what is Virtual Tape--- and few qiestions.

"Virtual tape" is a disk backup.  The disk is split into chunks that are presented to the backup server as if they were tapes.  The main advantage than just throwing backup data at a large chunk of disk is manageability, your disk is used and reused in a more efficient manner and in a way that is already familiar to you.

VTLs main advantage is that it overcomes the disadvantages of tape handling - requirement to bring tapes back on site for restores, time taken to load tapes, spool through tapes as well as general issues with tape being a physical media and sometimes getting inexplicably damaged (for example when one of my customers stored their tapes next to the shredder machine and all backup tapes were rendered inoperable due to dust getting in the shell) and issues with your tape library.

However, remember disk does not replace tape, it is an additional layer in the equation - you still need a long term offsite medium for your backups in the event of a disaster recovery scenario.

Apart from the above benefits, a Virtual Tape Library generally supports better compression that the physical tapes. It allocates small chunks at a time so if your backup is 100 GB against a 400 GB Virtual tape, you use only 100 GB disk though visible as 400 GB tape whereas in a physical tape, even if the backup is 100 GB only, you use the entire tape if dedicated for this backup. The biggest advantage that they give on Virtual devices these days is dedupelication whether at rest or on the wire.    

Hello Muneer,

Please check out these videos which will not only explain the differences/ advantages of disk backup vs tape, but will provide solid practical feedback from actual EMC customers.

VTL Technology with EMC Disk Library

"Tape Sucks" - EMC World 2010

EMc Customer Presentation - Wentworth Douglas