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Add documentation of DIB_IMAGE_CACHE variable

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It is an established convention to use DIB_IMAGE_CACHE in elements to
refer to the cache location. This convention has not been documented
in the README, unlike other element development practices. This change
adds documentation for DIB_IMAGE_CACHE both from an element user and
developer perspective.

This change was prompted by I76030654e7dfc7b7d4dfa87e082c14e7cee479b9

Change-Id: I6fcc2409450cdb45e5afcb292908fbaee1af258a
This commit is contained in:
stephane 2015-01-23 14:50:03 -08:00 committed by vigneshvar
parent bbeafa80bf
commit fe7823a249
1 changed files with 26 additions and 12 deletions
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38
README.rst
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@ -97,16 +97,20 @@ Caches and offline mode
=======================
Since retrieving and transforming operating system image files, git
repositories, Python or Ruby packages, and so on can be a significant overhead,
we cache many of the inputs to the build process in ~/.cache/image-create/. The
writing an element documention describes the interface within
disk-image-builder for caching. When invoking disk-image-builder the --offline
option will instruct disk-image-builder to not refresh cached resources.
repositories, Python or Ruby packages, and so on can be a significant
overhead, we cache many of the inputs to the build process. The cache
location is read from DIB\_IMAGE\_CACHE. The *Writing an element*
section of this document describes the interface within
disk-image-builder for caching. When invoking disk-image-builder, the
``--offline`` option will instruct disk-image-builder to not refresh
cached resources.
Note that we don't maintain operating system package caches, instead depending
on your local infrastructure (e.g. Squid cache, or an APT or Yum proxy) to
facilitate caching of that layer, so you need to arrange independently for
offline mode.
Note that we don't maintain operating system package caches, instead
depending on your local infrastructure (e.g. Squid cache, or an APT or
Yum proxy) to facilitate caching of that layer, so you need to arrange
independently for offline mode. For more information about setting up
a squid proxy, consult the [TripleO
documentation](http://docs.openstack.org/developer/tripleo-incubator/devtest_setup.html#f3).
Base images
-----------
@ -239,7 +243,7 @@ named after the element itself. Elements *should* have a README.md in the root
of the element directory describing what it is for.
Writing an element
-----------------
------------------
Conform to the following conventions:
@ -267,8 +271,15 @@ Conform to the following conventions:
and not remounted into the filesystem image - if the mount point is needed
again, your element will need to remount it at that point.
Phase Subdirectories
^^^^^^^^^^^^^^^^^^^^
* If caching is required, elements should use a location under
$DIB\_IMAGE\_CACHE.
* Elements should allow for remote data to be cached. When
$DIB\_OFFLINE is set, this cached data should be used if
possible. See the *Global image-build variables* section of this
document for more information.
### Phase Subdirectories ###
Make as many of the following subdirectories as you need, depending on what
part of the process you need to customise. The subdirectories are executed in
@ -416,6 +427,9 @@ Global image-build variables
diskimage-builder is building a disk image. This works only for ext
filesystems.
* DIB\_IMAGE\_CACHE : path to where cached inputs to the build process
are stored. Defaults to ~/.cache/image_create.
Structure of an element
-----------------------