BTRFS-IOCTL(2)

NAME
btrfs-ioctl - documentation about btrfs ioctls

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];
    };

    #define BTRFS_SUBVOL_RDONLY                  (1ULL << 1)
    #define BTRFS_SUBVOL_QGROUP_INHERIT          (1ULL << 2)
    #define BTRFS_DEVICE_SPEC_BY_ID              (1ULL << 3)
    #define BTRFS_SUBVOL_SPEC_BY_ID              (1ULL << 4)
    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];
    };

    #define BTRFS_QGROUP_INHERIT_SET_LIMITS         (1ULL << 0)
    struct btrfs_qgroup_inherit {
            __u64 flags;
            __u64 num_qgroups;
            __u64 num_ref_copies;
            __u64 num_excl_copies;
            struct btrfs_qgroup_limit lim;
            __u64 qgroups[];
    };

    #define BTRFS_QGROUP_LIMIT_MAX_RFER             (1ULL << 0)
    #define BTRFS_QGROUP_LIMIT_MAX_EXCL             (1ULL << 1)
    #define BTRFS_QGROUP_LIMIT_RSV_RFER             (1ULL << 2)
    #define BTRFS_QGROUP_LIMIT_RSV_EXCL             (1ULL << 3)
    #define BTRFS_QGROUP_LIMIT_RFER_CMPR            (1ULL << 4)
    #define BTRFS_QGROUP_LIMIT_EXCL_CMPR            (1ULL << 5)
    struct btrfs_qgroup_limit {
            __u64 flags;
            __u64 max_rfer;
            __u64 max_excl;
            __u64 rsv_rfer;
            __u64 rsv_excl;
    };

    /* Request information about checksum type and size */
    #define BTRFS_FS_INFO_FLAG_CSUM_INFO                    (1U << 0)
    /* Request information about filesystem generation */
    #define BTRFS_FS_INFO_FLAG_GENERATION                   (1U << 1)
    /* Request information about filesystem metadata UUID */
    #define BTRFS_FS_INFO_FLAG_METADATA_UUID                (1U << 2)
    struct btrfs_ioctl_fs_info_args {
            __u64 max_id;                           /* out */
            __u64 num_devices;                      /* out */
            __u8 fsid[BTRFS_FSID_SIZE];             /* out */
            __u32 nodesize;                         /* out */
            __u32 sectorsize;                       /* out */
            __u32 clone_alignment;                  /* out */
            /* See BTRFS_FS_INFO_FLAG_* */
            __u16 csum_type;                        /* out */
            __u16 csum_size;                        /* out */
            __u64 flags;                            /* in/out */
            __u64 generation;                       /* out */
            __u8 metadata_uuid[BTRFS_FSID_SIZE];    /* out */
            __u8 reserved[944];                     /* pad to 1k */
    };

    #define BTRFS_INO_LOOKUP_PATH_MAX               4080
    struct btrfs_ioctl_ino_lookup_args {
            __u64 treeid;
            __u64 objectid;
            char name[BTRFS_INO_LOOKUP_PATH_MAX];
    };

    /* Specify the subvolid. */
    #define BTRFS_SUBVOL_SYNC_WAIT_FOR_ONE         (0)
    /* Wait for all currently queued. */
    #define BTRFS_SUBVOL_SYNC_WAIT_FOR_QUEUED      (1)
    /* Count number of queued subvolumes. */
    #define BTRFS_SUBVOL_SYNC_COUNT                (2)
    /*
     * Read which is the first in the queue (to be cleaned or being cleaned already),
     * or 0 if the queue is empty.
     */
    #define BTRFS_SUBVOL_SYNC_PEEK_FIRST           (3)
    /* Read the last subvolid in the queue, or 0 if the queue is empty. */
    #define BTRFS_SUBVOL_SYNC_PEEK_LAST            (4)
    struct btrfs_ioctl_subvol_wait {
           __u64 subvolid;
           __u32 mode;
           __u32 count;
    };
Constant name | Value
BTRFS_UUID_SIZE | 16
BTRFS_FSID_SIZE | 16
BTRFS_SUBVOL_NAME_MAX | 4039
BTRFS_PATH_NAME_MAX | 4087
BTRFS_VOL_NAME_MAX | 255
BTRFS_LABEL_SIZE | 256
BTRFS_FIRST_FREE_OBJECTID | 256

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.

File descriptors of regular files are obtained by int fd = open(), directories opened as DIR *dir = opendir() can be converted to the corresponding file descriptor by fd = dirfd(dir).

