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btrfs-progs: docs: clarify inode numbers

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Update wording and add an example.

[ci skip]

Issue: #729
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
David Sterba 2024-02-20 10:25:29 +01:00
parent d03594b031
commit 93cdc1c2fc
1 changed files with 22 additions and 7 deletions
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29
Documentation/ch-subvolume-intro.rst
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@ -1,7 +1,8 @@
A BTRFS subvolume is a part of filesystem with its own independent
file/directory hierarchy and inode number namespace. Subvolumes can share file
extents. A snapshot is also subvolume, but with a given initial content of the
original subvolume. A subvolume has always inode number 256.
original subvolume. A subvolume has always inode number 256 (see more in
:docref:`Inode numbers <Subvolumes:subvolume-inode-numbers>`).
.. note::
A subvolume in BTRFS is not like an LVM logical volume, which is block-level
@ -15,8 +16,8 @@ changed.
A subvolume in BTRFS can be accessed in two ways:
* like any other directory that is accessible to the user
* like a separately mounted filesystem (options *subvol* or *subvolid*)
- like any other directory that is accessible to the user
- like a separately mounted filesystem (options *subvol* or *subvolid*)
In the latter case the parent directory is not visible and accessible. This is
similar to a bind mount, and in fact the subvolume mount does exactly that.
@ -143,13 +144,27 @@ the 4th column:
27 21 0:19 /subv1 /mnt rw,relatime - btrfs /dev/sda rw,space_cache
^^^^^^
.. duplabel:: subvolume-inode-numbers
Inode numbers
-------------
A proper subvolume has always inode number 256. If a subvolume is nested and
then a snapshot is taken, then the cloned directory entry representing the
subvolume becomes empty and the inode has number 2. All other files and
directories in the target snapshot preserve their original inode numbers.
A directory representing a subvolume has always inode number 256 (sometimes
also called a root of the subvolume):
.. code-block:: none
$ ls -lis
total 0
389111 0 drwxr-xr-x 1 user users 0 Jan 20 12:13 dir
389110 0 -rw-r--r-- 1 user users 0 Jan 20 12:13 file
256 0 drwxr-xr-x 1 user users 0 Jan 20 12:13 snap1
256 0 drwxr-xr-x 1 user users 0 Jan 20 12:13 subv1
If a subvolume is nested and then a snapshot is taken, then the cloned
directory entry representing the subvolume becomes empty and the inode has
number 2. All other files and directories in the target snapshot preserve their
original inode numbers.
.. note::
Inode number is not a filesystem-wide unique identifier, some applications