Seen one of those 1.5 GB CDs recently ?
Amazed ? Don't be. Those who do such things make "optimized" CD images (or ISO files). This article explains how they do it.


Basically, this is how an "optimized" ISO image of a CD works:

1. Any CD (or ISO image) has something called a TOC (the Table of Contents of the files on the CD).

2. All files are written to the CD and a corresponding entry to that file is made in the TOC (which tells your CD drive where to find that file).

3. Normally for any file or folder, there's one entry in the TOC. The exact file if put in another folder will acquire another entry in the TOC in addition to being written in the CD's "contents" area.

4. Look at the graphic in the attachment below. In it, assume the similar geometrical shapes to be identical files. Now, if the black circle was a file 100 MB in size, writing it to a CD in 3 separate places would cost you 300 MB. And, if each saffron square was 25 MB, 2 squares would have amounted to 50 MB. Let's say the irregular yellow shape is 200 MB. In total, this CD (of those shapes as files) would total up to 550 MB.

5. Somebody then got a brainwave and said, what if I just wrote the file once, and told the cd drive (through the TOC) that the same file is in X number of positions.
So now, the 550 MB CD above becomes just 325 MB (how ? each file is written just once, and additional occurrences of the same file is re-directed to point to the same location.

6. Thus, we see that the software making the ISO sees that there are repetitions of the same file many times in many folders, and encodes the file itself only once, but makes hard-link entries to that file in the TOC.

7. The result is that the user sees that there are 35,000 files occupying 6 GB (for example) on a CD, but due to optimization, only the 4,000 unique files are really written, the rest are all "virtual" links contained in the TOC of the CD, so the CD is actually just 630 MB (just for example, of course).



Please post any questions you have about this matter, or links to other places where "optimization" of a CD image is very useful (so that people can see where and how such ISO images are used).

Here are some links to good guides that make use of optimized cd images.
1. Making an MS-Office 2003 Pro Edition AIO (all-in-one) CD.
http://www.MSFN.org/board/index.php?&act=ST&f=70&t=13943

Go here for a poll on the best CD-optimization tool.

old news cdimage has been being used by people on this board for over 6 months

I kno....
I have also mentioned that that post would be a good starting point for newbies to understand optimized ISO9660 cd images.

You may have known it since a long time, but not everyone. And I bet not everyone uses UltraISO or knows it can be used to optimize CDs (it's easier to use than cdimage.exe and does lots more).

btw I cannot see the image??

huh ???????
Image attachement added.

aa yea u added it after jan 24!!!!!!!!!!!!!!!!!!!!!!!!

The idea behind cdimage now seems so obvious (since i found out how to use it here about a month ago) that i'm quite surprised that the same technique isn't used more widely in other compression/burning programs, or even a standard feature of the file system on the hard drive... am i asking too much?

'tis a cool program... unless there is some easier way, maybe i should start using cdimage for burning my incremental backups too, it would save me worrying about duplicate files and i could just burn the folders as they are...

@LotusAstra :
Just something to note - as far as cdimage or UltraISO is concerned, 2 files are duplicate files only if their byte-size and time-stamp is the same.
So if your incremental backups contain files that are exactly the same size, and same contents, but different modification dates (can happen when simply printing one of those duplicate files in Word and Word resets the last modification date while exiting), those files won't be treated as identical/duplicate files.

This is exactly the reason that when making a winXP AIO CD, you can cut down the ISO file size by 74 MB when having the same drivers.cab in both, XP home and pro I386 folders (because, the XP home drivers.cab has a modification date later than Pro - but their contents are *exactly* the same - don't ask me how or why, I don't know). The other files in both the I386 folders are mostly of the same file-size and time-stamp.

Where on earth can I find CDImage??

Here you go.

Note: you will have to modify the batch file - i changed the paths for my installation.

appreciated mate, i did think that's how it worked, thx for the comfirmation... for my lazy backup methods it could be a useful tool for archiving some of my old stuff that i would otherwise delete... But anyway, i haven't tried it to much extent yet, so i'm not sure how useful it would be in the real world, long file names seem to cause concern too...

...i mainly mentioned it to see what other people thought, if anyone else used it for that, or if anyone strongly recommended that i don't use it for what i want

Sorry, don't mean to drag your thread in another direction, just pondering any other uses for cdimage

@Stealth870 -
....... LOL ........
Here on earth, as you saw.


@LotusAstra -Not really, look at the output of "cdimage.exe /?"personally, I don't have ideas of using it for archival/backing-up anytime soon, but I don't see (any reason as to) why you can't/shouldn't use it for that purpose. It's a stable little program and gives very few reasons to frown (although, difficult to use until you know all the switches).


:clap clap:
ok class, as we were saying........
:gets back on topic:

OK, cool, took a look at that /? switch and very interesting... i'll keep what u say in mind and see what i can do in a bit... thx

pardon my ignorance, but is this the same cdimage referenced in the guide, or is this a completely different application?

The cdimage.exe referenced here is the same one referenced in the guide.
But wherever it says "CD-image" there you know that it refers to your ------.iso which you created.

So it would be prudent to backup data in an optimizied iso, and then compress it with an arhiver (like .rar or .7z or ** i heard there's even some with higer compression rate)?

It's not really an "optimized CD" it's just a normal ISO standard CD which accurately reflects a disc filesystem with hard-links. Hard-linking is a standard feature of most file systems outside the Windows world. In fact, MS needed it to get their POSIX compliance, but their implementation is the "shortcut" which isn't even on a par with "symbolic links" as the underlying os/fs should make their use transparent to applications, where as Windows doesn't.

As Windows doesn't do hard-linking, it seems that some burning software can do the linking on-the-fly while it creates the ISO. However I read further in this thread that at least one of them only compares file size & date stamp. I wasn't aware of them when I created one of these CDs so I wrote a short script to do all the linking based on POSIX 1003.2 CRC checksums which is much safer.

so that the reduction of size depends on the number of identical files. I wonder in original xp cd how many identical files it has. i had an integrated xp cd image but after applying some methods listed in forum, my image was still too big to write on cd. i had to buy a larger-capacity cd and thought of overburning it.

The XP installation CD doesn’t have so much double files, especially when most files are packet in the CAB files. Did you try to use nLite?

I have used Nlite but after adding the drivers for several systems the iso is to large for cd. I could use a dvd but not all systems have a dvd-rom. Any ideas?