So I was building a Windows 7 virtual machine, so I could play “Gardens of Time” as GoT doesn’t work on my Windows 2008 R2 box …, and I went looking for MaxPatchCacheSize setting in the registry.

I didn’t have access to the internet* and I wanted to set the setting so I could save space on my VM.  But the setting isn’t in the registry by default!

So as it’s a “policies” registry setting, I was able to find it in the Local Group Policy Editor (gpedit.msc):
Baseline file cache maximum size

The policy is called “Baseline file cache size”:

This policy controls the percentage of disk space available to the Windows Installer baseline file cache.

The Windows Installer uses the baseline file cache to save baseline files modified by binary delta difference updates. The cache is used to retrieve the baseline file for future updates. The cache eliminates user prompts for source media when new updates are applied.

If you enable this policy setting you can modify the maximum size of the Windows Installer baseline file cache.

If you set the baseline cache size to 0, the Windows Installer will stop populating the baseline cache for new updates. The existing cached files will remain on disk and will be deleted when the product is removed.

If you set the baseline cache to 100, the Windows Installer will use available free space for the baseline file cache.

If you disable this policy setting or if it is not configured the Windows Installer will uses a default value of 10 percent for the baseline file cache maximum size.

Changing the setting in the Local Group Policy Editor created the HKLM\Software\Policies\Microsoft\Windows\Installer\MaxPatchCacheSize key.

* – if I did have access to the internet, I would have remembered that I could have done this with a simple reg.exe command:
reg add HKLM\Software\Policies\Microsoft\Windows\Installer /v MaxPatchCacheSize /t REG_DWORD /d 0 /f

I wrote about that here.