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VTL data management issues

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Quantum's new DXi7500 uses different management software and is configurable as a VTL, a NAS backup target or both. When used as a NAS backup target, the DXi7500 appears as a disk pool to the backup application; when it copies data from its disk pool to physical tape, it optimizes the amount of data stored on each tape by filling it. This alleviates a current problem when copying data from a virtual to a physical cartridge: If a virtual tape cartridge isn't completely filled, the corresponding physical tape cartridge won't be either.

Using a VTL to manage data copying from disk to tape creates other possible problems (see "

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Six questions to ask before buying a VTL"). When copying data from disk to tape, the data stored to the physical tape may be in the same format as that stored on disk. In this format, it lacks a tape header and other information needed by the backup software to read the physical tape. This means you have to recover data from the physical tape to the VTL before the software can recover the data. Additionally, some mechanism is also required to copy the VTL's catalog from the production site to a VTL at the disaster recovery site so it can recover the data.

The greater concern with permitting the VTL to manage the data copy from disk to tape is that the VTL needs to connect to the backup software to update the backup software's catalog with the information about the new physical tape. Usually, admins manually update the backup software catalog with the information each time the VTL creates a physical tape copy, although some VTLs can handle the chore; for example, Quantum's DXi7500 has an interface to Symantec Corp.'s Veritas NetBackup 6.5 for updates. If the catalog isn't updated, you can only recover data from physical tape by first using the VTL to read the tape or forcing the software to read each tape and then catalog the data on it.

This was first published in December 2007

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