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btrfs-progs: docs: add example for nested subvolumes

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Make it more visible what the result of snapshotted subvolume is. This
partially duplicates the other section.

[ci skip]

Issue: #644
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
David Sterba 2024-07-30 18:30:30 +02:00
parent fca2fe3316
commit 4cab3f8c97
1 changed files with 22 additions and 1 deletions
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Documentation/ch-subvolume-intro.rst
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@ -86,11 +86,32 @@ descendants of the toplevel one), or nested.
What should be mentioned early is that a snapshotting is not recursive, so a
subvolume or a snapshot is effectively a barrier and no files in the nested
appear in the snapshot. Instead there's a stub subvolume (also sometimes
appear in the snapshot. Instead there's a stub subvolume (also sometimes called
*empty subvolume* with the same name as original subvolume, with inode number
2). This can be used intentionally but could be confusing in case of nested
layouts.
.. code-block:: bash
$ btrfs subvolume create subvol1
$ btrfs subvolume create subvol1/subvol2
$ btrfs subvolume snapshot subvol1 snap1
$ find -ls
121093 0 drwxr-xr-x 1 user users 24 Jul 30 12:34 .
256 0 drwxr-xr-x 1 user users 14 Jul 30 12:34 ./subvol1
256 0 drwxr-xr-x 1 user users 0 Jul 30 12:34 ./subvol1/subvol2
257 0 -rw-r--r-- 1 user users 0 Jul 30 12:34 ./subvol1/subvol2/file
256 0 drwxr-xr-x 1 user users 14 Jul 30 12:34 ./snap1
2 0 drwxr-xr-x 1 user users 0 Jul 30 12:34 ./snap1/subvol2
The numbers in the first colums are inode numbers, 256 is for a regular
subvolume (or snapshot), 2 is the *empty subvolume*. The snapshotted directory
representing *subvol2* does not contain the *file*.
.. note::
The *empty subvolume* will not be sent (:doc:`btrfs-send`) and thus will not be created on
the receive side (:doc:`btrfs-receive`).
Case study: system root layouts
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^