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Comment by iforgotpassword | Hacker News Reader

Exactly, some of the quirks are just a result of designing this for these old systems. ZIP needed to work acceptably on machines with less than a megabyte of RAM. The file format contains ambiguity and redundancy yes, but given well behaving software it's the "good enough" of archive formats. Even in the *nix world we didn't get anything better, tar isn't even seekable and has its own idiosyncrasies, especially if you compress it because the compression happens afterwards and is applied to the whole archive.

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Tar IS seekable, as it was designed especially for tapes.

The modern usage of it adds compression, which you have to undo first before doing your archive operations ;)

Zip was > tar (and cpio) in that regard because you didn't have that two step process and only had to have enough spare disk space for the extracted file, rather than extracted file + decompressed tar

Tar is seekable in the literal sense, you have to seek around until you find the file you want. You have to look at each header, if it's not the file you want you skip over that one file and read the next header. Because you only know how long that one file is. So you cannot even know where the third file is without having processed the headers of the previous two. It's "read, skip, read, skip, read skip" until you maybe find the file you wanted. ZIP otoh has the central directory index where you can look things up much faster.

Zip still > tar in that regard, since you can list archive contents (and extract individual files) without having to go through the entire thing every time, hence zip being suitable as a local file system (à la opendocument or ooxml).

Zip is terrible as a local files system, unless you consider read only (randomly, and not many files). Writing/append to a file requires some form of garbage organization/compaction - which requires rewrite of the entire CEN. Unlike the regular files/entries, CEN doesn't have a checksum, so corruption there is pretty bad.

Each file being compressed individually means low compression rates, the deflate overhead might be acceptable for slow mediums (although in that case the memory overhead would be non-trivial).

The massive downsize of zip is that each file is compressed individually, which means very poor compression. Overall the compression rate is significantly lower, same with an attempt to decompress it all. It's only useful for random reads of few select files.

Tar -> read, skip, read, till you find what you need, compression doesn't change much, as most stream compression algorithms have a flush method.

> The massive downsize...

I expect you meant "downside", because when talking about file compression, a "massive downsize" sounds like a good thing :-)

indeed, I suck at typing

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