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When I use Unetbootin to put a Linux ISO on a USB drive, it proceeds quite quickly until it gets to filesystem.squashfs, which takes longer to process than absolutely everything else combined.

Is this writing a new filesystem to the USB, or is it copying some huge filesystem-dependent file? If so, is there a way to only do it once in the event that I will be trying many distros and want to speed this step up?

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    That file is "the distro"... you can't do it "only once" for several distros... If you were curious to open the iso and see the content you'd see that squashfs takes 90% of the size so it's normal it takes a lot of time to write it. Jun 3, 2016 at 13:06

2 Answers 2

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Most major distributions use squashfs to hold their live CD. squashfs is intended to be used for read-only filesystems, which is exactly what a live CD is.

Decompressing filesystem.squashfs takes longer than any other process because filesystem.squashfs contains the entire system.

For more information, look at the wikipedia page: https://en.wikipedia.org/wiki/SquashFS

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It's probably copying files after mounting the squashfs which are files or directories that need editing. Example some files in /usr or /var will be copied over to ram during the boot process, unless the liveCD implemented an overlay filesystem like auFS or overlayfs with the former being used in the Gentoo Linux liveCD which doesn't required copying directories from its squashfs image.

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  • @user1717828's question concerns the process of creating the bootable USB drive, i.e. copying the distro's files onto the USB stick. Copying the files into RAM (with the purpose of keeping them there) as you describe happens when the USB drive is booted from.
    – n.st
    Jun 3, 2016 at 15:53

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