The fastest way to do a backup is to back up the physical storage devices without regard to logical structure of the files, the nature of the application being backed up or much of anything else. This involves streaming data off the device and onto the backup device at the maximum throughput the system can produce. Often a raw device backup can offer speeds several times those possible with more structured backup approaches.
Because of its sheer speed, raw device backup seems attractive to organizations with a lot of data and short backup windows. While it is a useful technique, this kind of backup has some serious drawbacks that become important when you actually need to use the backups.
The biggest problem is the lack of structure in raw device backups. Not only do they ignore the file structure on the disk, raw device backups usually can't distinguish used and unused sectors. Everything gets backed up, whether there is data there or not.
Another consideration is that a raw device backup requires that the system be inactive while the backup is being run. A straight disk-to-tape data dump has no way of handling activity while the backup going on.
Nor does a raw device backup necessarily make the best use of limited backup windows. If the size of the window varies regularly, as in the case of an organization with less activity -- and hence a wider backup window -- on the weekends, it may be more efficient to do incremental backups during the week and a full backup on the weekends.
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.