LIST OF IOCTLS
Name | Description | Data
BTRFS_IOC_SNAP_CREATE | (obsolete) create a snapshot of a subvolume | struct btrfs_ioctl_vol_args
BTRFS_IOC_DEFRAG |  |
BTRFS_IOC_RESIZE |  |
BTRFS_IOC_SCAN_DEV | scan and register a given device path with filesystem module | struct btrfs_ioctl_vol_args
BTRFS_IOC_SYNC | Sync the filesystem, possibly process queued up work | NULL
BTRFS_IOC_CLONE |  |
BTRFS_IOC_ADD_DEV | add a device to the filesystem by path | struct btrfs_ioctl_vol_args
BTRFS_IOC_RM_DEV | delete a device from the filesystem by path | struct btrfs_ioctl_vol_args
BTRFS_IOC_BALANCE |  |
BTRFS_IOC_CLONE_RANGE |  |
BTRFS_IOC_SUBVOL_CREATE | (obsolete) create a subvolume | struct btrfs_ioctl_vol_args
BTRFS_IOC_SNAP_DESTROY | (obsolete) delete a subvolume | struct btrfs_ioctl_vol_args
BTRFS_IOC_DEFRAG_RANGE |  |
BTRFS_IOC_TREE_SEARCH |  |
BTRFS_IOC_TREE_SEARCH_V2 |  |
BTRFS_IOC_INO_LOOKUP | resolve inode number to path, or lookup containing subvolume id | struct btrfs_ioctl_ino_lookup_args
BTRFS_IOC_DEFAULT_SUBVOL | set the default subvolume id | uint64_t
BTRFS_IOC_SPACE_INFO |  |
BTRFS_IOC_START_SYNC |  |
BTRFS_IOC_WAIT_SYNC |  |
BTRFS_IOC_SNAP_CREATE_V2 | create a snapshot of a subvolume | struct btrfs_ioctl_vol_args_v2
BTRFS_IOC_SUBVOL_CREATE_V2 | create a subvolume | struct btrfs_ioctl_vol_args_v2
BTRFS_IOC_SUBVOL_GETFLAGS | get flags of a subvolume | uint64_t
BTRFS_IOC_SUBVOL_SETFLAGS | set flags of a subvolume | uint64_t
BTRFS_IOC_SCRUB |  |
BTRFS_IOC_SCRUB_CANCEL |  |
BTRFS_IOC_SCRUB_PROGRESS |  |
BTRFS_IOC_DEV_INFO |  |
BTRFS_IOC_FS_INFO | get information about filesystem (device count, fsid, ...) | struct btrfs_ioctl_fs_info_args
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 | read filesystem label | char buffer[BTRFS_LABEL_SIZE]
BTRFS_IOC_SET_FSLABEL | set the filesystem label | char buffer[BTRFS_LABEL_SIZE]
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 | struct btrfs_ioctl_get_subvol_info_args
BTRFS_IOC_GET_SUBVOL_ROOTREF |  |
BTRFS_IOC_INO_LOOKUP_USER |  |
BTRFS_IOC_SNAP_DESTROY_V2 | destroy a (snapshot or regular) subvolume | struct btrfs_ioctl_vol_args_v2
BTRFS_IOC_ENCODED_READ |  |
BTRFS_IOC_ENCODED_WRITE |  |
BTRFS_IOC_SUBVOL_SYNC_WAIT | Wait until a deleted subvolume is cleaned or query the state. | struct btrfs_ioctl_subvol_wait

DETAILED DESCRIPTION

  BTRFS_IOC_SNAP_CREATE

Note: obsoleted by BTRFS_IOC_SNAP_CREATE_V2

(since: 3.0, obsoleted: 4.0) Create a snapshot of a subvolume.
Field | Description
ioctl fd | file descriptor of the parent directory of the new subvolume
ioctl args | struct btrfs_ioctl_vol_args
args.fd | file descriptor of any directory inside the subvolume to snapshot, must be on the same filesystem
args.name | name of the subvolume, although the buffer can be almost 4KiB, the file size is limited by Linux VFS to 255 characters and must not contain a slash ('/')

  BTRFS_IOC_SCAN_DEV

Scan and register a given device in the filesystem module, which can be later used for automatic device and filesystem association at mount time. This operates on the control device, not files from a mounted filesystem. Can be safely called repeatedly with same device path.
Field | Description
ioctl fd | file descriptor of the control device /dev/btrfs-control
ioctl args | struct btrfs_ioctl_vol_args
args.fd | ignored
args.name | full path of the device

  BTRFS_IOC_SYNC

Sync the filesystem data as would sync() syscall do, additionally wake up the internal transaction thread that may trigger actions like subvolume cleaning or queued defragmentation.
Field | Description
ioctl fd | file descriptor of any file or directory in the filesystem
ioctl args | NULL

  BTRFS_IOC_ADD_DEV

Add a given block device to the filesystem. Unlike the command btrfs device add there's are no safety checks (like existence of another filesystem on the device), device preparation (like TRIM or zone reset), so use it with care.

