A better approach is to spool the data to disk before moving it to tape. Spooled files are temporarily stored in cache memory until moved to tape. The backup software tracks the files the entire time. (When the files are dumped, the backup software assumes the files will be stored on disk permanently; if they're moved, the backup software won't know where the files now reside.) Using disk as a staging area speeds up the backup, but may slow the recovery; if the backup software has already moved the data to tape, users need to recover the data from tape.
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Vendors still employ disk caches to satisfy users who execute multiple backup jobs at the same time and send the data directly to a single tape drive, requiring the backup software to multiplex or interleave backup jobs to put all of the data on tape. While this speeds up the backup job and allows the job to complete within the backup window, it also makes recoveries slower since the data is fragmented; it may also cause backhitches, a stop and go of the tape drive, during the recovery.
Atempo's Time Navigator has the ability to defragment large data blocks. It first creates virtual buckets on disk where data from multiple backup jobs is sent. Data from each of the multiple running backup jobs is then separated by individual backup job and put on tape. This eliminates the need to interleave the data on the tape during the backup and improves recovery performance.
A third way vendors implement D2D2T is by first backing up to disk and then managing the creation, movement and retention periods of the backup on different types of media. Keeping a backup on disk for 30 days or 60 days satisfies most user recovery requirements, as most recovery requests will occur within that time period. It also permits the backup software to make a copy of the backup to tape for offsite disaster recovery needs, while allowing the backup software to manage and track the tape so it will know which tape to recall if a recovery is required at a later time. All current versions of enterprise backup software products implement this type of D2D2T, although some SMB products lack these advanced tape library integration and tape vaulting features.
However, one of the most popular ways users implement disk-based backups is by using virtual tape libraries (VTLs). While this approach normally requires the purchase of a separate hardware appliance that bundles the VTL software with it, a couple of backup software vendors offer packages that allow users to circumvent this requirement.
Atempo's Time Navigator Virtual Library System (VLS) treats existing storage arrays as a VTL with virtual tapes and allows users to transparently move data between their VTL image and normal tape libraries. BakBone Software Inc. offers a Shared Virtual Tape Library option with its NetVault:Backup product that lets administrators create and share a virtual disk library with multiple NetVault servers to centralize all of the backups in a single VTL.
It behooves you to check out some of the smaller backup software vendors. As traditional ties to tape and tape libraries are broken, products offered by smaller backup vendors may be better architected to handle the new requirements. Obviously, many users are loyal to their familiar backup software and switching to another program may be too daunting a task. Those companies should explore the possibility of using another backup program in a complementary role with their primary backup software to take advantage of new features and possibly cut costs.
About the author: Jerome M. Wendt is a storage analyst specializing in the field of open-systems storage and SANs. He has managed storage for small- and large-sized organizations in this capacity.