linux支持的内存根文件系统

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linux支持两种内存根文件系统：ramdisk和initramfs。

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Document/early-userspace/README

128 The kernel has currently 3 ways to mount the root filesystem:
129
130 a) all required device and filesystem drivers compiled into the kernel, no
131    initrd. init/main.c:init() will call prepare_namespace() to mount the
132    final root filesystem, based on the root= option and optional init= to run
133    some other init binary than listed at the end of init/main.c:init().
134
135 b) some device and filesystem drivers built as modules and stored in an
136    initrd. The initrd must contain a binary '/linuxrc' which is supposed to
137    load these driver modules. It is also possible to mount the final root
138    filesystem via linuxrc and use the pivot_root syscall. The initrd is
139    mounted and executed via prepare_namespace().
140
141 c) using initramfs. The call to prepare_namespace() must be skipped.
142    This means that a binary must do all the work. Said binary can be stored
143    into initramfs either via modifying usr/gen_init_cpio.c or via the new
144    initrd format, an cpio archive. It must be called "/init". This binary
145    is responsible to do all the things prepare_namespace() would do.
146
147    To maintain backwards compatibility, the /init binary will only run if it
148    comes via an initramfs cpio archive. If this is not the case,
149    init/main.c:init() will run prepare_namespace() to mount the final root
150    and exec one of the predefined init binaries.
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