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disk image: A "disk image" is a file containing an image (copy) of the contents of a disk. Usually, if you copy a disk, you copy first one file, then the next, and so on. If you create a image file, you copy the first sector, then the second, ...

This needs work -- see #Problems_with_this_page for some of the problems.

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Most Recent Definition (from this page)

disk image: A "disk image" is the image file of a disk. Usually, if you copy a disk, you copy first one file, then the next, and so on. If you create a image file, you copy the first sector, then the second, ...

By definition, a disk image will always have the same size as the disk. If the disk contains only a tiny file and all the rest empty space, the disk image will still contain all the data of all sectors on the disk, it doesn't matter that 99% of them happen to be zeroes.

What is it good for, you ask? Well, one common use, and the reason why the term is in this glossary, is installation media. Many Linux floppies are formatted in the "MINIX" filesystem, however, most users have nothing but DOS/Windows to start with and cannot write such a floppy. This is where a image file comes in handy: you can download it from the web, and using a utility like RAWRITE, a MS-DOS user can write the image file to a disk, bit by bit, and thence create an alien filesystem his OS knows nothing about.

To make matters more impressive, I said "bit by bit". However, "sector by sector" is more to the truth. Unfortunately, the word "sector" bears several different meanings according to context, and would deserve at least two more entries in the glossary. But in order to understand the concept of a disk image, "bit by bit" is good enough.

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Additional Tidbits

from schnobs (portions snipped):

I'm missing a word or two about images being "larger" than the disk. But I can't get the words right (ie foolproof to a certain degree -- I want to explain, not confuse even more).

Images contain not only the data (=capacity) but also the filesystems' accounting information. So a 1.44 disk image won't fit on a 1.44 floppy. Sounds silly, but makes sense.

<separate post>

All one needs to know about disk geometry can be found in the "system administrator's guide" (sag): http://www.linuxdoc.org/LDP/sag/x1001.html

Old Definition

Will probably delete in a few days, unless something should be merged above.

disk image: Data (or programs) can be stored on things like disks or backup tapes (or CD-Roms) in various formats (and the word format can be applied at different levels, which I will ignore for this first pass at a definition). In the normal case of files on a disk, the files are stored on a file by file basis, and may not be contiguous (I'm getting off the key point). The computer knows how to find the non-contiguous parts of the file, put them together, and present them to the CPU of the computer as a contiguous file (if required).

Data can be backed up to a backup media either in a similar fashion as described above, or as a "disk image". In a "disk image" backup, the same data is stored at the same place on the backup media. If you create a "disk image" of a bootable disk, it will also be bootable, because the necessary boot sectors will be in the proper location to allow it to be bootable. (I may be stretching this a little, maybe a "bootable image" is a step or two beyond a "disk image".)

As another way of looking at it, if you do a backup that is not a disk image, when a non-contiguous file is copied to the backup media, it will (probably) be rearranged to be contiguous -- the computer starts reading one file, finds all its pieces, and then writes the whole file before proceeding to the next file. In a disk image, the computer reads the data in the first physical sector of the disk, writes it to the first physical sector of the backup, then reads the next physical sector of the disk, regardless of whether the next sector is part of the file in the previous sector or not.

It is quite possible for a disk image backup to be made to a backup media with a different configuration, like disk to tape, or disk to cdrom, or disk to a hard disk with a different physical or logical geometry (of tracks / sectors / heads).

What I've brushed over:

  • Is a bootable image just a disk image of a bootable disk, or is there something more involved? IIUC, yes, a bootable image is a disk image of a bootable disk.

Problems with this page

  • First read it
  • Two definitions -- make one
  • The first definition almost makes me think there can only be disk images of floppies not hard drives
  • Tie into disk imaging -- and putting an image on another hard drive without (or with) an intermediate file

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Topic revision: r4 - 2003-03-07 - RandyKramer
 
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