How disk has changed backup


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While IBM's Tivoli Storage Manager (TSM) backs up directly to disk quite well, TSM administrators will experience provisioning and fragmentation issues if they begin storing all onsite backups on disk. (Most TSM disk storage pools aren't fragmented because they're immediately migrated to tape every night.) So, the advantages of virtual tape libraries (VTLs) apply to TSM as much as they apply to other backup products. In addition, a VTL would let TSM users create thousands of small virtual tapes, allowing them to turn on collocation for all clients without the usual penalty of hundreds of partially used tapes. It would also allow users to have dozens of virtual tape drives to perform reclamation at any time without causing contention.
You also need to realize that this process is happening without the knowledge of the backup software, so if something happens with a tape copy, the VTL will need to notify you of the problem. This results in another reporting interface, which might be considered a disadvantage. Another potential problem arises if the VTL puts more data on the virtual tape than can fit on the physical tape, preventing creation of a physical copy of the tape. Integrated VTL vendors ensure that this doesn't happen by stopping before the normal PEOT. However, standalone vendors might say this practice increases the number of tapes to purchase and handle, and adds to your costs.

Important VTL features
There are a number of differences among the major VTLs. Some (Alacritus, Diligent, FalconStor) are software only, so you can buy the software and run it on a regular disk array. Other VTL vendors (Maxxan, Neartek) sell a VTL head, which is analogous to a filer head. You use their software and head, but supply your own disk. Finally, some VTL vendors (ADIC, EMC, Quantum, Sepaton and Spectra Logic) offer an entire solution: software, head and disk. Software-only and filer head vendors allow you to redeploy an existing array, reducing your cost. Turnkey products cost more, but have the fewest integration issues.

Most VTLs offer replication or cascading, which replicates one VTL's backups to another VTL. But the tapes in the second VTL won't be considered duplicates by your backup software because they'll have the same bar codes as the original tapes. Also, remember that you'll probably be replicating the entire backup, and most backups aren't block level. Even incremental backups take up roughly 1% to 5% of the amount of data being backed up. This means you'll need to replicate 1% to 5% of your data center every night--a significant undertaking for many environments. Therefore, it may only be possible to use this feature within a campus, as opposed to including data from remote sites. Today, replication is offered by Alacritus- and FalconStor-based VTLs.

Some VTL vendors are beginning to offer a feature where their VTLs will examine the incremental backup, identify the changed blocks within that backup and replicate only the changed blocks. When that functionality becomes more widely available, replication between data centers will be much easier to accomplish. Diligent is the first to announce such a product with its ProtecTier offering.

If you have a heterogeneous environment with mainframe, AS/400 and open systems, you might consider a VTL that supports all three environments. Only Neartek currently offers this functionality.

A few integrated VTLs (FalconStor and Neartek) offer a feature called stacking. Stacking copies multiple virtual tapes onto one physical tape, a feature borrowed from mainframe virtual tape systems (VTS). Stacking was important to mainframes because apps were unable to append to a tape. The VTS would present hundreds of small virtual tapes to the app and then stack those virtual tapes onto one physical tape, significantly cutting media costs.

This was first published in November 2005

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