CompuServe Messages

Chas, DAT tapes use the same helical scan technology that VCRs use, although improved for reliability. As a result, they can pack a significant amount of data into a small area and the head-to-tape speeds are quite high..even though the tape speed (ips) is relatively slow. And, of course, with the high head-to-tape speed, they can also write a lot of data quickly. As to where directory or file information is kept….that's a function of the backup program. The drives themselves are streaming devices…that is you just "pour" the data into them and they store whatever it is you sent them. If you wrote a filesystem onto the tape…yes, that can cause significant slow-downs as it would have to find a directory sector(s) every time you wanted to read a file. The backup systems I'm familiar with (the software portion) simply put a header on each file that contains the file path and other necessary information (such as file size, ownership, filedate, last access date, etc.). There's usually an end-of-file marker also so that if one file is corrupt, the software can seek to the EOF and then look for the next valid file header. To recover a file from the tape, the software simply reads the tape until it finds the correct header and then does the recovery. If you're recovering multiple files…the software just looks for any one of the filenames as it reads the tape..and recovers files as it locates them. That avoids the 'find one file, rewind, find the next file, rewind, etc.' scenario. Don