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ref-manual: add overlayfs class

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(From yocto-docs rev: 5bd2f3c0bbf4178e381aec2b7de57ef8289c2271)

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
2021-08-22 22:14:54 +02:00
committed by Richard Purdie
parent 70059b37c7
commit 62b13474bf
1 changed files with 49 additions and 0 deletions
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@@ -1710,6 +1710,55 @@ one such example. However, being aware of this class can reduce the
proliferation of different versions of similar classes across multiple
layers.
.. _ref-classes-overlayfs:
``overlayfs.bbclass``
=======================
It's often desired in Embedded System design to have a read-only rootfs.
But a lot of different applications might want to have read-write access to
some parts of a filesystem. It can be especially useful when your update mechanism
overwrites the whole rootfs, but you may want your application data to be preserved
between updates. The :ref:`overlayfs <ref-classes-overlayfs>` class provides a way
to achieve that by means of ``overlayfs`` and at the same time keeping the base
rootfs read-only.
To use this class, set a mount point for a partition ``overlayfs`` is going to use as upper
layer in your machine configuration. The underlying file system can be anything that
is supported by ``overlayfs``. This has to be done in your machine configuration::
OVERLAYFS_MOUNT_POINT[data] = "/data"
.. note::
* QA checks fail to catch file existence if you redefine this variable in your recipe!
* Only the existence of the systemd mount unit file is checked, not its contents.
* To get more details on ``overlayfs``, its internals and supported operations, please refer
to the official documentation of the `Linux kernel <https://www.kernel.org/doc/html/latest/filesystems/overlayfs.html>`_.
The class assumes you have a ``data.mount`` systemd unit defined elsewhere in your BSP
(e.g. in ``systemd-machine-units`` recipe) and it's installed into the image.
Then you can specify writable directories on a recipe basis (e.g. in my-application.bb)::
OVERLAYFS_WRITABLE_PATHS[data] = "/usr/share/my-custom-application"
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.
In your machine configuration::
OVERLAYFS_MOUNT_POINT[mnt-overlay] = "/mnt/overlay"
and then in your recipe::
OVERLAYFS_WRITABLE_PATHS[mnt-overlay] = "/usr/share/another-application"
.. note::
The class does not support the ``/etc`` directory itself, because ``systemd`` depends on it.
.. _ref-classes-own-mirrors:
``own-mirrors.bbclass``