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To understand the performance advantages of VTLs, think of how backup applications write data to tape. A backup app typically continues writing to a tape until it hits the physical end of tape (PEOT). It will append to a tape, even if some of the previously written data has expired. Once the backup app hits PEOT, the tape is considered full. Most backup apps leave everything on the tape until all of the backups on that tape have expired; then they expire the whole tape and write to it from the beginning. Other backup apps wait until a certain percentage of the backups on a tape have expired before "reclaiming" that tape by migrating the non-expired backups to a second tape. The first tape is then expired and ready to be overwritten. The bottom line is that portions of a tape can't be overwritten.
|NETBACKUP'S INLINE TAPE COPY|
|Veritas NetBackup (now owned by Symantec) supports a feature called inline tape copy, which allows sending a backup to two tape drives simultaneously--creating an original and a copy in one step. An alternative is to use a standalone virtual tape library (VTL), and to send one copy to the physical tape and one to the VTL. The shortcoming with this approach is that it causes the VTL to run at the speed of the tape drive--defeating the purpose of going to disk backup in the first place. A more interesting approach would be to use an integrated VTL, send both backups to virtual tape, and then use the VTL to create the physical tape in the background.|
Because a VTL treats disk like tape, it eliminates fragmentation by writing backups to contiguous sections of disk. The blocks allocated to a tape stay allocated to that tape until the backup app starts overwriting that tape, at which point the VTL can once again write to contiguous sections of disk--just like data is written to tape. Because VTL vendors control the RAID volumes, they ensure that a given RAID group is only written to by a single virtual tape. A disk can perform much better if it's only writing/reading for a single app using contiguous sections of disk. This key difference explains why the fastest file systems write in hundreds of megabytes per second, while the fastest VTLs write in thousands of megabytes per second.
VTLs offer other advantages, as well. With one exception (see the next section), VTLs work with all existing backup software, processes and procedures (see "NetBackup's inline tape copy," this page, and "Do IBM Tivoli Storage Manager users need a VTL?"). In other words, everything works exactly as it would with a physical tape library (PTL). That isn't the case with disk-as-disk targets, where backup software can behave quite differently.
The disadvantage of VTLs cited by most storage admins is cost. They believe that if a disk array costs x, a disk array made to look like a VTL will cost x + y. But the y factor can vary from one VTL vendor to another. Most VTLs use capacity-based pricing, which means the cost is $/GB. At least one VTL vendor uses throughput-based pricing, so the price is determined by the number of Fibre Channel (FC) connections. The actual price of VTLs with disk included ranges from less than $4/GB to a little more than $12/GB. Disk-as-disk units fall into roughly the same price range, so it's basically a misconception that a VTL will always cost more than a disk-as-disk device.
Another issue is the price of backup software licensing. If a VTL sits next to an existing tape library, it will most likely require an additional tape library license for a library that's actually not there. This adds to the price of the VTL. How much you pay is based on how the VTL is configured and how your backup software charges for libraries. Some backup software products (e.g., IBM's TSM) have a single license for all tape libraries, while others charge for the number of slots or drives. When deciding how to configure your VTL, consider how your backup software charges for libraries. When comparing VTLs to disk-as-disk targets, you also need to remember that backup software products are beginning to charge to back up to disk-as-disk targets. These licensing challenges will probably go away as backup software vendors move toward capacity-based pricing in an effort to appear more VTL friendly. (NetBackup offers this kind of pricing today.)
This was first published in November 2005