A 398MB directory was only compressed to 393MB using 7Z and Normal ZIP compression. Is this normal? If so, why do people continue to use ZIP on Windows?
Windows Compression – Why Doesn’t ZIP Compress Anything?
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Related Solutions
I would assume that 7-Zip and other archiving tools compress before they encrypt, for the reasons stated in the linked blog post. But I was unable to find any documentation that confirms that, nor could I immediately ascertain it from looking at the 7-Zip source code.
However, I can explain why filenames aren't encrypted. As you might be aware, the 7z format contains a header with the file information and other metadata. 7-Zip will not encrypt this header unless you explicitly enable it. You can do this by checking the Encrypt file names box at the bottom of the Encryption segment of the archive creation screen on Windows, highlighted in red below.
On Linux and other Unix-like operating systems (and presumably the command line 7-Zip tool on Windows), you can enable header encryption by adding a -mhe=on switch to the 7z command.
Yes, it is possible to have different compression methods for different files within the same 7-Zip archive.
Use the command line version of 7-Zip with the -m parameter to specify the method as Copy, which means no compression.
Example :
7za a -m0=Copy tmp7.7z *.txt
For more info, see -m (Set compression Method) switch.
Best Answer
If you're compressing things that are already compressed (AVI, JPEG, MP3), you won't gain much other than packing everything in a single file.