I first read about MaxPatchCacheSize, and how to use it to save space in Virtual Machines, over at Jeremy Jameson’s blog.

From the Microsoft MSDN Library entry for MaxPatchCacheSize:

“The value of the MaxPatchCacheSize policy is the maximum percentage of disk space that the installer can use for the cache of old files. For example, a value of 20 specifies no more than 20% be used. If the total size of the cache reaches the specified percentage of disk space, no additional files are saved to the cache. The policy does not affect files that have already been saved.

If the value of the MaxPatchCacheSize policy is set to 0, no additional files are saved.”

But how much can it save really??? Well there is only one way to know how much it will save, and that is to test it, and measure the savings.

The Tests.
1. Office 2010 upgrade
2.Windows XP Windows Update.

I set the MaxPatchCacheSize entry to 0 by doing the following on a command line:
reg add HKLM\Software\Policies\Microsoft\Windows\Installer /v MaxPatchCacheSize /t REG_DWORD /d 0 /f

I setup a Virtual Machine with no MaxPatchCacheSize, ran the test, then reset the PC back to it’s unpatched state (aka reverted a snapshot).  Then ran the test again with MaxPatchCacheSize set to 0.

Office 2010 Upgrade
I upgraded Office 2007 to Office 2010, by accepting the default Upgrade option.
The result was disappointing, only 20 megabytes was saved.
So that’s a FAIL for MaxPatchCacheSize.

Windows XP Windows Update
I took a Windows XP SP3 PC, with no additional post SP3 security patches, and visited WindowsUpdate.  90 minutes later, I had a fully patched machine.
And a far better result for MaxPatchCacheSize.  448 megabytes.
On a 16GB hard disk, 448 megabytes is a worthwhile saving.
That’s a SUCCESS for MaxPatchCacheSize.

So in conclusion.
MaxPatchCacheSize is useful for Windows Installer based patches.  Not so much for a product upgrade such as Office 2010.

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