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Unix Backup and Recovery by W. Curtis Preston

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Simultaneous Backup of Many Clients to One Drive

This is also a relatively new problem for backup software products. It used to be that the backup drive was the bottleneck in any backup system. For many years, the optimum speed of most backup drives was less than 1 MB per second. Since disks were able to supply data much faster than that, the only other potential bottleneck would be the network. Therefore, even a 10-MB network could stream a backup device at its full speed.

Those days are gone. There are now backup drives that can write three times as fast as many disk drives. Backup drives now use Fast-Wide SCSI, large buffers, and sometimes write multiple streams of data to the drive at once. Fibre-channel backup drives are now available. The result is that many backup drives can write at 10 MB/s or faster. The difficulty with most of these drives, though, is that they are "streaming” tape drives. This means that you must supply them with a steady stream of incoming data equivalent to their maximum throughput. If not, the drive will not stream, resulting in a significantly slower transfer rate than it is capable of. This problem, of course, affects only streaming tape drives. It does not affect backup drives that simulate a disk drive, such as Magneto-Optical, CD, and DVD drives.

All streaming tape drives are designed to write most effectively at their optimum speed. If they are supplied with data at a slower rate than that, the result may be surprising. What begins to happen ...

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