• Michael Shigorin's avatar
    stage2: tunable squashfs compression · fe58c46e
    Michael Shigorin authored
    The existing implementation would handle kernel differences
    just fine but a bit too automatically: if it sees xz support,
    that's what will end up being used (and if there's -Xbcj binary
    compression filter available for the target platform, it will
    be applied unequivocally either).
    
    It's perfectly suitabe for getting fine-tuned release images
    but is also a bit too resource-consuming while developing the
    image configuration which has no business with its compression.
    
    The one and only knob is SQUASHFS (see doc/variables.txt);
    to give an idea of the differences, here are some numbers
    for a mostly-binary (43% as per 99-elf-stats) webkiosk livecd
    and a rather less so (18%) flightgear one on a dual quad-core
    X5570 node (each mksquashfs run used up all the cores):
    
    SQUASHFS | live-webkiosk.iso | live-flightgear.iso
    ---------+-------------------+---------------------
    fast     | 3:30 / 130M       | 5:11 / 852M
    normal * | 3:37 / 100M       | 5:35 / 688M
    tight    | 3:50 / 98M        | 6:47 / 683M
    
    Thus if the knob isn't fiddled with, the defaults will allow
    for a reasonably fast build of a pretty slim image; if one is
    building a release or if a particular image is very sensitive
    being close to the media capacity then just add SQUASHFS=tight
    and see it a percent or two down on size.
    
    Please note that lzo/gzip-compressed images are also quicker
    to uncompress thus further helping with test iterations.
    
    Thanks to led@ and glebfm@ for helpful hints and questions.
    fe58c46e
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