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The btrfs filesystem supports setting file attributes or flags. Note there are
old and new interfaces, with confusing names. The following list should clarify
that:
* *attributes*: ``chattr(1)`` or ``lsattr(1)`` utilities (the ioctls are
FS_IOC_GETFLAGS and FS_IOC_SETFLAGS), due to the ioctl names the attributes
are also called flags
* *xflags*: to distinguish from the previous, it's extended flags, with tunable
bits similar to the attributes but extensible and new bits will be added in
the future (the ioctls are FS_IOC_FSGETXATTR and FS_IOC_FSSETXATTR but they
are not related to extended attributes that are also called xattrs), there's
no standard tool to change the bits, there's support in ``xfs_io(8)`` as
command **xfs_io -c chattr**
Attributes
^^^^^^^^^^
a
*append only*, new writes are always written at the end of the file
A
*no atime updates*
c
*compress data*, all data written after this attribute is set will be compressed.
Please note that compression is also affected by the mount options or the parent
directory attributes.
When set on a directory, all newly created files will inherit this attribute.
This attribute cannot be set with 'm' at the same time.
C
*no copy-on-write*, file data modifications are done in-place
When set on a directory, all newly created files will inherit this attribute.
.. note::
Due to implementation limitations, this flag can be set/unset only on
empty files.
d
*no dump*, makes sense with 3rd party tools like ``dump(8)``, on BTRFS the
attribute can be set/unset but no other special handling is done
D
*synchronous directory updates*, for more details search ``open(2)`` for *O_SYNC*
and *O_DSYNC*
i
*immutable*, no file data and metadata changes allowed even to the root user as
long as this attribute is set (obviously the exception is unsetting the attribute)
m
*no compression*, permanently turn off compression on the given file. Any
compression mount options will not affect this file. (``chattr`` support added in
1.46.2)
When set on a directory, all newly created files will inherit this attribute.
This attribute cannot be set with *c* at the same time.
S
*synchronous updates*, for more details search ``open(2)`` for *O_SYNC* and
*O_DSYNC*
No other attributes are supported. For the complete list please refer to the
``chattr(1)`` manual page.
XFLAGS
^^^^^^
There's overlap of letters assigned to the bits with the attributes, this list
refers to what ``xfs_io(8)`` provides:
i
*immutable*, same as the attribute
a
*append only*, same as the attribute
s
*synchronous updates*, same as the attribute *S*
A
*no atime updates*, same as the attribute
d
*no dump*, same as the attribute
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