2016, WDS, Deduplication: Disable RES.WIM usage??


  • This is a weird question, probably not possible without a feature request to Microsoft..

    A bit of history here. Windows Deployment Services has been around for quite a number of years, and it uses a file-based deduplication mechanism to conserve disk space.

    When you capture an image with WDS, what it is actually doing is creating hashes for all files on the disk and then checking to see if those hashes are present in RES.WIM. Any hashed files not present in RES.WIM are copied up to the server, added to RES.WIM, and then the disk image itself is just a list of hashes to pull out of RES.WIM to reconstruct the image.

    So it is to your benefit to only create a single huge imaging group, because for each additional image group there is a new RES.WIM which is independent of the other and may duplicate hashed files from the other RES.WIM. Throw everything into it, XP, Vista, 7, 8, 10, and all of them automatically combine into a single huge RES.WIM hash pool that greatly reduces file storage.

    Except RES.WIM has a problem in that you can never shrink it, because it cannot know what image files you've deleted and what hashed files it needs to keep. Even if you've deleted all your Win XP images, those files for XP will live on forever in the RES.WIM hash pool.

    Apparently the only real way to shrink RES.WIM is to create a new image group, export images from the first one, and import to the new one, which takes up even more disk space, before you can finally remove the old image group to recover disk space. It's extremely unhandy and inconvenient.


    Though, Microsoft has now come up with sector-based deduplication. Scan in the background, deduplicate identical sectors, and regain free disk space. This accomplishes the same thing as the file-based deduplication of WDS, but this is now even better, because you can delete deduplicated files and you can recover the disk space used by the non-duplicate parts of the image file.

    Sure, you can run both WDS and sector-based deduplication on the same drive, but then you still cannot ever shrink RES.WIM.

    It seems better to just disable WDS RES.WIM file hashing, save all images as raw huge file collections, and then let the nightly optimization shrink the image to nothing, merging its content with other images saved to the deduplicated drive.

    If you then delete unneeded images, you will recover disk space as expected, though not necessarily as much as you might think due to the deduplicated parts of it.

    Thursday, March 23, 2017 4:15 PM

All replies

  • Hi,

    Thank you for share the great experience to us.It seems related to the mechanism by design.And please also give the advice and suggestion on  the Windows Server User Voice site,thank you.

    Best Regards
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    Friday, March 24, 2017 3:29 AM