Question-1 Explain the file system backups.
If a computer’s file system is irrevocably lost, where due to hardware or software, restoring all the information will be difficult, time consuming, and in many cases,impossible.
Backups to tape are generally made to handle one of two potential problems:
- Recover from disaster, Recover from stupidity.
Making a backup takes a long time and occupies a large amount of space, these consideration raises the following issues (Issues of Backup):
- Issue: 1: First should the entire file system be backup or only part of it? It is usually desirable to back up only specific directories and everything in them rather than the entire file system.
- Issue: 2: It is wasteful to back up files that have no change since the previous backup which leads to the idea of incremental dumps.
- Issue: 3: Third since immense amount of data are typically dumped, it may be desirable to compress the data before writing them to tape.
- Issue: 4: It is difficult to perform a backup on an active file system.
- Issue: 5: Making backups introduces many nontechnical problems into an organization.
Two strategies can be used for dumping a disk to tape: Physical dump, Logical dump
- It is started 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 program is so simple that it can probably be made 100% bug free, something that can probably not be said about any other useful program.
- Main advantages of physical dumping are simplicity and great speed.
- It is started at one or more specified directories and recursively dumps all files and directories found there that have changed since some given base date.
- In logical dump, the dump tape gets a series of identifed directories and file.
In figure above A file system to be dumped. The squares are directories and the circles are files. The shaded items have been modified since the last dump. Each directory and file is labeled by its inode number.
- Restoring a file system from the dump tapes is straightforward. To start with, an empty file system is created on the disk. Then the most recent full dump is restored.
- Since the directories appear first on the tape, they are all restored first, giving a skeleton of the file system.
- Then the files themselves are restored. This process is then repeated with the first incremental dump made after the full dump, then the next one, and so on.
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