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Why can I futher compress an MSI with a compressed CAB?

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Question

Why can I futher compress an MSI with a compressed CAB?

I discovered that our MSI can be further compressed with WinZip from 1798K to 1488K.

Answer

The resulting MSI file you’re getting has two pieces. One is the MSI database which is not compressed. It is an OLE structured storage file. The other part is the CAB (which I believe ends up being a stream in the OLE structured storage file) which contains all of the actual files to be installed. The further compression you’re seeing in WinZip is probably just compression that is done on the OLE structured storage file. Unfortunately, MSI doesn’t support compressed databases unless you also provide a bootstrapper (such as a self extracting exe) to decompress the database before you pass it to msiexec.

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  • Scott, Microsoft Dev

[Author: Robert Flaming]

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