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)