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ref-manual: First draft of the "Image Generation" section 

Fixes [YOCTO #2808]

Added the first draft of the expanded explanation for generating
an image.

(From yocto-docs rev: 72e4db25d89cad62e1cd9ee1d638af374ec1bfc0)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
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Scott Rifenbark 2013-09-13 13:57:28 -07:00 committed by Richard Purdie
parent da7a072aa0
commit b577e74096
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documentation/ref-manual/closer-look.xml
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@ -830,6 +830,97 @@
</note>
</para>
</section>
<section id='image-generation-dev-environment'>
<title>Image Generation</title>
<para>
Once packages are split and stored in the Package Feeds area,
the OpenEmbedded build system uses BitBake to generate the
root filesystem image:
<imagedata fileref="figures/image-generation.png" align="center" width="6in" depth="7in" />
</para>
<para>
The image generation process consists of several stages and
depends on many variables.
The <filename>do_rootfs</filename> uses these key variables
to help create the list of packages to actually install:
<itemizedlist>
<listitem><para><link linkend='var-IMAGE_INSTALL'><filename>IMAGE_INSTALL</filename></link>:
Lists out the base set of packages to install from
the Package Feeds area.</para></listitem>
<listitem><para><link linkend='var-IMAGE_FEATURES'><filename>IMAGE_FEATURES</filename></link>:
Specifies features to include in the image.
These features map to additional packages for
installation.</para></listitem>
<listitem><para><link linkend='var-PACKAGE_CLASSES'><filename>PACKAGE_CLASSES</filename></link>:
Specifies the package manager to use and consequently
helps determine where to locate packages within the
Package Feeds area.</para></listitem>
<listitem><para><link linkend='var-PACKAGE_EXCLUDE'><filename>PACKAGE_EXCLUDE</filename></link>:
Specifies packages that should not be installed.
</para></listitem>
</itemizedlist>
</para>
<para>
In addition to the previously mentioned variables, the image
generation process uses
<link linkend='var-IMAGE_LINGUAS'><filename>IMAGE_LINGUAS</filename></link>
to determine what language for which packages are being
installed.
</para>
<para>
Part of the image generation process includes compressing the
root filesystem image.
Compression is accomplished through several optimization
routines designed to reduce the overall size of the image.
</para>
<para>
The process runs as many post installation scripts as possible.
Any scripts that cannot be run are run when the system is
first booted.
If you are using a read-only root filesystem, all the post
installation scripts are run during the package installation
phase since the root filesystem cannot be written into.
</para>
<para>
During the Prelink phase, optimization processes are run across
the image.
These processes include <filename>mklibs</filename> and
<filename>prelink</filename>.
The <filename>mklibs</filename> optimizes the size of the
libraries.
The <filename>prelink</filename> process optimizes the dynamic
linking of shared libraries to reduce start up time of the
executables.
See the
<link linkend='var-IMAGE_POSTPROCESS_COMMAND'><filename>IMAGE_POSTPROCESS_COMMAND</filename></link>
and
<link linkend='var-ROOTFS_POSTPROCESS_COMMAND'><filename>ROOTFS_POSTPROCESS_COMMAND</filename></link>
variables for additional information.
</para>
<para>
After the root filesystem has been constructed, the image
generation process turns everything into an image file or
a set of image files.
we need to turn into an image file or set of image files.
The formats used for the root filesystem depend on the
<link linkend='var-IMAGE_FSTYPES'><filename>IMAGE_FSTYPES</filename></link>
variable.
</para>
<note>
The entire image generation process is run under Pseudo.
Running under Pseudo ensures that the files in the root
filesystem have correct ownership.
</note>
</section>
</section>
<section id="package-feeds-dev-environment">