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Dealing with deduplication
New VTL?features dramatically increase the amount of data that can be stored on disk, but they add to the complexity of copying data from disk to tape. The compression algorithm in the VTL may not be the same as the one used by the target tape drive. This forces an admin to do one of three things when copying data to tape:
  1. Decompress the data on the VTL and then compress it again at the tape drive.
  2. Copy data directly from disk to tape with compression on the tape drive turned off.
  3. Turn off disk compression on the VTL.
None of these options is particularly desirable. The first adds overhead during the copy to the VTL and tape drive, although it may be the most acceptable option depending on the time admins have to copy data from disk to tape and the performance on the VTL. The second option eliminates performance overhead during the tape copy, but forces firms to first recover data from tape to the VTL and then from the VTL. Turning off compression on the VTL may double or triple the amount of capacity needed to store backed up data.

Deduplication on VTLs creates similar issues. Because tape drives don't natively support deduplication, a VTL with deduplicated data must first reconstruct the data in its native format before sending it to tape. This requires reserving sufficient

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time and ensuring that the VTL's performance is sufficient to reconstruct the deduplicated data before copying it off to tape. Technically, the deduplicated data can be copied to tape, but that reintroduces the dependency on the VTL for recoveries.

This was first published in December 2007

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