This is a filesystem-exclusive operation and it will fail if there's another one already running, with one exception, when there's a paused balance.

Required permissions: CAP_SYS_ADMIN
Field | Description
ioctl fd | file descriptor of any file or directory in the filesystem
ioctl args | struct btrfs_ioctl_vol_args
args.fd | ignored
args.name | full path of the block device to be added

  BTRFS_IOC_RM_DEV

Remove a device from the filesystem specified by it's path, or cancel a running device deletion by special path cancel.

This is a filesystem-exclusive operation and it will fail if there's another one already running.

Required permissions: CAP_SYS_ADMIN
Field | Description
ioctl fd | file descriptor of any file or directory in the filesystem
ioctl args | struct btrfs_ioctl_vol_args
args.fd | ignored
args.name | full path of the block device to be deleted or string "cancel"

  BTRFS_IOC_SUBVOL_CREATE

Note: obsoleted by BTRFS_IOC_SUBVOL_CREATE_V2

(since: 3.0, obsoleted: 4.0) Create a subvolume.
Field | Description
ioctl fd | file descriptor of the parent directory of the new subvolume
ioctl args | struct btrfs_ioctl_vol_args
args.fd | ignored
args.name | name of the subvolume, although the buffer can be almost 4KiB, the file size is limited by Linux VFS to 255 characters and must not contain a slash ('/')

  BTRFS_IOC_SNAP_DESTROY

Note: obsoleted by BTRFS_IOC_SNAP_DESTROY_V2

(since: 2.6.33, obsoleted: 5.7) Delete a subvolume.
Field | Description
ioctl fd | file descriptor of the parent directory of the new subvolume
ioctl args | struct btrfs_ioctl_vol_args
args.fd | ignored
args.name | name of the subvolume, although the buffer can be almost 4KiB, the file size is limited by Linux VFS to 255 characters and must not contain a slash ('/')

  BTRFS_IOC_INO_LOOKUP

Resolve inode number to a path (requires CAP_SYS_ADMIN), or read a containing subvolume id of the given file (unrestricted, special case). The size of the path name buffer is shorter than PATH_MAX (4096), it's possible that the path is trimmed due to that. Also implemented by btrfs inspect-internal rootid <#man-inspect-rootid>.

The general case needs CAP_SYS_ADMIN and can resolve any file to its path. The special case for reading the containing subvolume is not restricted:

    struct btrfs_ioctl_ino_lookup_args args;
    fd = open("file", ...);
    args.treeid = 0;
    args.objectid = BTRFS_FIRST_FREE_OBJECTID;
    ioctl(fd, BTRFS_IOC_INO_LOOKUP, &args);
    /* args.treeid now contains the subvolume id */
Field | Description
ioctl fd | file descriptor of the file or directory to lookup the subvolumeid
ioctl args | struct btrfs_ioctl_ino_lookup_args
args.treeid | subvolume id against which the path should be resolved (needs CAP_SYS_ADMIN), or 0 so the subvolume containing fd will be used
args.objectid | inode number to lookup, INODE_REF_KEY with that key.objectid, or BTRFS_FIRST_FREE_OBJECTID as special case to read only the tree id and clear the args.name buffer
args.name | path relative to the toplevel subvolume, or empty string

  BTRFS_IOC_DEFAULT_SUBVOL

Set the given subvolume id as the default one when mounting the filesystem without subvol=path or subvolid=id options.
Field | Description
ioctl fd | file descriptor of the directory inside which to create the new snapshot
ioctl args | numeric value of subvolume to become default (uint64_t)

  BTRFS_IOC_SNAP_CREATE_V2

Create a snapshot of a subvolume.
Field | Description
ioctl fd | file descriptor of the directory inside which to create the new snapshot
ioctl args | struct btrfs_ioctl_vol_args_v2
args.fd | file descriptor of any directory inside the subvolume to snapshot, must be on the filesystem
args.transid | ignored
args.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
args.name | the name, under the ioctl fd, for the new subvolume

