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Why use disk-based backup instead of tape?

What reasons, if any, do you see for going with disk-based backup over tape?
There are a number of compelling arguments in favor of disk-based backups. Performance is one, considering some 2 GB FC hard drives offer performance exceeding 200 MBps versus 40 MBps for some of the fastest tape devices. However, data access mode is where disk-based backup appeals to many. Tape is a sequential access device; data is written one block after another until end-of-tape (EOT) is reached. When a tape is mounted for a read operation, there is a delay for seek time as the tape must be forwarded to the correct location. In contrast, a disk is a random access device. This means that data can be written or read without having to forward to the location, simply by repositioning the head.

Because of the random access nature of disk devices, it also enables multiple concurrent write (backup) sessions. While this is also possible with tape devices, it often involves "data interleaving", which can introduce a performance penalty during subsequent read (restore) sessions.

The introduction of lower cost disk technology (i.e.: SATA) is also making disk-based backups increasingly popular. Of course, these are only some of the advantages. There are also disadvantages to disk-based backups. As mentioned on numerous occasions, offsite storage represents one of the biggest challenges facing disk-based backup users.

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