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ref-manual: update overlayfs class

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Describe how to use a generated helper service to set up a proper
systemd dependency chain.

(From yocto-docs rev: c8c1f7e852830dae4468da70e1ac732d7bbce06f)

Signed-off-by: Vyacheslav Yurkov <uvv.mail@gmail.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
This commit is contained in:
Vyacheslav Yurkov
2021-11-02 14:56:14 +01:00
committed by Richard Purdie
parent fdcbca09f8
commit 683a7db46d
1 changed files with 15 additions and 1 deletions
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documentation/ref-manual/classes.rst
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@@ -1742,7 +1742,8 @@ Then you can specify writable directories on a recipe basis (e.g. in my-applicat
To support several mount points you can use a different variable flag. Assuming we
want to have a writable location on the file system, but do not need that the data
survives a reboot, then we could have a ``mnt-overlay.mount`` unit for a ``tmpfs`` file system.
survives a reboot, then we could have a ``mnt-overlay.mount`` unit for a ``tmpfs``
file system.
In your machine configuration::
@@ -1752,6 +1753,19 @@ and then in your recipe::
OVERLAYFS_WRITABLE_PATHS[mnt-overlay] = "/usr/share/another-application"
On a practical note, your application recipe might require multiple
overlays to be mounted before running to avoid writing to the underlying
file system (which can be forbidden in case of read-only file system)
To achieve that :ref:`overlayfs <ref-classes-overlayfs>` provides a ``systemd``
helper service for mounting overlays. This helper service is named
``${PN}-overlays.service`` and can be depended on in your application recipe
(named ``application`` in the following example) ``systemd`` unit by adding
to the unit the following::
[Unit]
After=application-overlays.service
Requires=application-overlays.service
.. note::
The class does not support the ``/etc`` directory itself, because ``systemd`` depends on it.