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

struct btrfs_ioctl_vol_args {
        __s64 fd;
        char name[BTRFS_PATH_NAME_MAX + 1];
};
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;
         };
};
struct btrfs_ioctl_get_subvol_info_args {
     /* Id of this subvolume */
     __u64 treeid;

     /* Name of this subvolume, used to get the real name at mount point */
     char name[BTRFS_VOL_NAME_MAX + 1];

     /*
      * Id of the subvolume which contains this subvolume.
      * Zero for top-level subvolume or a deleted subvolume.
      */
     __u64 parent_id;

     /*
      * Inode number of the directory which contains this subvolume.
      * Zero for top-level subvolume or a deleted subvolume
      */
     __u64 dirid;

     /* Latest transaction id of this subvolume */
     __u64 generation;

     /* Flags of this subvolume */
     __u64 flags;

     /* UUID of this subvolume */
     __u8 uuid[BTRFS_UUID_SIZE];

     /*
      * UUID of the subvolume of which this subvolume is a snapshot.
      * All zero for a non-snapshot subvolume.
      */
     __u8 parent_uuid[BTRFS_UUID_SIZE];

     /*
      * UUID of the subvolume from which this subvolume was received.
      * All zero for non-received subvolume.
      */
     __u8 received_uuid[BTRFS_UUID_SIZE];

     /* Transaction id indicating when change/create/send/receive happened */
     __u64 ctransid;
     __u64 otransid;
     __u64 stransid;
     __u64 rtransid;
     /* Time corresponding to c/o/s/rtransid */
     struct btrfs_ioctl_timespec ctime;
     struct btrfs_ioctl_timespec otime;
     struct btrfs_ioctl_timespec stime;
     struct btrfs_ioctl_timespec rtime;

     /* Must be zero */
     __u64 reserved[8];
};
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_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 -- create a snapshot of a subvolume

  • BTRFS_IOC_SUBVOL_CREATE_V2 -- create a subvolume

  • BTRFS_IOC_SUBVOL_GETFLAGS -- get flags of a subvolume

  • BTRFS_IOC_SUBVOL_SETFLAGS -- set flags of a subvolume

  • 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

  • BTRFS_IOC_RM_DEV_V2

  • BTRFS_IOC_LOGICAL_INO_V2

  • BTRFS_IOC_GET_SUBVOL_INFO -- get information about a subvolume

  • BTRFS_IOC_GET_SUBVOL_ROOTREF

  • BTRFS_IOC_INO_LOOKUP_USER

  • BTRFS_IOC_SNAP_DESTROY_V2 -- destroy a (snapshot or regular) subvolume

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_SNAP_CREATE_V2

Note

obsoletes BTRFS_IOC_SNAP_CREATE

Create a snapshot of a subvolume.

ioctl fd

file descriptor of the directory inside which to create the new snapshot

argument type

struct btrfs_ioctl_vol_args_v2

fd

file descriptor of any directory inside the subvolume to snapshot

transid

ignored

flags

any subset of BTRFS_SUBVOL_RDONLY to make the new snapshot read-only, or BTRFS_SUBVOL_QGROUP_INHERIT to apply the qgroup_inherit field

name

the name, under the ioctl fd, for the new subvolume

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

BTRFS_IOC_SUBVOL_GETFLAGS

Read the flags of a subvolume. The returned flags are either 0 or BTRFS_SUBVOL_RDONLY.

ioctl fd

file descriptor of the subvolume to examine

argument type

uint64_t

BTRFS_IOC_SUBVOL_SETFLAGS

Change the flags of a subvolume.

ioctl fd

file descriptor of the subvolume to modify

argument type

uint64_t, either 0 or BTRFS_SUBVOL_RDONLY

BTRFS_IOC_GET_SUBVOL_INFO

Get information about a subvolume.

ioctl fd

file descriptor of the subvolume to examine

argument type

struct btrfs_ioctl_get_subvol_info_args

BTRFS_IOC_SNAP_DESTROY_V2

Destroy a subvolume, which may or may not be a snapshot.

ioctl fd

if flags does not include BTRFS_SUBVOL_SPEC_BY_ID, or if executing in a non-root user namespace, file descriptor of the parent directory containing the subvolume to delete; otherwise, file descriptor of any directory on the same filesystem as the subvolume to delete, but not within the same subvolume

argument type

struct btrfs_ioctl_vol_args_v2

fd

ignored

transid

ignored

flags

0 if the name field identifies the subvolume by name in the specified directory, or BTRFS_SUBVOL_SPEC_BY_ID if the subvolid field specifies the ID of the subvolume

name

only if flags does not contain BTRFS_SUBVOL_SPEC_BY_ID, the name (within the directory identified by fd) of the subvolume to delete

subvolid

only if flags contains BTRFS_SUBVOL_SPEC_BY_ID, the subvolume ID of the subvolume to delete

AVAILABILITY

btrfs is part of btrfs-progs. Please refer to the documentation at https://btrfs.readthedocs.io.

SEE ALSO

ioctl(2)