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I wanted to know what type of memory is consumed when a disk image is mounted in a virtual CD-ROM (for example, when we mount a .ISO file using Daemon Tools) and where is the data inside the disk image stored on a computer when the image is mounted. Also if an image is mounted, will it affect the computer's performance?

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It doesn't move the ISO files anywhere. It stays on the hard drive (or wherever it's stored) and when mounted to a virtual disc drive, it just de-abstracts the ISO container.

When the PC reads the first block of data from the virtual disc drive, the ISO mounting application redirects that read request to the appropriate block of data within the ISO file. This process continues for each additional read request.

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    So this should also mean that the size of the ISO file should always be exactly same as the size of the disk in the virtual CD rom
    – Sid
    Jul 19, 2015 at 14:42
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    It depends. There is the size and then the size on disk. Because of the way data is stored on hard drives, sometimes a file may take a larger amount of space on the disk then it physically takes on other media.
    – Geruta
    Jul 19, 2015 at 14:52
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    (one obvious way to make the "size" and "size on disk" different numbers is to turn on NTFS compression for the file)
    – user253751
    Jul 19, 2015 at 22:03
  • @user20574 Turning on compression doesn't always work... If the ISO is badly compressible (e.g. video-DVD) it will just be stored "as-is" regardless of NTFS compression settings. I had some discussions a few months ago with someone who was certain his Windows was broken because NTFS compression didn't give him more disk-space. All the files he wanted to compress where zip/rar archives and/or MKV videos. It took a while to convince him you can't repeat compression on a file and expect the file to get smaller on every run.
    – Tonny
    Jul 20, 2015 at 9:09

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