On Tue, Sep 29, 2015 at 8:23 AM, <[email protected]> wrote: > As we all know fxos apps are compressed and packed in zip format. My > question is why zip format was chosen when we have a better compressing > format tar.xz ? tar.xz has highest compression ratio than zip & other > existing one's. even Linux kernel is distributed in this format due to its > efficient compression. My opinion is that user can save some extra > bandwidth when downloading apps are packed & distributed in tar.xz & for > developers, compressing & decompressing tools are available for windows > (7zip), Mac & linux also. So there is no problem. >
[disclaimer: I wasn't involved in the decision-making process, but the below is what I think might have been part of the discussion] It's not just about the compression ratio: you also have to think about resource consumption (time and memory) when decompressing. A quick Google search suggests that xz is somewhat slower and more resource hungry than the compression algorithm used in zip. And that would negatively affect app startup time and performance. An app also may not require all of its files at once: starting up the app may only require a single HTML page plus a few JavaScript files and maybe some images. Another part of the app may require a slightly different (but still small) subset). So your compression format should be able to support finding arbitrary files quickly. The zip format is ideal for this, since it has a directory-like listing of all the files contained in the archive. The tar format has no such index (the files are stored one after another with a small header prior to each file's data describing the name of the file, size, etc.), so finding an arbitrary file is quite expensive: you have to uncompress part or all of the .tar.xz to find each file. You could cache some of the uncompressed data, but it's not clear that would be effective. You could, of course, imagine some "xip" format that is like .tar.xz, but with a zip-like directory attached to the front so you can find arbitrary files quickly. But you then have to weigh the engineering effort involved in creating that, testing it, etc. You also have to consider whether xz is a good compression algorithm in this context: resource usage, as detailed above, compression ratios on small files (since you're probably not compressing the entire archive at one, but smaller chunks, even at the level of individual files), etc. Once all those factors are thrown into the mix, zip starts to look like a reasonable choice. -Nathan
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