btrfs-progs/Documentation/btrfs-ioctl.rst

btrfs-ioctl(3)
==============

NAME
----

btrfs-ioctl - documentation for the ioctl interface to btrfs

DESCRIPTION
-----------

The ioctl() system call is a way how to request custom actions performed on a
filesystem beyond the standard interfaces (like syscalls).  An ioctl is
specified by a number and an associated data structure that implement a
feature, usually not available in other filesystems. The number of ioctls grows
over time and in some cases get promoted to a VFS-level ioctl once other
filesystems adopt the functionality. Backward compatibility is maintained
and a formerly private ioctl number could become available on the VFS level.


DATA STRUCTURES AND DEFINITIONS
-------------------------------

.. code-block::

   struct btrfs_ioctl_vol_args {
           __s64 fd;
           char name[BTRFS_PATH_NAME_MAX + 1];
   };

.. code-block::

   struct btrfs_ioctl_vol_args_v2 {
           __s64 fd;
           __u64 transid;
           __u64 flags;
           union {
                   struct {
                           __u64 size;
                           struct btrfs_qgroup_inherit __user *qgroup_inherit;
                   };
                   __u64 unused[4];
           };
           union {
               char name[BTRFS_SUBVOL_NAME_MAX + 1];
               __u64 devid;
               __u64 subvolid;
            };
   };

.. code-block::

   BTRFS_SUBVOL_NAME_MAX = 4039
   BTRFS_PATH_NAME_MAX = 4087

OVERVIEW
--------

The ioctls are defined by a number and associated with a data structure that
contains further information. All ioctls use file descriptor (fd) as a reference
point, it could be the filesystem or a directory inside the filesystem.

An ioctl can be used in the following schematic way:

.. code-block::

   struct btrfs_ioctl_args args;

   memset(&args, 0, sizeof(args));
   args.key = value;
   ret = ioctl(fd, BTRFS_IOC_NUMBER, &args);

The 'fd' is the entry point to the filesystem and for most ioctls it does not
matter which file or directory is that. Where it matters it's explicitly
mentioned. The 'args' is the associated data structure for the request. It's
strongly recommended to initialize the whole structure to zeros as this is
future-proof when the ioctl gets further extensions. Not doing that could lead
to mismatch of old userspace and new kernel versions, or vice versa.
The 'BTRFS_IOC_NUMBER' is says which operation should be done on the given
arguments. Some ioctls take a specific data structure, some of them share a
common one, no argument structure ioctls exist too.

The library 'libbtrfsutil' wraps a few ioctls for convenience. Using raw ioctls
is not discouraged but may be cumbersome though it does not need additional
library dependency. Backward compatibility is guaranteed and incompatible
changes usually lead to a new version of the ioctl. Enhancements of existing
ioctls can happen and depend on additional flags to be set. Zeroed unused
space is commonly understood as a mechanism to communicate the compatibility
between kernel and userspace and thus zeroing is really important. In exceptional
cases this is not enough and further flags need to be passed to distinguish
between zero as implicit unused initialization and a valid zero value. Such
cases are documented.

LIST OF IOCTLS
--------------

* BTRFS_IOC_SUBVOL_CREATE -- (obsolete) create a subvolume
* BTRFS_IOC_SNAP_CREATE
* BTRFS_IOC_DEFRAG
* BTRFS_IOC_RESIZE
* BTRFS_IOC_SCAN_DEV
* BTRFS_IOC_SYNC
* BTRFS_IOC_CLONE
* BTRFS_IOC_ADD_DEV
* BTRFS_IOC_RM_DEV
* BTRFS_IOC_BALANCE
* BTRFS_IOC_CLONE_RANGE
* BTRFS_IOC_SUBVOL_CREATE
* BTRFS_IOC_SNAP_DESTROY
* BTRFS_IOC_DEFRAG_RANGE
* BTRFS_IOC_TREE_SEARCH
* BTRFS_IOC_TREE_SEARCH_V2
* BTRFS_IOC_INO_LOOKUP
* BTRFS_IOC_DEFAULT_SUBVOL
* BTRFS_IOC_SPACE_INFO
* BTRFS_IOC_START_SYNC
* BTRFS_IOC_WAIT_SYNC
* BTRFS_IOC_SNAP_CREATE_V2
* BTRFS_IOC_SUBVOL_CREATE_V2 -- create a subvolume
* BTRFS_IOC_SUBVOL_GETFLAGS
* BTRFS_IOC_SUBVOL_SETFLAGS
* BTRFS_IOC_SCRUB
* BTRFS_IOC_SCRUB_CANCEL
* BTRFS_IOC_SCRUB_PROGRESS
* BTRFS_IOC_DEV_INFO
* BTRFS_IOC_FS_INFO
* BTRFS_IOC_BALANCE_V2
* BTRFS_IOC_BALANCE_CTL
* BTRFS_IOC_BALANCE_PROGRESS
* BTRFS_IOC_INO_PATHS
* BTRFS_IOC_LOGICAL_INO
* BTRFS_IOC_SET_RECEIVED_SUBVOL
* BTRFS_IOC_SEND
* BTRFS_IOC_DEVICES_READY
* BTRFS_IOC_QUOTA_CTL
* BTRFS_IOC_QGROUP_ASSIGN
* BTRFS_IOC_QGROUP_CREATE
* BTRFS_IOC_QGROUP_LIMIT
* BTRFS_IOC_QUOTA_RESCAN
* BTRFS_IOC_QUOTA_RESCAN_STATUS
* BTRFS_IOC_QUOTA_RESCAN_WAIT
* BTRFS_IOC_GET_FSLABEL
* BTRFS_IOC_SET_FSLABEL
* BTRFS_IOC_GET_DEV_STATS
* BTRFS_IOC_DEV_REPLACE
* BTRFS_IOC_FILE_EXTENT_SAME
* BTRFS_IOC_GET_FEATURES
* BTRFS_IOC_SET_FEATURES
* BTRFS_IOC_GET_SUPPORTED_FEATURES

DETAILED DESCRIPTION
--------------------

BTRFS_IOC_SUBVOL_CREATE
~~~~~~~~~~~~~~~~~~~~~~~

.. note::
   obsoleted by BTRFS_IOC_SUBVOL_CREATE_V2

*(since: 3.0, obsoleted: 4.0)* Create a subvolume.

ioctl fd
    file descriptor of the parent directory of the new subvolume
argument type
    struct btrfs_ioctl_vol_args
fd
    ignored
name
    name of the subvolume, although the buffer can be almost 4k, the file
    size is limited by Linux VFS to 255 characters and must not contain a slash
    ('/')

BTRFS_IOC_SUBVOL_CREATE_V2
~~~~~~~~~~~~~~~~~~~~~~~~~~

.. note::
   obsoletes BTRFS_IOC_SUBVOL_CREATE

*(since: 3.6)* Create a subvolume, qgroup inheritance can be specified.

ioctl fd
    file descriptor of the parent directory of the new subvolume
argument type
    struct btrfs_ioctl_vol_args_v2
fd
    ignored
transid
    ignored
flags
    ignored
size
    ...
qgroup_inherit
    ...
name
    name of the subvolume, although the buffer can be almost 4k, the file size
    is limited by Linux VFS to 255 characters and must not contain a slash ('/')
devid
    ...

AVAILABILITY
------------

**btrfs** is part of btrfs-progs.  Please refer to the documentation at
https://btrfs.readthedocs.io or wiki http://btrfs.wiki.kernel.org for further
information.

SEE ALSO
--------
ioctl(2)