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Key considerations
There are many other questions to consider when deciding whether a data-reduction product is right for your organization. These include:

Where's data reduction performed? Data reduction can occur in three places: the application server, inline in front of the backup server and after uncompressed backup data is stored on disk. Data reduction at the application server takes compute cycles from application processing and may impact application performance. But the backups are highly efficient and network traffic is drastically reduced. The overall throughput may be impacted and you can't use your existing backup software--your backup and restore procedures have to change significantly--but the overall payoff can be excellent. Avamar's Axion and HP's RISS are the only two products on the market that fall into this category.

Data Domain and Diligent offer inline solutions with an appliance that sits in front of the backup server, intercepts the data stream and performs data reduction. These vendors use different data-reduction methods, but the placement of their appliances is in-band; both require additional disk space beyond the size of the reduced data.

Sepaton's S2100-ES2 with DeltaStor performs backups at full speed, unimpeded by the data-reduction engine. In the background, the capacity-optimization engine takes over and creates new "virtual cartridges" that replace those containing the original data.

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Disk space is released and the process repeats itself. This approach requires more disk capacity than other techniques, but delivers the best backup speed. ExaGrid's InfiniteFiler is also in this category.

When is data reduction performed? Avamar, Data Domain and Diligent do their data reduction whenever a backup happens. Backup isn't complete until the data reduction is finished; overall throughput is largely determined by the efficiency of the data-reduction engine. With ExaGrid and Sepaton, the backup speed is limited only by the normal efficiencies of the backup infrastructure. All capacity optimization happens after the fact, and the speed of data reduction can be throttled via policy to ensure backups always enjoy a higher priority.

How is data reconstructed? Because files are often broken down into blocks or chunks and stored only once, the integrity of the software that constructs an object from these smaller elements is critical. So it's imperative that the meta data required to construct a file is highly available. The design of the file system that reconstructs files or databases is just as important as the data-reduction algorithm.

This was first published in July 2006

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