Tape or Tape files and their geometry.

[Kamel Rana]

Hi,

I've heard and used Tapes only in mainframes. Do we use them in other open system technologies too? If they are really cheap alternatives for DASD why it is not that popular with other technologies?

[Kamel Rana]

Hi,

I have one more question, can someone please guide me to a "tutorial" kind of a material on tape-geometry. This will be a great help.

[Robert Sample]

There are a variety of tape devices available for different platforms, not just mainframes. Tape is frequently used for back up since tapes are removable (and hence there is no practical limit to the bytes that can be stored on a tape drive -- just a limit to the bytes stored on each tape). The reason tapes are not used to replace DASD devices is that tape is a SEQUENTIAL media -- in order to read the 1 billionth byte, you have to read every byte in front of it first. You cannot skip to a cylinder or track as you can with a disk drive (which implies, by the way, that a database cannot reside on tape -- although tape is frequently used to back up the database). Furthermore, if you have 100,000 tapes in your library, you have to be able to identify which of those tapes contains the data you need, which is usually done with a tape management system.

I'm not sure what you think you're going to get by asking about "tape geometry". A tape is basically so many bits wide (36 bits is common for IBM mainframes but other widths are possible) and so many bits deep (or long). If a tape stores 38,000 bits per inch and is 1200 feet long, the capacity of a single tape drive would be 38000 times 12 times 1200 (divide by 8 to convert bits to bytes). Most tape drives these days express capacity in GB (200GB or 400GB for example) instead of giving bits per inch.

Google windows server tape drive to get an idea of the tape drives available for non-mainframe platforms. Google ts7700 to see information about the current IBM tape drives.

[Kamel Rana]

Thanks for your excellen treply Robert. t37700 has directed me to "IBM Virtualization Engine" manuals...the internet connection is slow and the pdf failed to download multiple times. But I am just wondering how tape and "Virtualization Engine" might be related...

[Robert Sample]

A virtual tape system allows a computer to use tape drives and mount tapes no matter what the underlying technology is. With the decreasing cost of disk drives, virtual tape systems often use hard disks as the physical media; the virtual tape drive acts like a physical tape drive processing data sequentially.