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Virtual Tape Emulation

The Virtual Tape Emulation software that NexiTech currently has available to OEMs and end-users runs as an application on a Windows server.  The code sits on top of custom SCSI Target Mode device drivers that transform an ordinary PC or rackmount server into a Storage Virtualization Appliance.  Support for Fibre Channel and iSCSI is currently accomplished via hardware and software bridges.  This code has been ported to other platforms, including a proprietary embedded OS running on a custom RAID controller.  The code is modular and portable, and can be ported to other platforms, based on specific customer requirements.

A real automated tape library consists of one or more physical tape drives, some number of slots for holding tape media, and a medium changer device that moves tapes between the slots and the drives.  For example, a simple autoloader may have one tape drive and 15 slots for holding tape media (i.e. a 1x15 tape library).  The medium changer device is necessary because it would be cost prohibitive to allocate a separate physical tape drive to each of the 15 slots.  But in the virtual tape world, why not just allocate a separate virtual tape drive to each and every slot?  It doesn't cost any more or less to do so.  It is then not necessary to implement a medium changer device, because each slot has its own drive, and the drive is either simply loaded or unloaded.  Continuing with the example above, a VTAng 1x15 autoloader emulation actually appears more like a 15x15 virtual tape library (i.e. 15 drives and 15 slots), but without the medium changer.  More expensive automated tape libraries may have two to four tape drives and up to 30 or 40 slots.  A VTAng backup appliance can emulate a 30-slot tape library by defining 30 virtual tape drives and assigning each of them to its own virtual slot, or virtual tape media.  If one were using a competing VTL, one would have to define the virtual tape media for all 30 slots anyway, so this is no more or less difficult to use than that.  And from the host's point of view, the operation is the same, with the exception that no medium changer software is necessary.  In the case of a Windows host, this means that the medium changer class driver is not running, and there are no commands that are issued involving the movement of tape media.  In the case of other hosts (e.g. AS/400), software that supports medium changers may not be included in the OS, but is available from third party vendors at additional cost.  The VTAng backup appliance obviates the need for such software, but at the same time is compatible with it.

When a host computer boots up and discovers an automated tape library or a VTL, it needs to inventory the library by reading each tape.  In the absence of a bar code scanner, this means that each tape must be moved from its slot into an available tape drive, and the tape label is then read.  Then the tape is moved back to its original slot.  The end result is a pool of tape media that is known to the host.  With a VTAng backup appliance, the end result is the same.  After booting, the host has a pool of available tape media, but it did not need to issue any medium changer commands to move media between slots and drives.  The entire inventory operation is obviously much faster than with an automated tape library, and slightly faster than with a VTL due to the absence of media movement commands.

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