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Dumpster diving
The dump utility is very filesystem-specific, so there may be slight variations in how it works on various Unix platforms. For the most part, however, the following description should cover how it works because most versions of dump are generally derived from the same code base. Let's first look at the output from a real dump. We're going to look at an incremental backup because it has more interesting messages than a level-0 backup:

# /usr/sbin/ufsdump 9bdsfnu 64 80000 150000 /dev/null /

    DUMP: Writing 32 Kilobyte records
    DUMP: Date of this level 9 dump: Mon Feb 15 22:41:57 2006
    DUMP: Date of last level 0 dump: Sat Aug 15 23:18:45 2005
    DUMP: Dumping /dev/rdsk/c0t3d0s0 (sun:/) to /dev/null.
    DUMP: Mapping (Pass I) [regular files]
    DUMP: Mapping (Pass II) [directories]
    DUMP: Mapping (Pass II) [directories]
    DUMP: Mapping (Pass II) [directories]
    DUMP: Estimated 56728 blocks (27.70MB) on 0.00 tapes.
    DUMP: Dumping (Pass III) [directories]
    DUMP: Dumping (Pass IV) [regular files]
    DUMP: 56638 blocks (27.66MB) on 1 volume at 719 KB/sec
    DUMP: Level 9 dump on Mon Feb 15 22:41:57 2006

In this example, ufsdump makes four main passes to back up a filesystem; note that Pass II was performed three times. Here's what dump did during each pass.

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Pass I
Based on the entries in the dumpdates file (usually /etc/dumpdates) and the dump level specified on the command line, an internal variable named DUMP_SINCE is calculated. Any file modified after the DUMP_SINCE time is a candidate for the current dump. dump then scans the disk and looks at all inodes in the filesystem. Note that dump "understands" the layout of the Unix filesystem and reads all of its data through the raw disk device driver.

Unallocated inodes are skipped. The modification times of allocated inodes are compared to DUMP_SINCE. Modification times of files greater than or equal to DUMP_SINCE are candidates for backup; the rest are skipped. While looking at the inodes, dump builds:

  • A list of file inodes to back up
  • A list of directory inodes seen
  • A list of used (allocated) inodes

This was first published in August 2007

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