There are a number of problems with tape, which manufacturers are trying to solve.
Its quite difficult to fill up a high capacity tape in a reasonable time. Take a 200GB tape. It takes 4 hours to fill, writing at 50GB/hour.
Getting a specific piece of data off a large tape can take a while (I've waited 40 minutes to recover a critical file from the end of a 3480) this is partly because of the fact that a tape is read sequentially from start to finish.
Only one task can use a tape at a time. This causes problems when several people want to recall migrated data which is stored on one tape.
The cost differential between disk and tape is eroding. Its possible to buy a terabyte of good quality disk for £30,000. A terabyte of TS1120 tapes costs about £500, but if you factor in the cost of a tape silo to hold them, then a terabyte costs nearer £3,000. But the cost differential is not 10:1 Its easy to fill a disk to 80-90%, in fact the problem is not filling the disk, the problem is keeping some disk space free. Its difficult to fill a tape by much more that 40%. So the true cost differential is nearer 5:1.
So how are these problems to be fixed?
Servo tracks or external buffer memories will enable an application to rapidly locate a specific point on a tape without having to read right down in. This is happening with 3590s and AIT.
Virtual tape allows us to fill tapes, and could allow concurrent access, but at the expense of eroding the disk/tape ratio even more. Virtual tape consists of virtual tape drives which write to a disk buffer. The disk buffer is then flushed out to tape when enough data has been stored. The Virtual Tape section explains this in detail
