From 2be874d5b401d271e9ae5eef46bbb0f64546f86b Mon Sep 17 00:00:00 2001 From: Michael Opdenacker Date: Sun, 17 Sep 2023 16:38:11 +0200 Subject: ref-manual: add Initramfs term Backported from the master branch (From yocto-docs rev: f5ecf1f407585617d258b6afc706d43fdbb33547) Signed-off-by: Michael Opdenacker Signed-off-by: Steve Sakoman --- documentation/ref-manual/terms.rst | 42 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 42 insertions(+) diff --git a/documentation/ref-manual/terms.rst b/documentation/ref-manual/terms.rst index ba13f77f19..a7ae8e1801 100644 --- a/documentation/ref-manual/terms.rst +++ b/documentation/ref-manual/terms.rst @@ -192,6 +192,48 @@ universal, the list includes them just in case: of the supported image types that the Yocto Project provides, see the ":ref:`ref-manual/images:Images`" chapter. + :term:`Initramfs` + An Initial RAM Filesystem (:term:`Initramfs`) is an optionally compressed + :wikipedia:`cpio ` archive which is extracted + by the Linux kernel into RAM in a special :wikipedia:`tmpfs ` + instance, used as the initial root filesystem. + + This is a replacement for the legacy init RAM disk ("initrd") + technique, booting on an emulated block device in RAM, but being less + efficient because of the overhead of going through a filesystem and + having to duplicate accessed file contents in the file cache in RAM, + as for any block device. + + .. note:: + + As far as bootloaders are concerned, :term:`Initramfs` and "initrd" + images are still copied to RAM in the same way. That's why most + most bootloaders refer to :term:`Initramfs` images as "initrd" + or "init RAM disk". + + This kind of mechanism is typically used for two reasons: + + - For booting the same kernel binary on multiple systems requiring + different device drivers. The :term:`Initramfs` image is then customized + for each type of system, to include the specific kernel modules + necessary to access the final root filesystem. This technique + is used on all GNU / Linux distributions for desktops and servers. + + - For booting faster. As the root filesystem is extracted into RAM, + accessing the first user-space applications is very fast, compared + to having to initialize a block device, to access multiple blocks + from it, and to go through a filesystem having its own overhead. + For example, this allows to display a splashscreen very early, + and to later take care of mounting the final root filesystem and + loading less time-critical kernel drivers. + + This cpio archive can either be loaded to RAM by the bootloader, + or be included in the kernel binary. + + For information on creating and using an :term:`Initramfs`, see the + ":ref:`dev-manual/building:building an initial ram filesystem (Initramfs) image`" + section in the Yocto Project Development Tasks Manual. + :term:`Layer` A collection of related recipes. Layers allow you to consolidate related metadata to customize your build. Layers also isolate information used -- cgit v1.2.3-54-g00ecf