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VTL data management issues

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Six questions to ask before buying a VTL
Before buying a virtual tape library (VTL), you should consider the following.
  1. Is backup software supported within the VTL? Installing backup software on the VTL lets it function as a media server recognizable by the backup software. The backup app can then automatically record the creation of physical tape copies in its catalog. The downside is that finger-pointing between vendors can result if support issues arise and backup software upgrades become dependent on what operating system the VTL supports.


  2. How is the backup software catalog updated? Most VTLs don't internally support backup software, so backup software needs to be made aware of tape-copy processes to ensure it knows a tape copy exists and where it's located. Although some VTLs manage the disk-to-tape copy themselves, they lack any mechanism to update most backup software catalogs. This requires admins to manually update the catalog or the backup software must read each tape in the tape library to determine what information is on each tape.


  3. When

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  1. does compression and deduplication occur? Compression and deduplication are becoming must-haves when implementing VTLs, but they're problematic when copying data from disk to tape. Data must be decompressed, reconstituted or both when moved from disk to tape, introducing performance overhead and lengthier tape copy windows. The best way to avoid this is to use a VTL that can first back up data in an uncompressed, native format for easy movement to tape.


  2. In what format is the data stored on tape? Data that's copied to tape must be in a format recognized by the backup software. In situations where the VTL controls copying data from disk to tape, the backup software may not recognize the format of the data on the tape, rendering it unreadable or requiring the VTL that created the tape to first recover the data. This creates a dependency on the VTL in recovery scenarios.


  3. How does the VTL device manage existing physical tape libraries? VTLs may treat physical tape libraries in one of three ways: no knowledge of their existence, treat them as a backup target or virtualize them. While one way isn't necessarily better than another, knowing how or if a VTL manages tape libraries can help you optimize your tape library in conjunction with the VTL.


  4. Does the VTL present itself as a NAS target, a VTL or both? NAS targets appear as one large disk pool to the backup software, while VTLs present virtual tape targets and manage disk as virtual tape cartridges. Using a large disk pool eliminates some of the inefficiencies associated with virtual tape cartridges (unused space on virtual cartridges) and allows admins to entirely fill physical tape cartridges when data is moved to tape. However, VTLs associate a bar code with each virtual tape cartridge; with a NAS target, the bar code is created only when the data is moved to a physical tape.

This was first published in December 2007

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