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Choosing the right VTL

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Coping with capacity
All VTLs have limited amounts of capacity. They address this limitation through compression, data deduplication, scalable back-end disk capacity, and tape libraries that integrate disk and real tape.

Data is compressed using the backup software on the host or compression software on the VTL. Compression allows VTLs to increase their total storage capacity by ratios of 2:1 or more, but compression introduces latency during backups and recoveries on the backup server or VTL that most users will find unacceptable.

Quantum's DX-Series of VTL storage appliances circumvents the performance problems that compression normally introduces. Using a combination of its software and hardware, Quantum embeds its Optyon compression technology (a $2,500 option) into its DX-Series controllers and then dedicates hardware adapter cards to compress incoming data and decompress outgoing data.

Another technology some vendors are starting to employ to reduce the amount of data stored is data deduplication, which stores similar blocks of data together and identical blocks only once. The technology uses meta data to track specific blocks of data and reconstruct the data in the appropriate order during recoveries. Similar to compression, the primary concern with this approach is the performance hit that comes with the deduplication process. Vendors address this issue in one of the following two ways.

Diligent Technologies' ProtecTier VT software is loaded onto a Red Hat Linux server, configured as a VTL server appliance and processes data in real time. Its HyperFactor technology detects recurring data within sets of data and then creates a single-instance store. Unlike most hashing techniques that introduce significant performance overhead when data is stored and retrieved, HyperFactor maintains an index in the server's RAM that allows the ProtecTier appliance to support a high throughput rate because there's no disk I/O.

This was first published in August 2006

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