In a D2D2T scenario, the disk subsystem serves as an intermediary caching device, letting you first write quickly to a disk system, then allowing backup data to be transferred to streaming tape at a later time. This provides the convenience and performance of D2D backup, with the additional security of removable tape that can be run at your convenience.
A few years ago, a number of D2D2T systems were marketed as turnkey appliances that combined disk and tape, but after an initial burst of popularity, many of the vendors making these systems were acquired. Breece Hill remains one of the few companies that continues to offer a dedicated D2D2T system; its iStora appliance targets the small-business market.
A number of backup software products from CA, EMC, Symantec and other companies provide D2D2T functionality (see "Escape the Tape" ). Software can support the use of existing tape and disk storage and offer the performance enhancements of disk without your replacing much of your backup infrastructure. D2D2T offers the same RPO/RTO benefits as D2D, as well as the flexibility of letting you choose the length of time the backup data resides on disk, plus the offline, remote storage benefits of removable tape media.
Finally, VTL disk backup is enjoying increased popularity for both D2D and D2D2T applications. VTLs evolved to provide D2D2T capabilities in large-scale Fibre Channel environments, where backups direct to disk weren't supported, and to enable disk access to large-scale backup applications that could write only to tape libraries. Although similar to other D2D methodologies, a VTL storage environment is highly optimized for backup purposes and not susceptible to the fragmentation problems of conventional file-based systems.
A VTL is software or a disk appliance capable of emulating an existing physical tape library in every respect, including tape format, drive type, drive count, tray specifications, cartridge count, barcode scheme and tape rotation methodology. By presenting the disk subsystem to the backup application as a virtual library that is indistinguishable from a physical library, a VTL provides fast write capabilities to the disk system and buffered writes to the multidrive tape library at its own pace.
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