By the looks of it romfs is the clear winner for space savings. 180483 initrd.cram.gz 161209 initrd.iso.gz 161023 initrd.ext2.gz 160621 initrd.minix.gz 159622 initrd.rom.gz romfs adds about 4kB to the kernel when built in cramfs adds ? the initrd filesystem will have to be builtin, which will be smaller than module size, so these are only usefull for relative comparison to other modules. 44599 ext2.o 30358 minix.o 24729 isofs.o