Patch building with MsiMsp.exe -- can target MSI differ from original MSI?

Question

I'm building an msp patch for a product that has already been delivered. The product has a large number of components/files.

I'm building the patch from the original target MSI and an upgrade MSI (modified by hand via orca). So far so good.

Building and applying the patch takes a lot of time with that many files so I'm thinking about removing all the unchanged files from both MSIs to speed it up. I know that I can make some modifications to the target MSI and it still applies fine, but I'm wondering about changes to files/components/directories. Can anyone tell me

1. if removing files is going to work in the first place, and
2. if I'm inviting trouble with further patches (both building on baseline and patched version) if I do that?

I've pretty much hit every pothole so far, I thought I'd ask first this time. And I freely admit that I don't have a full grasp of the windows installer yet, so I may be approaching it all wrong.
Thanks!

Answer 1

Removing entries from the original MSI is not recommended because it usually breaks the patch rules (directories,components or features do not match). Also, the patch may not be applied correctly.

Instead, you can try this:

• perform an administrative installation for both the target and upgraded packages
• for the extacted files manually delete the ones which haven't changed
• build your patch

This may speed up the build process. Unfortunately you cannot speed up the installation because the delay is most likely caused by a high component count. In this case a good approach is to use a major upgrade which minimizes the number of components in Component table.