The what and why of virtual tape: hard disk augments that storage standby. (Storage Networking).

Before I get into why there is virtual tape I need to explain why tape even exists. Tape started off being a primary storage device. In the early days we did not have disk drives, so the only way data could be stored beyond the memory of the computer was tape. As computers evolved and the need for faster access became a must for the business world, IBM invented the disk drive. These disk drives were really fast but had a couple of problems. First, they cost a lot of money and, second, they had this issue called "head crashes." This happens when the read/write head literally crashes into the magnetic oxide platter. In almost all cases the data was lost and the repair required the replacement of the disk drive. Now you know where the term "system crash" came from. In those days, the disk drive was the most common cause for system error or crash. These crashes stopped the system from working and forced users to regenerate al their data--not a very efficient use of time or money. IBM's solution, at least for the data loss, was taking the now-displaced tape drives use them to backup the hard disks. This allowed data to be easily restored after a crash. In some machines, you could still run off the tape drives until the hard disk was replaced. It was slow, but it got your system working again.

Now let's fast forward 50 years and see that tape has been the primary way that all large computers or networks backup their data. The main reason tape has never been threatened by other technologies is that it is still the cheapest way to backup and archive data. Also, there are boatloads of software packages that can not only backup and archive your data but can now intelligently move data off your disk drives to tape so that your system can run at optimum efficiency.

Now back to virtual tape. Let me start off with the definition from Whatis.com: Virtual tape is the use of a special storage device that manages less-frequently needed data so that it appears to be stored entirely on tape cartridges when some parts of it may actually be located in faster, hard disk storage. The programming for a virtual tape system is sometimes called a virtual tape server (VTS). Virtual tape can be used with a hierarchical storage management (HSM) system in which data is moved as it falls through various usage thresholds to slower but less-costly forms of storage media. Virtual tape may also be used as part of a storage area network (SAN) where less-frequently used or archived data can be managed by a single virtual tape server for a number of networked computers.

This is a great definition and really addresses new applications like HSM (which might be old to some people) and provisioning but does not really address why most people are moving to tape virtualization.

So Why Virtualize Tape?

Because there has been this one aspect of tape and backup that has haunted the IT professional for years. It is called the "backup window." It is the amount of time it takes to backup your system. In the old days, you would do your backups in the evening when everyone was off the system and data could be backed up at your leisure. Now we live in a world were we have so much data that the tape system does not have the time or backup window to complete the backup or, worse, our environment is 24/7 and there is not backup window.

Virtualization to the Rescue!

Most virtual tape systems or servers can minimize or even eliminate the backup window. They do this by using the same technology that gave them their start--hard disks. Putting hard disks in front of the tape drive or library and then fooling the backup or archiving software that it is a tape drive and not a hard disk allows the software to rapidly put data on the tape. Think of the disk drives as a large cache that eliminates the time to do tape media changes and can handle nonstreaming data. Non-streaming data is data that does not keep up with the tape drive, thus making the tape drive reposition the media. These repositions can take seconds and can slow the data transfer to tape thus slowing the backup or archiving process.

Virtual Tape Configuration

Besides the performance benefits, the other technology that is used and that can totally eliminate the backup window is mirroring. By setting up a synchronous mirror of your disk drives, you have a complete mirror of your data at all times. This allows you to have an up-to-the-minute image of your data. When you start the backup or archiving, the mirror is broken and the backup or archiving is now done with the mirrored data. This allows the backup to run without affecting the primary data that is still being accessed. When the backup is complete, the mirror is re-engaged and maintains the mirror until the next backup. This process works extremely well in SANs and in heterogeneous environments where multiple backup program may be run to completely backup the data.

Virtual Tape Configuration With Mirror

One of the real benefits of virtual tape is that it allows you to use your current backup and archive software. This means that you do not have to worry about losing the ability to read your old tapes, yet it gives you the benefits of smaller backup windows and even eliminating backup windows altogether.

With the onset of provisioning software and even lower-cost disk systems, the need for virtual tape might go away, but tape always seems to come back. Its demise has been predicted many times. It was in 1972 that IBM predicted the death of tape, and, in so doing, helped a company called StorageTek come out of nowhere to become a dominate provider of tape technology. Now, both IBM and StorageTek are leaders in virtual tape products, and tape might still be around for some time to come.