Microsoft took a serious look at how to save space from the operating system files with Windows 8.1 Update. What they came up with at the time was WIMBoot, which used the recovery partition’s compressed WIM image file as the basis for most of the system files needed by Windows. Since the recovery partition is at least 4 GB in size, this is a pretty substantial savings especially on the lower cost devices which WIMBoot was targeted at.

I’ve discussed the changes with Windows 10 a couple of times, but a recent blog post from Michael Niehaus outlines how the new system works, what it is called, and how to manually enable it.

The last bit there is pretty important, since moving to WIMBoot was not something that could be done easily. It had to be done at the time the system image was put onto the computer, and there were a couple of extra steps OEMs could take in order to incorporate their own software into the WIMBoot.

Standard Partition with Windows 8.1

WIMBoot Enabled Windows 8.1

This also lead to some of the first issues with WIMBoot. The actual size of the recovery partition, if it was just Windows, would be around 4 GB, but once an OEM adds in their own software, along with maybe a copy of Microsoft Office, and all of a sudden the recovery partition could bloat to 10 GB or more. This was a major issue because unlike with a standard install of Windows, the recovery partition can not be removed on a WIMBoot system leaving a large chunk of a possibly small drive used up with no way to reclaim that space.

The other issue was that over time the WIMBoot partition would become less and less used, since when there were security updates to the operating system, key system files would be replaced with a full uncompressed version, but the original version would still be part of the WIM which could not be modified. Over time, Windows would grow and grow to fill more and more of the drive, and the WIMBoot concept was clearly not working out as intended.

So with Windows 10, Microsoft has moved away from the recovery partition altogether. When you do a system reset, Windows will be rebuilt from the components in the \Windows\winsxs folder. This means that the system will also be almost fully patched after a reset, unlike with earlier versions of Windows where any restore off of the recovery partition would revert you back to whatever set of files was used to create the WIM. Only the most recent 30 days of patches will be installed, and this was a design decision in case the reset itself is due to something going wrong within the last 30 days.

The other part of the space savings is from a compression tool Microsoft is calling Compact OS. This kind of goes back to WIMBoot in a way, since the system files are compressed into what amounts to a WIM file. The big difference here is that unlike WIMBoot, CompactOS can be enabled and disabled on the fly.

From an administrative command prompt, simply use the commands:

Compact.exe /CompactOS:query

This will query Windows to see if CompactOS is enabled or not

Compact.exe /CompactOS:always

This will enable CompactOS

Compact.exe /CompactOS:never

This will disable CompactOS

I ran CompactOS on an ASUS TP200S which has 64 GB of eMMC storage. Windows 10 did not enable CompactOS automatically since it was not needed, but manually enabling it saved over 3 GB of space on the C: drive. Luckily ASUS has included enough storage in the TP200S where it’s not really necessary out of the box, but on any system with 32 GB or less this could be a big help.

There is going to be a performance impact of course since the files will need to be decompressed when accessed and the actual differences are something I hope to have a chance to test and document at some point in the not too distant future.

In the end, CompactOS looks to be a nice upgrade over WIMBoot which had a lot of promise, but was not as effective as hoped.

Source: TechNet

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  • eddman - Friday, October 2, 2015 - link

    Too many apple articles -> anandtech is paid by apple.
    Too many intel articles -> anandtech is intel's marketing wing.
    Too many MS articles -> anandtech's writers are MS shills.
    Too many nvidia articles -> anandtech is nvidia's PR division.
    Too many AMD articles -> anandtech's writers are on AMD's payroll.

    There is always something, isn't it?
  • inighthawki - Friday, October 2, 2015 - link

    "There is always something, isn't it?"

    Don't forgot Google.
  • iLLz - Friday, October 2, 2015 - link

    Didn't Win 10 just cross the 100 million install mark in less than 2 months? How is that slow adoption?
  • Gigaplex - Friday, October 2, 2015 - link

    It was initially fast, but it has tailed off and they're no longer on target to meet their projections.
  • Michael Bay - Saturday, October 3, 2015 - link

    I`d very much like to know the current activation count. Show your sources.
  • DanNeely - Saturday, October 3, 2015 - link

    MS numbers are likely not available publicly (the occasional press release not withstanding); but Steam's Hardware Survey is updated for the end of last month and shows W10 at 26% (vs IIRC 18% last month, and 5 or 6% 2 months ago). Looking at shares from the general browsing public, StatsCounter shows it at 7.6% vs 5.4% last month, NetMarketshare 6.6 vs 5.2%, W3Schools has 3.5% in August but doesn't have September data up yet.

    I'm not overly concerned that month 2 adoption was much less than month 1 because an initial surge much larger than following months is a fairly common pattern; especially since W10's launch was impacted by back to school shopping and a likely sales bump as a result. I've seen at least on news site that paid for weekly data from a webstats company and wrote an article on it; but the graphs they showed were noisy as hell and unsuitable for any sort of analysis in the raw format. Which didn't stop the person writing the article from writing an article and acting like a several percent jump in W7 numbers on the graph (at 8.x's expense) was a real signal and not one of the reasons why the companies don't release finer grained data to the general public.
  • Michael Bay - Sunday, October 4, 2015 - link

    Yes, Steam is what I was looking at myself.
    I guess we`ll have to wait for their next survey or the one after that for a proper trend to emegre.
  • lilmoe - Saturday, October 3, 2015 - link

    If we're assuming fair distribution of articles based on OS relevance in Anantech's article archive, then Windows coverage is pretty lacking...

    Anandtech is very short on technical articles about Windows and Windows technologies. For example, they just recently posted a tiny article about .NET Native after quite a while of its announcement.

    You sound like Apple fans on The Verge who outrageously claim "lack of coverage of Apple related articles"....
  • Michael Bay - Saturday, October 3, 2015 - link

    Who in his right mind would even read that SJW-infested rag?
    When they got mossberg`d, I thought even the most dense will see that it`s dead.
  • Mumrik - Sunday, October 4, 2015 - link

    There is no more important piece of software a site with its roots in PC tech could cover.

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