  BTRFS_IOC_SUBVOL_CREATE_V2

(since: 3.6) Create a subvolume, qgroup inheritance and limits can be specified.
Field | Description
ioctl fd | file descriptor of the parent directory of the new subvolume
ioctl args | struct btrfs_ioctl_vol_args_v2
args.fd | ignored
args.transid | ignored
args.flags | flags to set on the subvolume, BTRFS_SUBVOL_RDONLY for readonly, BTRFS_SUBVOL_QGROUP_INHERIT if the qgroup related fields should be processed
args.size | number of entries in args.qgroup_inherit
args.qgroup_inherit | inherit the given qgroups (struct btrfs_qgroup_inherit) and limits (struct btrfs_qgroup_limit)
name | name of the subvolume, although the buffer can be almost 4KiB, the file size is limited by Linux VFS to 255 characters and must not contain a slash ('/')

  BTRFS_IOC_SUBVOL_GETFLAGS

Read the flags of a subvolume. The returned flags are either 0 or BTRFS_SUBVOL_RDONLY.
Field | Description
ioctl fd | file descriptor of the subvolume to examine
ioctl args | uint64_t

  BTRFS_IOC_SUBVOL_SETFLAGS

Change the flags of a subvolume.
Field | Description
ioctl fd | file descriptor of the subvolume to modify
ioctl args | uint64_t, either 0 or BTRFS_SUBVOL_RDONLY

  BTRFS_IOC_GET_FSLABEL

Read the label of the filesystem into a given buffer. Alternatively it can be read from /sys/fs/btrfs/FSID/label though it requires to know the FSID of the filesystem.
Field | Description
ioctl fd | file descriptor of any file/directory in the filesystem
ioctl args | char buffer[BTRFS_LABEL_SIZE]

  BTRFS_IOC_SET_FSLABEL

Set the label of filesystem from given buffer. The maximum length also accounts for terminating NUL character. Alternatively it can be also set by writing to /sys/fs/btrfs/FSID/label though it requires to know the FSID of the filesystem (and an explicit commit before the change is permanent).

Required permissions: CAP_SYS_ADMIN
Field | Description
ioctl fd | file descriptor of any file/directory in the filesystem
ioctl args | char buffer[BTRFS_LABEL_SIZE]

  BTRFS_IOC_FS_INFO

Read internal information about the filesystem. The data can be exchanged both ways and part of the structure could be optionally filled. The reserved bytes can be used to get new kind of information in the future, always depending on the flags set.
Field | Description
ioctl fd | file descriptor of any file/directory in the filesystem
ioctl args | struct btrfs_ioctl_fs_info_args

  BTRFS_IOC_GET_SUBVOL_INFO

Get information about a subvolume.
Field | Description
ioctl fd | file descriptor of the subvolume to examine
ioctl args | struct btrfs_ioctl_get_subvol_info_args

  BTRFS_IOC_SNAP_DESTROY_V2

Destroy a subvolume, which may or may not be a snapshot.
Field | Description
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
ioctl args | struct btrfs_ioctl_vol_args_v2
args.fd | ignored
args.transid | ignored
args.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
args.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
args.subvolid | only if flags contains BTRFS_SUBVOL_SPEC_BY_ID, the subvolume ID of the subvolume to delete

  BTRFS_IOC_SUBVOL_SYNC_WAIT

(since: 6.13) Wait until a deleted subvolume is cleaned or query the state.

There are several modes of operation, where the most common ones are to wait on a specific subvolume or all currently queued for cleaning. This is utilized e.g. in backup applications that delete subvolumes and wait until they're cleaned to check for remaining space.

The other modes are for flexibility, e.g. for monitoring or checkpoints in the queue of deleted subvolumes, again without the need to use SEARCH_TREE.

Notes:

•   waiting is interruptible, the timeout is set to 1 second and is not configurable

•   repeated calls to the ioctl see a different state, so this is inherently racy when using e.g. the count or peek next/last

    Use cases (definition of constants):

•   a subvolume A was deleted, wait for cleaning (WAIT_FOR_ONE)

•   a bunch of subvolumes were deleted, wait for all (WAIT_FOR_QUEUED or PEEK_LAST + WAIT_FOR_ONE)

•   count how many are queued (not blocking), for monitoring purposes

•   report progress (PEEK_NEXT), may miss some if cleaning is quick

•   own waiting in user space (PEEK_LAST until it's 0)
    Field | Description
    ioctl fd | file descriptor of any file or directory in the filesystem
    ioctl args | struct btrfs_ioctl_subvol_wait
    args.subvolid | Depending on the mode, the numeric id of subvolume to wait for, or the one queried by PEEK modes
    args.mode | mode of operation described above
    args.count | if mode is set to COUNT the number of subvolumes queued for cleaning

AVAILABILITY

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

SEE ALSO

ioctl(2) <https://man7.org/linux/man-pages/man2/ioctl.2.html>