Send stream format

Send stream format represents a linear sequence of commands describing actions to be performed on the target filesystem (receive side), created on the source filesystem (send side). The stream is currently used in two ways: to generate a stream representing a standalone subvolume (full mode) or a difference between two snapshots of the same subvolume (incremental mode). The stream can be generated using a set of other subvolumes to look for extent references that could lead to a more efficient stream by transferring only the references and not full data.

The stream format is abstracted from on-disk structures (though it may share some BTRFS specifics), the stream instructions could be generated by other means than the send ioctl.

Data types

Raw data types. Integer values are stored in little endian byte order.

  • unsigned int 8bit (u8)

  • unsigned int 16bit (u16)

  • unsigned int 32bit (u32)

  • unsigned int 64bit (u64)

  • variable length binary data (data)

  • variable length string (string)

  • UUID, 16 bytes (uuid)

  • time specification, 64bit seconds, 32bit nanoseconds (timespec)

Stream structure

The stream starts with a descriptor bytes btrfs-stream followed by version stored as little endian u32. Then the sequence of commands starts. The expected start is a subvolume or snapshot followed by commands to change the data.

Command structure

  • u32 length

  • u16 command type

  • u32 checksum (CRC32C)

Note: the checksum initial seed is not 0xFFFFFFFF but 0x0. That is a slight mistake and not the recommended way, overlooked when the protocol was implemented. This does not have a big impact though.

Command data are structured as sequence of type-length-value (TLV):

  • TLV type

  • TLV length of the data

  • raw data

The TLVs are stored in the command data sequentially and the order is not mandatory, expected data for each command is looked up based on the type. Unrecognized TLV type may be ignored on the receive side. In the following documentation the TLVs are listed in the order as they’re emitted by kernel side generating the stream.

Subvolumes from the source and target filesystems are matched by the UUIDs.

All file or directory changes are matched by path, inode numbers may be different on the source and target filesystems but are present in the TLVs.

Attributes, TLV types

The TLV types are also called attributes of the command. The raw data type is one of the Data types listed above, the actual named type may be a generic one (like PATH) or specific for set of command (like CLONE_OFFSET).

The attributes represent the same raw data type when used in various commands, though this is not strictly necessary.

Version 1

  • BTRFS_SEND_A_UNSPEC (0) - invalid

  • BTRFS_SEND_A_UUID (1) - uuid

  • BTRFS_SEND_A_CTRANSID (2) - u64

  • BTRFS_SEND_A_INO (3) - u64

  • BTRFS_SEND_A_SIZE (4) - u64

  • BTRFS_SEND_A_MODE (5) - u64

  • BTRFS_SEND_A_UID (6) - u64

  • BTRFS_SEND_A_GID (7) - u64

  • BTRFS_SEND_A_RDEV (8) - u64

  • BTRFS_SEND_A_CTIME (9) - timespec

  • BTRFS_SEND_A_MTIME (10) - timespec

  • BTRFS_SEND_A_ATIME (11) - timespec

  • BTRFS_SEND_A_OTIME (12) - timespec

  • BTRFS_SEND_A_XATTR_NAME (13) - string

  • BTRFS_SEND_A_XATTR_DATA (14) - data

  • BTRFS_SEND_A_PATH (15) - string

  • BTRFS_SEND_A_PATH_TO (16) - string

  • BTRFS_SEND_A_PATH_LINK (17) - string

  • BTRFS_SEND_A_FILE_OFFSET (18) - u64

  • BTRFS_SEND_A_DATA (19) - data

  • BTRFS_SEND_A_CLONE_UUID (20) - uuid

  • BTRFS_SEND_A_CLONE_CTRANSID (21) - u64

  • BTRFS_SEND_A_CLONE_PATH (22) - string

  • BTRFS_SEND_A_CLONE_OFFSET (23) - u64

  • BTRFS_SEND_A_CLONE_LEN (24) - u64

Version 2

  • BTRFS_SEND_A_FALLOCATE_MODE (25) - u32

  • BTRFS_SEND_A_FILEATTR (26) - u64

  • BTRFS_SEND_A_UNENCODED_FILE_LEN (27) - u64

  • BTRFS_SEND_A_UNENCODED_LEN (28) - u64

  • BTRFS_SEND_A_UNENCODED_OFFSET (29) - u64

  • BTRFS_SEND_A_COMPRESSION (30) - u32

  • BTRFS_SEND_A_ENCRYPTION (31) - u32

Special cases

File creation (MKFILE) is done in two phases, where first a file with special name like o-123-45678 is created and then renamed (RENAME) to the target location.

Parent directory where file changes are currently done gets update of times (UTIMES) after each operation that affects it. This is not optimal as only the last one would be needed.

Raw data type is processed in a different way in protocol version 1 and 2. In 1 the maximum size of the TLV is 64KiB as the size is stored in u16. This is not sufficient for encoded write (ENCODED_WRITE) command. In 2 the data length is up to 4GiB (using the type u32) but the TLV must be last and the actual length is calculated as the delta between the whole command and the TLV (i.e. ignoring the TLV header length).

Stream version 1

BTRFS_SEND_C_UNSPEC (0)

Placeholder, invalid or ignored command.

