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A Virtual Tape Library (VTL) is a dedicated computing appliance that emulates the drives of a physical tape library and stores backup images to disk. Backup applications, like HP Data Protector, use the VTL emulated tape and library devices for backups when in fact it is an array-based appliance. The VTL consists of three components: computer hardware, application software, and a RAID-based array of disk drives. The application software emulates a tape library. The tape drives and the RAIDbased array of drives verify that no backup data is lost if a hard drive fails. These components are frequently bundled by a single vendor into an appliance.
The VTL allows a customer to configure virtual tape drives and virtual tape cartridges, and to specify cartridge capacity. The maximum number of supported virtual tape drives varies by vendor, ranging from single digits to an unlimited number of drives. And, unlike physical tape libraries, which require that additional tape drives be purchased and installed, virtual tape drives can be added to the VTL by changing the software configuration, with no additional hardware costs.
Because the VTL emulates a tape library and its drives, it does not require a change to the backup paradigm. When using HP Data Protector software, you would configure the device just as you would configure any other direct/LAN/SAN–attached tape library and drive.
Problems with physical tape drives, robotic failures, and media lead to the failure of several backup jobs. These problems can be difficult to diagnose. Write errors, reported by an operating system, do not indicate whether the media or drive is at fault. The administrator must then spend time determining whether the media or drive caused the problem. In addition, restoring from physical tape can involve multiple tape cartridges. If one of these cartridges fails, the restore most likely will be incomplete or fail altogether.
Because all VTLs use RAID storage, read and write failures are extremely unlikely, so the VTL reduces drive and media issues from the backup and recovery process.
Base VTL throughput can also be improved by adding more capacity (disk drives), controllers, and Fibre Channel (FC) ports. However, using newer tape drives capable of backing up data, with compression, at greater than 50 MB/s, backing up large amounts of multi-streamed data to physical tape may still be faster than the VTL.
Performing restores from the VTL can also be faster than using physical tape. This is likely to be the case when recovering specific files, due to the random access of disk as compared to the sequential access of tape. However, if huge amounts of data are being restored, and multiple tape drives are reading data in parallel, physical tape may be faster than the VTL.
Multiplexing or interleaving of client backup jobs to a single tape drive is often used to keep a tape drive streaming. But, if the tape drive cannot continue streaming, it either has to stop, reposition the tape, and start writing again—which has a huge impact on performance and reliability—or the drive has to slow down and write data at reduced speed. Either way, backup performance is compromised. Multiplexing also impacts restore performance. Restoring data from a multiplexed backup takes longer because one client’s data is interleaved with many others and spread over a larger area on the tape cartridge. A VTL uses disk and provides random access to data. Rather than multiplex backups, each client can be allocated a separate virtual drive. If the disk backup is then copied to physical tape, it will not be multiplexed. Restoring from this tape can be faster than restoring from a multiplexed backup.
With no penalty imposed when configuring additional VTL drives by using HP Data Protector’s capacity-based licensing model—assuming the maximum allowable number of drives has not been reached—virtual drives can be allocated specifically for restore operations. This facilitates that restores will be initiated quickly. Overall speed of the restore operation will still depend on available bandwidth and the size of the restore.
In most instances the VTL will be deployed as a front-end to a traditional physical tape library. Backup data can be object copied from the VTL to physical tape using the copy functions of HP Data Protector.
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