Windows – What does Windows 10 do while it is “preparing to install updates”

windows 10windows update

I notice that Windows 10 seems to spend a lot of time "preparing to install updates" before it actually gets on with it. I also note that for most of that time, the progress bar doesn't advance at all. Like how it just now stayed at 10% for a long time (I didn't clock it, but I don't think I'm exaggerating if I put it at a few minutes), then jumped to 68%, relating to a single update.

Because the text alone is apparently not clear enough, here is a screenshot of the specific Windows Update step that I am asking about (follow the link to see the full screenshot from which the below has been cropped). Note that the screenshot was taken during a different update session than the one I am talking about in the first paragraph, hence the percentage complete is different.

Windows Update is 33% done with "Preparing to install updates"

What does Windows actually do with all this time? What "preparations" are taking it such extreme amounts of time?

Note that I am not asking about the actual installation of the update.

While the particular time that prompted me to ask this question was in a VM, I'm seeing similar behavior with Windows 10 running on actual hardware, so I don't think it's related only to the fact that it's running in a VM (even though that could perhaps be a factor).

Best Answer

I've captured the Windows Update activity with the Microsoft-Windows-WindowsUpdateClient ETW provider.

Here I see no event prepare:

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I only see Start and Stop of Download and Install.

I now used WPRUI to trace DiskIO and CPU usage during the step "preparing to Install Updates".

Here I can see that the TrustedInstaller is busy checking hashes (wcp.dll!CCSDirectTransaction::VerifyFileHashes) of the update data:

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and I see disk IO where TRustedInstaller.exe copies data to C:\Windows\WinSxS\Temp\InFlight\ and C:\Windows\SoftwareDistribution\Download\.

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So basically this "preparing to install" is actually the installation of the update, so this an confusing output.

With 1809, Microsoft fixed the wrong status and shows directly the correct "installing text":

enter image description here