Physical Dump

By: Jesse Miller

Two strategies can be used for dumping a disk to a tape: a physical dump or a logical dump. A physical dump starts at block 0 of the disk, writes all the disk blocks onto the output tape in order, and stops when it has copied the last one. Such a program is so simple that it can probably be made 100% bug free, sometime that can probably not be said about any other useful program.
There is no value in backing up unused disk blocks.

If the dumping program can get access to the free block data structure, it can avoid dumping unused blocks. Skipping unused blocks requires writing the number of each block in front of the block, since it is no longer true that block k on the tape was block k on the disk.

A second concern is dumping bad blocks. If all bad blocks are remapped by the disk controller and hidden from the operating system then physical dumping would be an excellent choice. If they are visible to the operating system and maintained in one or more "bad block files" or bitmaps, then it is absolutely essential that the physical dumping program get access to this information and avoid dumping them to prevent endless disk read errors during the dumping process.

The main advantages of physical dumping are simplicity and great speed (basically, it can run at the speed of disk). The main disadvantage is the inability to skip selected directories, make incremental dumps, and restore individual files upon request. For these reasons, most installations make logical dumps.

It is difficult to perform a back up on an active file system. If files and directories are being added, deleted, and modified during the dumping process, the resulting dump may be inconsistent. However, since making a dump may take hours, it may be necessary to take the system offline especially when it is connected in a network.

Algorithms have been devised for making rapid snapshots of the file system state by copying critical data structures, and then requiring future changes to files and directories to copy the blocks instead of updating them in one place. File system is effectively frozen at the moment of snap shot.

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The best online security in the world may be useless if the system administrator keeps all the back up tapes in his office and leaves it open and unguarded.

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