live-initramfs
"live-initramfs is a fork from Casper by Canonical. live-initramfs is used for to build an initramfs suited to boot live systems. [...]
live-initramfs is a hook for the initramfs-tools, used to generate a initramfs capable to boot live systems, such as those created by live-helper(7). This includes the Debian Live isos, netboot tarballs, and usb stick images.
At boot time it will look for a (read-only) media containing a "/live" directory where a root filesystems (often a compressed filesystem image like squashfs) is stored. If found, it will create a writable environ‐ ment, using unionfs, for Debian like systems to boot from."
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