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documentation/poky-ref-manual/ref-variables.xml: image size glossary 

I added three new glossary entries for how the build process
figures out the image size.  Entries are for IMAGE_ROOTFS_SIZE,
IMAGE_OVERHEAD_FACTOR, and IMAGE_ROOTFS_EXTRA_SPACE.
I still need to create a usage section for this topic.
Likely this will be in the development manual in a new chapter
I am thinking about.

Fixes [YOCTO #1110] partially.

(From yocto-docs rev: ec6b9d77d10a95f0a2d460dca3cf8eab5dfbf0c0)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
This commit is contained in:
Scott Rifenbark 2012-01-05 09:52:18 -06:00 committed by Richard Purdie
parent 01d5729e13
commit 03df4ce093
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@ -510,6 +510,87 @@
</glossdef>
</glossentry>
<glossentry id='var-IMAGE_OVERHEAD_FACTOR'><glossterm>IMAGE_OVERHEAD_FACTOR</glossterm>
<glossdef>
<para>
Defines a multiplier that the build system might apply to the initial image
size to create free disk space in the image as overhead.
By default, the build process uses a multiplier of 1.3 for this variable.
This default value results in 30% free disk space added to the image when this
method is used to determine the final generated image size.
See <filename><link linkend='var-IMAGE_ROOTFS_SIZE'>IMAGE_ROOTFS_SIZE</link></filename>
for information on how the build system determines the overall image size.
</para>
<para>
The default 30% free disk space typically gives the image enough room to boot
and allows for basic post installs while still leaving a small amount of
free disk space.
If 30% free space is inadequate, you can increase the default value.
For example, the following setting gives you 50% free space added to the image:
<literallayout class='monospaced'>
IMAGE_OVERHEAD_FACTOR = "1.5"
</literallayout>
</para>
<para>
Alternatively, you can ensure a specific amount of free disk space is added
to the image by using
<filename><link linkend='var-IMAGE_ROOTFS_EXTRA_SPACE'>IMAGE_ROOTFS_EXTRA_SPACE</link></filename>
the variable.
</para>
</glossdef>
</glossentry>
<glossentry id='var-IMAGE_ROOTFS_EXTRA_SPACE'><glossterm>IMAGE_ROOTFS_EXTRA_SPACE</glossterm>
<glossdef>
<para>
Defines additional free disk space created in the image in Kbytes.
By default, this variable is set to "0".
This free disk space is added to the image after the build system determines
the image size as described in
<filename><link linkend='var-IMAGE_ROOTFS_SIZE'>IMAGE_ROOTFS_SIZE</link></filename>.
</para>
<para>
This variable is particularly useful when you want to ensure that a
specific amount of free disk space is available on a device after an image
is installed and running.
For example, to be sure 5 Gbytes of free disk space is available, set the
variable as follows:
<literallayout class='monospaced'>
IMAGE_ROOTFS_EXTRA_SPACE = "5242880"
</literallayout>
</para>
</glossdef>
</glossentry>
<glossentry id='var-IMAGE_ROOTFS_SIZE'><glossterm>IMAGE_ROOTFS_SIZE</glossterm>
<glossdef>
<para>
Defines the size in Kbytes for the generated image.
The Yocto Project build system determines the final size for the generated
image using an algorithm that takes into account the initial disk space used
for the generated image, a requested size for the image, and requested
additional free disk space to be added to the image.
Programatically, the build system determines the final size of the
generated image as follows:
<literallayout class='monospaced'>
if (du * overhead) &lt; IMAGE_ROOTFS_SIZE:
IMAGE_ROOTFS_SIZE = IMAGE_ROOTFS_SIZE + xspace
else:
IMAGE_ROOTFS_SIZE = (du * overhead) + xspace
</literallayout>
In the above example, <filename>overhead</filename> is defined by the
<filename><link linkend='var-IMAGE_OVERHEAD_FACTOR'>IMAGE_OVERHEAD_FACTOR</link></filename>
variable, <filename>xspace</filename> is defined by the
<filename><link linkend='var-IMAGE_ROOTFS_EXTRA_SPACE'>IMAGE_ROOTFS_EXTRA_SPACE</link></filename>
variable, and <filename>du</filename> is the results of the disk usage command
on the initially generated image.
</para>
</glossdef>
</glossentry>
<glossentry id='var-INC_PR'><glossterm>INC_PR</glossterm>
<glossdef>
<para>Defines the Package revision.