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Clarify '--one-file-system' for btrfs (1.1 backport) #5478

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Merged
ThomasWaldmann merged 1 commit into from
Nov 3, 2020
Merged

Clarify '--one-file-system' for btrfs (1.1 backport) #5478

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14 changes: 13 additions & 1 deletion src/borg/archiver.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3321,6 +3321,18 @@ def define_archive_filters_group(subparser, *, sort_by=True, first_last=True):
and not include any other contents of the containing folder, this can be enabled
through using the ``--keep-exclude-tags`` option.

The ``-x`` or ``--one-file-system`` option excludes directories, that are mountpoints (and everything in them).
It detects mountpoints by comparing the device number from the output of ``stat()`` of the directory and its
parent directory. Specifically, it excludes directories for which ``stat()`` reports a device number different
from the device number of their parent. Be aware that in Linux (and possibly elsewhere) there are directories
with device number different from their parent, which the kernel does not consider a mountpoint and also the
other way around. Examples are bind mounts (possibly same device number, but always a mountpoint) and ALL
subvolumes of a btrfs (different device number from parent but not necessarily a mountpoint). Therefore when
using ``--one-file-system``, one should make doubly sure that the backup works as intended especially when using
btrfs. This is even more important, if the btrfs layout was created by someone else, e.g. a distribution
installer.


.. _list_item_flags:

Item flags
Expand Down Expand Up @@ -3415,7 +3427,7 @@ def define_archive_filters_group(subparser, *, sort_by=True, first_last=True):

fs_group = subparser.add_argument_group('Filesystem options')
fs_group.add_argument('-x', '--one-file-system', dest='one_file_system', action='store_true',
help='stay in the same file system and do not store mount points of other file systems')
help='stay in the same file system and do not store mount points of other file systems. This might behave different from your expectations, see the docs.')
fs_group.add_argument('--numeric-owner', dest='numeric_owner', action='store_true',
help='only store numeric user and group identifiers')
fs_group.add_argument('--noatime', dest='noatime', action='store_true',
Expand Down