Large Capacity CDs
March 30, 2005 2:06 AM Subscribe
I have a CD with some software on it. When I copy the files from that CD, the size of the files on my hard drive are far greater than the capacity of the 700mb CD it came from!
There is just about 1gb worth of data resulting from the copy. I know about mode 2 or XCD as described here, but there is much more data than the 800mb theoretical capacity. How is this possible?
There is just about 1gb worth of data resulting from the copy. I know about mode 2 or XCD as described here, but there is much more data than the 800mb theoretical capacity. How is this possible?
Sounds like a data protection scheme.
Someone will be along with a better explanation in a minute, but essentially the index on the disc is carefully corrupted so that the same data blocks appear more than once in the directory structure of the disc.
It could also be a CD with some of it's error correction space traded for data storage space - I believe SVCDs do this (on checking your link, it seems that's called an XCD disc)
posted by Leon at 2:14 AM on March 30, 2005
Someone will be along with a better explanation in a minute, but essentially the index on the disc is carefully corrupted so that the same data blocks appear more than once in the directory structure of the disc.
It could also be a CD with some of it's error correction space traded for data storage space - I believe SVCDs do this (on checking your link, it seems that's called an XCD disc)
posted by Leon at 2:14 AM on March 30, 2005
could it be that you compressed the data while burning, and it's uncompressing as you transfer it back?
posted by amberglow at 5:30 AM on March 30, 2005
posted by amberglow at 5:30 AM on March 30, 2005
In most cases, software manufacturers compress the data on installation discs. It's rare that the data DOESN'T expand out into more than 700MB.
You don't say whether you created the software CD or not.
posted by stupidsexyFlanders at 5:34 AM on March 30, 2005
You don't say whether you created the software CD or not.
posted by stupidsexyFlanders at 5:34 AM on March 30, 2005
stupidsexyFlanders: "In most cases, software manufacturers compress the data on installation discs. It's rare that the data DOESN'T expand out into more than 700MB."
It sounds to me like viama is simply copying the files by hand from the CD to hir hard drive. I don't know of any compression scheme that automagically decompresses when it's copied, and I don't know if that would be either possible or desirable.
I'd lean towards Saydur's blocksize explanation. ISO9660 defaults to 1024-byte blocks, and NTFS generally defaults to something much larger. It varies depending on the drive size, but it could be as large as 64 kB.
In Windows, when you right-click and go to Properties, it lists both "size" and "size on disk". Are they significantly different? Are you even talking about Windows?
posted by Plutor at 5:47 AM on March 30, 2005
It sounds to me like viama is simply copying the files by hand from the CD to hir hard drive. I don't know of any compression scheme that automagically decompresses when it's copied, and I don't know if that would be either possible or desirable.
I'd lean towards Saydur's blocksize explanation. ISO9660 defaults to 1024-byte blocks, and NTFS generally defaults to something much larger. It varies depending on the drive size, but it could be as large as 64 kB.
In Windows, when you right-click and go to Properties, it lists both "size" and "size on disk". Are they significantly different? Are you even talking about Windows?
posted by Plutor at 5:47 AM on March 30, 2005
Best answer: If you're just copying the files than compression isn't a possibility, and even with zero error correction there's no way to get a gigabyte on there. The cluster size suggestion is only possible if the disk does contain thousands of tiny files and your hard disk is using abnormally large clusters.
The most likely possibility is, as already suggested, messing up the data table so that the entries for two files point to the same area of the disk, meaning the second one doesn't take up any more space until two separate files are created on the hard disk. Without knowing more about the disk it's hard to say why they'd do this.
posted by cillit bang at 5:56 AM on March 30, 2005
The most likely possibility is, as already suggested, messing up the data table so that the entries for two files point to the same area of the disk, meaning the second one doesn't take up any more space until two separate files are created on the hard disk. Without knowing more about the disk it's hard to say why they'd do this.
posted by cillit bang at 5:56 AM on March 30, 2005
Could be a GigaRec disc. This is Plextor's method of cramming 1GB of storage onto a CD-R, or 99 Min. of audio.
Seems like the standard never really got off the ground, but you can still buy GigaRec Drives.
