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Let's say files orig and copy are huge but identical files. In order to save storage space one could therefore either make copy a soft-link to orig or hard-link both files to the same data. orig must never be modified, while in the future copy may do so.

Can it be achieved that copy becomes a files of its own again the instance modifications are made to it, thus keeping orig unmodified at the expense of loosing the previously saved space again?

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The general scheme is called "copy on write". I am not aware of any file systems that do that, but it might help with searching. –  dmckee Feb 11 '12 at 0:00
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On Windows, "Single instance storage" is quite similar. (For Linux, btrfs comes to mind: cp --reflink.) –  grawity Feb 11 '12 at 0:02
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up vote 5 down vote accepted

Not with Windows as it normally works. You'd need something else to pull that particular stunt off.

If these files are being stored on a server, Windows Storage Server has a Single Instance Store feature that can give you something close to what you want.

OpenDedup is a cross platform file system implementation that does dedupe as well. You could run it on your desktop, if necessary.

There's also non-Windows server options as well. ZFS on Solaris and FreeBSD are particularly popular.

Finally, you can sometimes get lucky with just saving a file, but that depends on a particular application's save implementation.

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Some applications allow you to make a file a 'template'.

When you open that file it will create a new instance, with the content and formatting of the template.

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