Image of a DAT module from StorageWorks

Importance of backup
As we all know an accident can happen when we don't counting on it. Your hard drive can crash, someone may mess with your files on your hard drive or a burglary walks out with your computer. To solve these problems you can use a Tape Backup Drive for secure and backup your files. Many companies make backups daily and keep the backup tapes in a locked safe. A small DAT tape can store as much as 8 GB of data. Larger tapes stores a lot more. Let's have a look on the two most popular Tape Backup Drives and how they work.

How a Quarter-inch Cartridge (QIC) Tape Backup Drive works

Here is a step-by-step description:

1. The programs that are used with the QIC Tape Backup Driver are doing the  backup. The backup program reads your hard disk's file allocation table to find  the files you have told it to back up. The program transforms the information  from FAT to a 32K buffer in your PC's RAM. It then copies the files into the  same buffer. Each file is preceded with header information. This header  information identifies the file and its location on the hard drive's directory tree.

2. If the tape driver's controller includes chips that handle error correction, the  backup software dumps the full buffer from RAM to the controller's own buffer,  where the chips counts error correction (EC) codes. The software computes the  EC codes based on the pattern of 0 and 1 bits in the files, attaches the to the  end of the data in the RAM buffer and finally copies the contents of the RAM  buffer to the controller buffer. If the controller has built in error correction, it will  do all this work without this software. When the data is transferred to the  controller, the RAM buffer is free to receive the next block of data from the disk.

3. The tape drive's controller sends signals to the tape mechanism, which will start  to move the tape. The cartridge contains an elastic belt, which make the tape  roll. This makes the tape press against the drive with a constant pressure. The  result of this solution is that write and read errors are minimised. 

4. The controller sends a stream of data to the drive's write head. Many QIC drives  have a three-part read-while-write head. Two read heads flank a central write  head that transfers the data to the magnetic coating on the tape. The tape can  move forwards and backwards. One of the read heads reads the data in order  to verify the data that just had been written by the other write head. If the data  isn't complete, the controller's buffer empties, and the drive moves on to the  next section of disk data. If the data is corrupted, it will be rewritten on the next  stretch of tape.

5. The most common format of a QIC tape contains 20 to 32 parallel tracks. When  the tape reaches one of the ends of a spool, it reverses and the flow of data  loops back in a spiral fashion to the next outside track. Each of these tracks is  divided into blocks of 512 or 1,024 bytes. Each segment typically contains 32  blocks. Each segment contains eight error-checking codes. At the end of each  block, the drive computes a cyclic redundancy check (CRC) for further error  correction and appends it to the block. 

6. When one of the tape ends approaches the drive head, holes that are punched  in the tape signal the drive to reverse the direction of the tape and to shift the  active area of the recording head up or down to the next track and then  continue recording. Finally, when all the data has been written to the tape, the  backup software updates the tape's directory with the track and segment of the  files that it has backed up.
 

 How a Digital Audio-Tape (DAT) Backup Drive works
Here is a step-by-step description:

Image of the DAT drive's read/write head
 

1. The software starts to read from your hard disk's file allocation table to  find the files to back up. Then it copies the data, file by file, into the digital audio  tape drive's buffer. This buffer normally has room for 512K or 1MB of data. It  works like a QIC tape drive to perform an algorithm on the data to create error- correction code that it adds to the data in the buffer.

2. The design of the DAT drive's read/write head is what allows it to back up  many gigabytes of data into a small tape cartridge. The mechanism is a rotating  cylinder with four heads 90 degrees apart. Write heads A and B, write backup  data, and read heads A and B verify that data. The cylinder is tilted and rotates  at an angle to the tape. The cylinder rotates with a speed of 2,000 rounds per  minute while the tape passes in front of the cylinder in the opposite direction of  the cylinder's rotation.

3. When write head A is in contact with the tape, it writes about 128K of data and  error-correction codes from the drive's buffer to a track on the tape. The cylinder  is tilted, as you can see on the image above. This tilted form make let the head  encounters one edge of the tape at the beginning of the write head and move  diagonally across the tape until it reaches the other side. The result of this  operation will be a narrow diagonal track about eight times longer than the width  of the tape.

4. Read head A checks and verifies the data in track A, bit by bit, and compares it  with the data still in the buffer. If the data read head A reads are similar with the  data stored in the buffer, the data is flushed from the buffer and more data is  read from the hard disk. If the data in track a contains errors, it will be rewritten  on the next pass.
 

5. When write head B passes over the tape, it writes data in a track at a 40-degree  angle to track A. It makes a criss-cross pattern that overlaps track A. This  overlapping data packs more information per inch of tape.

6. The process continues until all data has been copied to the tape. Finally the  drive rewinds the tape and writes a directory of stored files. This directory is  stored on either in a special partition in the beginning of the tape or in a file on  the hard disk.