Tape life should be simple, we might think. A physical tape drive is simple to drive: start it, rewind it; stop it; write to it. All a virtual tape drive needs to do is respond appropriately to a backup application when it gets these messages and so ‘fool’ the application into thinking it’s dealing with a real tape drive.

When a virtual tape system (VTS) talks to tape libraries it needs to act as if it were a backup application and understand each individual library’s command set. Again, it should be simple.

But, as you’ve probably guessed already, it isn’t that simple. A virtual tape system abstracts tape drives or libraries to backup applications and backup applications to tape devices. Devices and software either side of this middleware need to be supported. And there are no firm standards beyond the tape device command sets and SCSI.

Non-existent drives
A VTS, like FalconStor’s VTL, can set up tape drives that couldn’t exist. For example, it could set up an apparent LTO 2 virtual cartridge on disk which has twice as much capacity as a real LTO 2 cartridge. If the backup application queries the LTO 2 drive it thinks it’s talking to and asks for its capacity, and gets back a figure twice as big as it should be, then it can declare an error and fail to run.

FalconStor’s John Lallier, VP of technology, explained that some backup applications are table-driven, with parameters for different tape formats coded in. The VTS has to know how backup applications deal with different tape formats as well as simply emulating the tape drive responses and actions.

What this means is that the VTS has to explicitly and fully support backup applications, tape drives and libraries. It does if it’s a VTS produced by an independent software vendor. An ADIC disk cache need only support ADIC libraries and their supported tape formats. FalconStor’s VTS supports more libraries and more formats. E.g. DLT/SDLT, LTO 1 and 2, SAIT and StorageTek’s 9840 and 9940 formats; ADIC, HP, IBM and Quantum libraries..

More drives than real drives
In a SAN where multiple hosts might back up to a shared library then the hosts have to have their backup streams controlled to keep the library operating. This can lengthen individual host’s backup times and drive the SAN owner to buy more drives to mitigate the problem.

By employing a virtual tape library extra ‘tape drives’ can be configured – they are only virtual cartridges on disk – and the SAN fabric used to better effect to backup the hosts concurrently. There is no need to share a virtual tape library. The extra ‘drives’ don’t cost money. So you can dupe the backup applications on the hosts into thinking you have more tape drives than you actually have. It saves money because you don’t actually have to buy new tape drives.

VTS suppliers
Virtual tape software is available from a number of suppliers, such as:-

- ADIC supplies its Pathlight VX disk backup system using its own software.
- Certance has its CP 3100 disk-to-disk-to tape system for DAT 72/DDS tapes
- CNT supplies its Ultranet Storage Server – which is really FalconStor’s software.
- EMC uses FalconStor’s software in its Clariion Disk Library.
- FalconStor supplies its virtual tape system through partners. Sometimes it’s Falconstore-branded, sometimes not.
- HP has certified FalconStor software for its Data Protector.
- MaXXan supplies FalconStor software on its intelligent switches.
- Overland has ‘in-house’ virtual tape software.
- Quantum’s DX30 and DX100 backup disk arrays use Quantum software.
- StorageTek’s MirrorStore uses FalconStor software

Customers don’t want backup; they want data protection
Customers can consider virtual tape if they use real tape. Virtual tape gives them faster backup and faster restore, if it is done from disk.

Are they ready for this? HP’s Steve Case, NSS manager for Europe, says, “Disk as a backup solution is in its infancy.” Where people do think about disk as a backup medium then, “the backup application can look at it as disk or it can look at it as tape. HP would recommend a disk-based strategy.”

What is being said here is that existing backup applications generally assume a tape destination device, hence the need for virtual tape systems. But customers don’t want tape per se, or even backup software. They want their data protected. If disk is being considered as the medium to store the protected data then it can be treated as a virtual tape or, a big or, it can be treated as disk and a snapshot-type technique used.

Lallier mentions another ‘gotcha’ here. Database’s need to be quiesced, stopped, for a copy to be made. Otherwise the copy isn’t valid as far as the database software is concerned and it will test the copy for integrity, taking several hours. A database-aware snap facility, like FalconStor’s TimeMark, will create a valid snap that can be used straight away.

“Storage array snap facilities don’t do this,” he says. The storage array snap function has no knowledge of the database.

Virtual tape systems end the ‘tape wait’ state that causes the backup window to be so long. If you are considering virtual tape as part of your data protection strategy, do understand the virtues of the different systems, to be sure to get the benefits appropriate for your particular situation.


Check out the virtual tape white paper.