BTRFS_SEND_C_SUBVOL (1)

  • string path - relative path of the subvolume

  • uuid uuid - uuid of the sent subvolume

  • u64 ctransid - creation transaction

BTRFS_SEND_C_SNAPSHOT (2)

Start of commands of a given snapshot.

  • string path - relative path of the subvolume

  • uuid uuid - uuid of the sent subvolume

  • u64 ctransid - creation transaction

  • uuid clone_uuid -

  • u64 clone_ctransid -

BTRFS_SEND_C_MKFILE (3)

Create regular file. See also section Special cases.

  • string path - relative file path to create

  • u64 ino - inode number

BTRFS_SEND_C_MKDIR (4)

Create a directory.

  • string path - relative directory path to create

  • u64 ino - inode number

File creation is done in two commands, the first one contains a special file name that is later renamed to the final name. (WHY)

BTRFS_SEND_C_MKNOD (5)

Create a special file of type device node (mknod).

  • string path - relative file path to create

  • u64 ino - inode number

  • u64 mode - file mode parameter of mknod(2)

  • u64 rdev - rdev parameter of mknod(2)

BTRFS_SEND_C_MKFIFO (6)

Create a special file of type fifo (mkfifo).

  • string path - relative file path to create

  • u64 ino - inode number

BTRFS_SEND_C_MKSOCK (7)

Create a special file of type socket (mknod S_IFSOCK).

  • string path - relative file path to create

  • u64 ino - inode number

BTRFS_SEND_C_RENAME (9)

Rename file path.

  • string path - relative source file path

  • string path_to - relative target file path

BTRFS_SEND_C_RMDIR (12)

Remove directory.

  • string path - relative directory path

BTRFS_SEND_C_SET_XATTR (13)

Set a value of extended attribute.

  • string path - relative file path

  • string xattr_name - name of the extended attribute

  • data xattr_data - value of the extended attribute

BTRFS_SEND_C_REMOVE_XATTR (14)

Remove an extended attribute.

  • string path - relative file path

  • string xattr_name - name of the extended attribute

BTRFS_SEND_C_WRITE (15)

Write file data to a given file offset.

  • string path - relative file path

  • u64 - file offset where to write data

  • data - raw file data (variable length)

BTRFS_SEND_C_CLONE (16)

Clone extents from another file.

  • string path - relative file path

  • u64 file_offset - offset in the source file to clone from

  • u64 clone_len - length of cloned data

  • uuid clone_uuid -

  • u64 clone_ctransid -

  • string clone_path - clone target relative file path

  • u64 clone_offset - clone offset in the target file

BTRFS_SEND_C_TRUNCATE (17)

Truncate file to a given length.

  • string path - relative file path

  • u64 size - truncate to given size

BTRFS_SEND_C_CHMOD (18)

Chmod a file or directory.

  • string path - relative file path

  • u64 mode - new mode

BTRFS_SEND_C_CHOWN (19)

Change file owner (uid) and group (gid), specified by numeric id. The uid/gid must exist on the target filesystem, no mapping is done.

  • string path - relative file path

  • u64 uid - numeric used id

  • u64 gid - numeric group id

BTRFS_SEND_C_UTIMES (20)

Change file atime and mtime, nanosecond precision. While the ctime is also sent it’s not possible to change it using utimensat. The creation time is sent since protocol version 2 but cannot be changed on the target filesystem.

  • string path - relative file path

  • timespec atime - file atime

  • timespec mtime - file mtime

  • timespec ctime - file ctime

  • timespec otime - (since v2) file otime (creation time)

BTRFS_SEND_C_END (21)

Special command to denote end of one logical stream inside the whole stream sequence. May or may not be processed by receiver.

BTRFS_SEND_C_UPDATE_EXTENT (22)

When send is generated without data (BTRFS_SEND_FLAG_NO_FILE_DATA), this command informs about changed extent but does not send the actual data.

  • string path - relative file path

  • u64 file_offset - file offset where data were updated

  • u64 size - length of the data

Stream version 2

BTRFS_SEND_C_FALLOCATE (23)

Change file extents to preallocated, punch hole or zero fill.

  • string path - relative file path

  • u32 fallocate_mode - which fallocate operation to do

  • u64 file_offset - file offset where to apply the operation

  • u64 size - length of the range

BTRFS_SEND_C_FILEATTR (24)

File attributes, representing various flags (SETFLAGS ioctl, XFLAGS, BTRFS specific inode flags). The value is set from BTRFS inode bits and the stream format inherits that. Note that some flags like IMMUTABLE or APPEND may affect ability to change other flags and that for some flags there’s ready interface to set them.

BTRFS_SEND_C_ENCODED_WRITE (25)

File data encoded by the source filesystem and written directly to the target filesystem, without any other transformation. The data can be compressed or encrypted and the payload depends on presence of the TLVs.

  • string path - relative file path

  • u64 file_offset - file offset where to write the data

  • u64 unencoded_file_len -

  • u64 unencoded_len -

  • u64 unencoded_offset -

  • u32 compression - (optional) compression type

  • u32 encryption - (optional) encryption type

  • data data - encoded payload