ref-manual: move Initramfs entry from variables to terms

It looks like this was accidentally added to the variable glossary
instead of the terms list, so move it to the latter.

(From yocto-docs rev: c9829dcae18cd2cb5f008081d841ca995b76b8bf)

Signed-off-by: Paul Eggleton <bluelightning@bluelightning.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 42 insertions, 0 deletions

diff --git a/documentation/ref-manual/terms.rst b/documentation/ref-manual/terms.rst
index 92cc723ff6..68313d0294 100644
--- a/documentation/ref-manual/terms.rst
+++ b/documentation/ref-manual/terms.rst
@@ -212,6 +212,48 @@ 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