Also known as: VTL, Virtual Tape
A disk-based backup appliance that presents itself to backup software exactly like a physical tape library or drive, so existing tape-based save jobs and scripts keep working unchanged.
A virtual tape library (VTL) is a storage appliance built on disk (spinning or flash) that emulates a physical tape library or tape drive at the software interface level. Backup software, including AS400/IBM i save commands and job schedulers built around tape, sees what looks like a real tape library: cartridges, slots, and drives, with the same device commands it already knows how to talk to. Underneath, the data actually lands on disk.
The appeal is that a VTL removes the physical parts of tape, media handling, drive wear, mount/dismount time, from the daily backup, while leaving the backup process itself untouched. Restores are typically faster too, since disk seek and read speeds beat physical tape positioning.
A VTL is not the same thing as generic disk-to-disk backup software with no tape emulation, and it is not the same as a deduplication appliance, though many VTL products include deduplication as a feature. The defining trait of a VTL specifically is that it emulates tape device commands closely enough that software written for physical tape does not need to be reconfigured. Many AS400/IBM i shops still eventually copy VTL contents to real physical tape for offsite/air-gapped retention, using the VTL as a fast primary target rather than a full tape replacement. See Virtual Tape Library vs. Physical Tape Library for the fuller comparison, and the VTL directory for specific appliances.