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btrfs-progs/Documentation/btrfs-ioctl.asciidoc
David Sterba 4a9b83c7a4 btrfs-progs: docs: add more general ioctl description 
Add general paragraphs and establish per-ioctl argument description
formatting, using [horizontal] where the struct member and the
description are on the same line, unlike ordinary ::.

Signed-off-by: David Sterba <dsterba@suse.com>
2021-03-24 22:20:19 +01:00

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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
-------------------------------
[verse]
struct btrfs_ioctl_vol_args {
__s64 fd;
char name[BTRFS_PATH_NAME_MAX + 1];
};
[verse]
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];
};
char name[BTRFS_SUBVOL_NAME_MAX + 1];
};
[verse]
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:
----------------
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_TRANS_START
BTRFS_IOC_TRANS_END
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.
[horizontal]
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.
[horizontal]
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 btrfs wiki http://btrfs.wiki.kernel.org for
further details.
SEE ALSO
--------
`ioctl`(2)
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