Logical Dump

By: Jesse Miller

A logical dump starts at one or more specified directories and recursively dumps all files and directories found there that have been changed since some given base date (the last backup for an incremental dump for a full dump). In a logical dump, the dump tape gets a series of carefully identified directories and files, which makes it easy to restore a specific file or directory upon request.

Logical dumping is the most common form. Most UNIX systems use algorithms. Algorithm also dumps all directories that lie on the path to a modified file or directory for two reasons. Restore the dumped files and directories to a fresh file system on a different computer.

In this way, the dump and restore programs can be used to transport entire file systems between computers.

The second reason for dumping unmodified directories above modified files is to make it possible to incrementally store a single file (possibly to handle recovery from mistake). If you had done a full system dump on a particular day and usually you removed the directory along with all the directories and files. After some days you want to restore the file.

Restoring a file is not possible out of blue. There is a procedure and space should be allocated to put the files back. Directories must be restored first. In order to have their owners, modes, times, etc., correct, these directories must be present on the dump tape even though they themselves were not modified since the previous full dump.

Technology has greatly advanced.
The dump of an algorithm maintains a bitmap indexed by i-node number with several bits per i-node. Bits will be set and cleared in the map as the algorithm proceeds. The algorithm operates in four phases. Phase 1 begins at the starting directory and examines all the entries in it. For each modified file, its node is marked in the bitmap. Each directory is also marked and then recursively inspected.

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At the end of phase 1, all modified files and all directories will be marked in the bitmap. Phase 2 marks any directories that have no modified files or directories in them or under them. For efficiency, phases 1 and 2 can be combined in one. At this point it is known which directories and files must be dumped. Phase 3 consists of scanning the i-nodes in numerical order and dumping the marked directories.

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