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+.. SPDX-License-Identifier: CC-BY-SA-2.0-UK
+
+Efficiently Fetching Source Files During a Build
+************************************************
+
+The OpenEmbedded build system works with source files located through
+the :term:`SRC_URI` variable. When
+you build something using BitBake, a big part of the operation is
+locating and downloading all the source tarballs. For images,
+downloading all the source for various packages can take a significant
+amount of time.
+
+This section shows you how you can use mirrors to speed up fetching
+source files and how you can pre-fetch files all of which leads to more
+efficient use of resources and time.
+
+Setting up Effective Mirrors
+============================
+
+A good deal that goes into a Yocto Project build is simply downloading
+all of the source tarballs. Maybe you have been working with another
+build system for which you have built up a
+sizable directory of source tarballs. Or, perhaps someone else has such
+a directory for which you have read access. If so, you can save time by
+adding statements to your configuration file so that the build process
+checks local directories first for existing tarballs before checking the
+Internet.
+
+Here is an efficient way to set it up in your ``local.conf`` file::
+
+   SOURCE_MIRROR_URL ?= "file:///home/you/your-download-dir/"
+   INHERIT += "own-mirrors"
+   BB_GENERATE_MIRROR_TARBALLS = "1"
+   # BB_NO_NETWORK = "1"
+
+In the previous example, the
+:term:`BB_GENERATE_MIRROR_TARBALLS`
+variable causes the OpenEmbedded build system to generate tarballs of
+the Git repositories and store them in the
+:term:`DL_DIR` directory. Due to
+performance reasons, generating and storing these tarballs is not the
+build system's default behavior.
+
+You can also use the
+:term:`PREMIRRORS` variable. For
+an example, see the variable's glossary entry in the Yocto Project
+Reference Manual.
+
+Getting Source Files and Suppressing the Build
+==============================================
+
+Another technique you can use to ready yourself for a successive string
+of build operations, is to pre-fetch all the source files without
+actually starting a build. This technique lets you work through any
+download issues and ultimately gathers all the source files into your
+download directory :ref:`structure-build-downloads`,
+which is located with :term:`DL_DIR`.
+
+Use the following BitBake command form to fetch all the necessary
+sources without starting the build::
+
+   $ bitbake target --runall=fetch
+
+This
+variation of the BitBake command guarantees that you have all the
+sources for that BitBake target should you disconnect from the Internet
+and want to do the build later offline.
+