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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.

Meaning Size Name
unsigned int 8 bit u8
unsigned int 16 bit u16
unsigned int 32 bit u32
unsigned int 64 bit u64
variable length binary data variable data
variable length string variable 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

Name Number Type
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

Name Number Type
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)

Type Name Description
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.

Type Name Description
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.

Type Name Description
string path relative file path to create
u64 ino inode number

BTRFS_SEND_C_MKDIR (4)

Create a directory.

Type Name Description
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).

Type Name Description
string path relative file path to create
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).

Type Name Description
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).

Type Name Description
string path relative file path to create
u64 ino inode number

BTRFS_SEND_C_SYMLINK (8)

Create a symlink.

Type Name Description
string path relative symlink path to create
u64 ino inode number
string path_link target of the symlink

BTRFS_SEND_C_RENAME (9)

Rename file path.

Type Name Description
string path relative source file path
string path_to relative target file path

BTRFS_SEND_C_LINK (10)

Create a file hardlink.

Type Name Description
string path relative source file path
string path_link relative target file path to link to

BTRFS_SEND_C_UNLINK (11)

Unlink file.

Type Name Description
string path relative file path

BTRFS_SEND_C_RMDIR (12)

Remove directory.

Type Name Description
string path relative directory path

BTRFS_SEND_C_SET_XATTR (13)

Set a value of extended attribute.

Type Name Description
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.

Type Name Description
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.

Type Name Description
string path relative file path
u64 file_offset where to write data
data data raw file data (variable length)

BTRFS_SEND_C_CLONE (16)

Clone extents from another file.

Type Name Description
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.

Type Name Description
string path relative file path
u64 size truncate to given size

BTRFS_SEND_C_CHMOD (18)

Chmod a file or directory.

Type Name Description
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.

Type Name Description
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.

Type Name Description
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.

Type Name Description
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.

Type Name Description
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.

Type Name Description
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