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So there are different compression methods in 7zip. Which method is best suited for what task?

For example: One difference between LZMA and LZMA2 is I can choose all my cpu cores, whereas in LZMA 2 cores is the max.

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benchmarks of compression algorithms : compressionratings.com/sort.cgi?rating_sum.brief+4n – akira Jun 3 '12 at 8:19
up vote 13 down vote accepted

Use LZMA 2 unless you are looking to extract the archive on a system that cannot deal with LZMA 2 archives.

Generally speaking most modern compression algorithms give roughly the same compression, and with regard to the number of cores that you can use at once, it is up to you to decide how many you want to use. Generally speaking (unless you are creating large archives) there is no reason to need more than one though. In addition, with multiple cores doing the compression, the bottleneck may become the hard drive.

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side note: the better compression results ("ultra") are mostly bought by cpu and (important) ram. lzma2-ultra-dictsize(64mb)-4threads will eat 2+gb of ram, bzip2-ultra-dictsize(900kb) will eat 69mb of ram. – akira Jun 3 '12 at 8:17

Lzma2 is faster when using 4 or more cores and it gives better compression. This document explains it all.

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Good document, thanks for this. – Horst Walter Jun 23 '12 at 0:16
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I'm not saying the document is unusable, but it has a range of problems. Prominently, the author does not even specify what kind of data is being compressed (text? pictures? encrypted data?), does not use relative sizes where applicable and does not seem to understand solid archives at all. – mafu May 16 at 13:24

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