You can find out more about GigaRec at this page.
posted by SlyBevel at 6:51 AM on March 30, 2005
Seems like the standard never really got off the ground, but you can still buy GigaRec Drives.
You can find out more about GigaRec at this page.
posted by SlyBevel at 6:51 AM on March 30, 2005
how would that work as a data protection scheme? is the idea that it makes cd - disk - cd copying difficult? because the disk copy is logically consistent with the cd, as far as i can see (you've effectively expanded some links as separate files, in quasi-unix speak, right?)
posted by andrew cooke at 6:53 AM on March 30, 2005
posted by andrew cooke at 6:53 AM on March 30, 2005
I don't know...when i copy stuffit files off of cds that i've previously burned at work onto the harddrive, a shield comes onto the screen and it decompresses/examines them. (i don't know what program it is/mac)
posted by amberglow at 7:07 AM on March 30, 2005
posted by amberglow at 7:07 AM on March 30, 2005
Response by poster: It's XP on NTFS. It's a software install CD. Size versus size on disk (1.13gb) is more or less the same. If I select all of the files on the CD and select properties, then I get the same answer. The block size and file system answers are probably on the right track. Does the ISO standard allow for 'pointers' to files in this manner?
posted by viama at 7:16 AM on March 30, 2005
posted by viama at 7:16 AM on March 30, 2005
Yeh, this sounds like multiple files pointing to the same place on disk. I'm pretty sure the ISO spec doesn't care about this, it just seems to deal with what type of names can point to data on disc. Your specific CD is probably copy protected or optimized .
One of the neat things to do with this is make a cd with Windows XP Home and Pro. Pro is just like home, except it has a few different files and a few extra ones. Microsoft has an app that finds all of the files that don't differ and make them point to the same data on the cd, thus allowing you to have two "copies" of windows on the same cd. You can make your own with this guide.
It's basically the same concept as hard links on NTFS or *nix, except you only know about them when writing. When you read, there's nothing telling you that x file is the same as y file. On NTFS or *nix, there's a file system flag that says that the file is a hard link.
Some older copy protection schemes use this so people don't copy all the files off of the cd and fail when burning them to a new cd, since it's bigger than the CD they have. Things are much more advanced now, since must cd burning programs just copy the bytes off of the cd to the other cd - copying raw data, not "files" or other abstractions.
As odinsdream said, sector size (the minimum size of a file) is also a problem. On a CD, it's at least half of most hard drive sector sizes. If you had a CD full of 700MB of one character text files, copying it to your hard drive would probably result in 1400MB "on disk".
posted by easyasy3k at 11:24 AM on March 30, 2005
One of the neat things to do with this is make a cd with Windows XP Home and Pro. Pro is just like home, except it has a few different files and a few extra ones. Microsoft has an app that finds all of the files that don't differ and make them point to the same data on the cd, thus allowing you to have two "copies" of windows on the same cd. You can make your own with this guide.
It's basically the same concept as hard links on NTFS or *nix, except you only know about them when writing. When you read, there's nothing telling you that x file is the same as y file. On NTFS or *nix, there's a file system flag that says that the file is a hard link.
Some older copy protection schemes use this so people don't copy all the files off of the cd and fail when burning them to a new cd, since it's bigger than the CD they have. Things are much more advanced now, since must cd burning programs just copy the bytes off of the cd to the other cd - copying raw data, not "files" or other abstractions.
As odinsdream said, sector size (the minimum size of a file) is also a problem. On a CD, it's at least half of most hard drive sector sizes. If you had a CD full of 700MB of one character text files, copying it to your hard drive would probably result in 1400MB "on disk".
posted by easyasy3k at 11:24 AM on March 30, 2005
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If the CD uses say, 512 byte cluster size, then you can't waste more than 511 bytes per file at very worst. However, on the hard drive, you could have cluster sizes as large as 4k. Even a text file with one sentence in it would therefore take up 4k of disk space. Depending on your operating system and disk size, you could even be talking 64k clusters, although that's highly unlikely.
Of course, the sheer size of the expansion does seem a little hefty. Are we talking 5,000+ files on the CD? If so, what OS and hard disk size do you have? If you know, what file system (FAT16, FAT32, NTFS) are you using?
posted by Saydur at 2:13 AM on March 30, 2005