Signed-off-by: Vyacheslav Yurkov <uvv.m...@gmail.com> --- documentation/ref-manual/classes.rst | 46 ++++++++++++++++++++++++++++ 1 file changed, 46 insertions(+)
diff --git a/documentation/ref-manual/classes.rst b/documentation/ref-manual/classes.rst index 09878c480f..6d80e7dc4f 100644 --- a/documentation/ref-manual/classes.rst +++ b/documentation/ref-manual/classes.rst @@ -1708,6 +1708,52 @@ 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 a read-write access to +some parts of a filesystem. It can be especially useful when your update mechanism +overwrites the whole rootfs, but you want your application data to be preserved +between updates. The ``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. 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 check fails to catch file existence if you redefine this variable in your recipe! + +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 to the image. + +Then you can specify writable directories on a recipe basis:: + + OVERLAYFS_WRITABLE_PATHS[data] = "/usr/share/my-custom-application" + +To support several mount points you can use a different variable flag. Assume we +want to have a writable location on the file system, but not interested where the data +survive 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 /etc directory itself, because systemd depends on it + .. _ref-classes-own-mirrors: ``own-mirrors.bbclass`` -- 2.28.0
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