With certain compressors, which level you choose makes a lot of difference (i.e. more than it does with e.g. gzip). Especially memory usage varies a lot between different levels, and could be need to change between files.

Adding another option for compression level would make this flexible, where the level varies from -1 (--fast) to -9 (--best) with the default level being somewhere inbetween and up to the compressor - usually by passing "-1"

I've added a --compression-args argument to specify the compression level: xar --compression-args=9 -cvf bin.xar /bin

This should be recognized by gzip, bzip2, and lzma compression. It should also be extensible enough so that additional tuning parameters can be passed, such as the block size for bzip2, and the memory limit for lzma.