

In many ways, host-based replication is array-based replication's polar opposite. Whereas array-based replication functions without any real knowledge of the server or application, host-based replication is intimately aware of the operating system and application particulars. That can be appealing to storage managers who want tight integration with the application—particularly if they're running replication in conjunction with server high-availability software like failover clustering.

"If you want application consistency, you have to participate with the application APIs to enforce consistency at set points in time," says Anup Tirumala, president and COO at InMage Systems Inc., a Santa Clara, CA, company whose DR-Scout replication and continuous data protection (CDP) suite requires a lightweight host agent. The agent, says Tirumala, integrates with the application API to force data in memory to be flushed and committed to disk. Array-based replication products often offer add-ons for specific applications like Microsoft Exchange or Oracle, "but those require an agent, too, and that detracts from their [agentless] value proposition," says Tirumala.

But the big knock against host-based replication is that it consumes CPU cycles on what are typically mission-critical application servers. "The one thing we really didn't like about [Veritas] Volume Replicator is that when you load software on the host you impact the performance of the system," says TradeCard's Ercolino. Furthermore, Ercolino bristled at the Symantec licensing model, which prices VVR licenses according to the class—the number and type of CPUs—of the system.

This was first published in May 2006

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