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Does host-based replication still make sense?

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Examining the host side
Host-based replication is often used in smaller environments or on a departmental basis. A host-based solution can be deployed at a fraction of the cost of a high-end, storage-based implementation, even considering the cost of software licenses, hardware and implementation services.

Host-based replication typically resides at the file system or logical volume level within the operating system. Like storage-based offerings, it's usually transparent to the application, but certainly not transparent to the host operating system or hardware. In most cases, asynchronous replication over the LAN or WAN to a similar (not necessarily identical) system is provided. Distance isn't usually an issue and heterogeneous storage can be deployed easily in a given configuration. Also, most products can transfer data at the byte level rather than the data block level, potentially requiring less bandwidth.

So, what's the downside? One issue is that it's a decentralized solution at a time when most of IT is trending toward centralization. We're consolidating data centers, servers and storage, so why would we decentralize an important DR function like replication? However, costs often outweigh this concern.

More practical concerns are scalability and performance. Host-based solutions, by definition, have some degree of impact on the CPU and other server resources. A good understanding of data change rates, resource

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utilization patterns and available capacity is important when determining if this type of solution will meet your needs.

Manageability and support must also be considered. It's easy to deploy host-based replication on a few servers but, if widely deployed, it can complicate server management.

Where does it fit?
When is host-based replication an option? If synchronous replication is required, most host-based solutions are ruled out. Similarly, host-based solutions don't support the notion of consistency groups. But there are many cases where a host-based option may fit, including:

  • DR protection for a few key applications, particularly where cluster-aware solutions are required.
  • Low-cost DR protection of file and print servers, often leveraging the many-to-one replication capabilities of these products.
  • Remote office data protection to centralize data and eliminate local backup.
  • To provide the missing replication capability for Windows-powered NAS devices.
  • Replication for heterogeneous storage devices.

All replication options should be evaluated with the same scrutiny. Key features, such as a CLI, can significantly impact the usability of any solution.

The replication landscape is evolving. Functionality once found only on high-end arrays is now offered on midtier Fibre Channel and iSCSI platforms. And advances in network-based solutions may change the economics of replication.

In any event, we should strive to find the appropriate tool for the job. For many, host-based replication is the right tool at the right price.

This was first published in November 2005

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