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ref-manual: add Initramfs term

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(From yocto-docs rev: f5ecf1f407585617d258b6afc706d43fdbb33547)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>
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Michael Opdenacker
2023-09-17 16:38:11 +02:00
committed by Steve Sakoman
parent 5ea10fc05b
commit 2be874d5b4
1 changed files with 42 additions and 0 deletions
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documentation/ref-manual/terms.rst
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@@ -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 <Cpio>` archive which is extracted
by the Linux kernel into RAM in a special :wikipedia:`tmpfs <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