Miksi Zip voi purkaa yksittäisiä tiedostoja paremmin kuin samat sisällöt?

Suppose that I have 10,000 XML files and want to send them to a friend. Before sending them, I would like to compress them.

Method 1: Do Not Compress Them

Results:

Method 2: Zip Every File Separately and Send Him 10,000 Zipped XML Files

Command:

Method 3: Create a Single Zip File Containing All 10,000 XML Files

Command:

Method 4: Concatenate the Files Into a Single File and Zip It

Command:

Questions

• Why do I get such dramatically better results when I am just zipping a single file?
• I was expecting to get drastically better results using method 3 rather than method 2, but I do not. Why is this?
• Is this behaviour specific to zip? If I tried using Gzip, would I get different results?

Additional Info

Meta Data

One of the answers given suggests that the difference is the system meta data that is stored in the zip file. I do not believe that this can be the case. To test it, I did the following:

Zip compression is based on repetitive patterns in the data to be compressed, and the compression gets better the longer the file is, as more and longer patterns can be found and used.

Simplified, if you compress one file, the dictionary that maps (short) codes to (longer) patterns is necessarily contained in each resulting zip file; if you zip one long file, the dictionary is ‘reused’ and grows even more effective across all content.

If your files are even a bit similar (as text always is), reuse of the ‘dictionary’ becomes very efficient and the result is a much smaller total zip file.

In zip, each file is compressed separately. The opposite is solid compression, that is, files are compressed together. 7-zip and Rar use solid compression by default. Gzip and Bzip2 cannot compress multiple files, so Tar is used first, having the same effect as solid compression.

As xml files have similar structure (and probably similar content), if the files are compressed together then the compression will be higher.

For example, if a file contains the string “” and the compressor has already found that string in another file, it will replace it with a small pointer to the previous match. If the compressor does not use solid compression, the first occurrence of the string in the file will be recorded as a literal, which is larger.