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Pass IV
Finally, file data is dumped. During Pass IV, dump writes only those files that were marked for backup. dump logically writes data during this pass as it did in Pass III for directory data:

    Header (TS_INODE)
    Disk blocks (file block[s])
    Header (TS_ADDR)
    Disk blocks (more file block[s])
    .
    .
    .
    Header (TS_ADDR)
    Disk blocks (more file block[s])
    Repeat the previous four steps for each file in the list of file inodes to back up.
Post-Pass IV
To mark the end of the backup, dump writes a final header using the TS_END record type. This header officially marks the end of the dump.

Summary of dump steps
The following is a summary of each of dump's steps:

    Pass I
    dump builds a list of the files it is going to back up.
    Pass II
    dump scans the disk multiple times to determine a list of the directories it needs to back up.
    Pre-Pass III
    dump writes a dump header and two inode maps.
    Pass III
    dump writes a header (which includes the directory inode) and the directory data blocks for each directory in the directory backup list.
    Pass IV
    dump writes a header (which includes the file inode), and the file data blocks for each file in the file backup list.

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    Post-Pass IV
    dump writes a final header to mark the end of the dump.
A final analysis of dump
As described earlier, using dump to back up a mounted filesystem can dump files that are found to be corrupt when restored. The likelihood of that occurring rises as the activity of the filesystem increases. There are also situations where data is backed up safely, but the information in the dump is inconsistent. For these inconsistencies to occur, certain events have to occur at the right time during the dump. And it is possible that the wrong file is dumped during the backup; if that file is restored, the admin will wonder how that happened!

The potential for data corruption to occur is low but still a possibility. For most people, dumping live filesystems that are fairly idle produces a good backup. Generally, you'll have similar success or failure performing a backup with dump as with tar or cpio.

This was first published in August 2007

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