SquashFS
Squashfs is a compressed read-only file system for Linux. Squashfs compresses files, inodes and directories, and supports block sizes from 4 KiB up to 1 MiB for greater compression. Several compression algorithms are supported.
The Live CD versions of various Linux distributions mount SquashFS as the root filesystem and mostly use OverlayFS to provide a writable layer over SquashFS.
In Alpine Linux Diskless Mode, the read-only squashfs image named modloop serves as the lower layer. The modloop contains kernel modules and essential system files. The modloop gets mounted as a loop device at /.modloop. The unpacked Apkovl content is loaded and mounted as its root filesystem into tmpfs, thus making it fully writable in RAM.