File formats
To maintain data persistence, Tarantool writes each data change request (insert,
update, delete, replace, upsert) to a write-ahead log (WAL) file in the
wal.dir directory.
Each data change request is assigned a continuously growing 64-bit log sequence
number. The name of the WAL file is based on the log sequence number of the first
record in the file, plus an extension .xlog.
A new WAL file is created
when the current one reaches the wal_max_size size.
Each WAL record contains:
- a log sequence number
- a data change request (formatted as in Tarantool’s binary protocol)
- a header
- some metadata
- the data formatted according to msgpack rules.
To see the hexadecimal bytes of the given WAL file, use the hexdump command:
$ hexdump 00000000000000000000.xlog
For example, the WAL file after the first INSERT request might look the following way:
Hex dump of WAL file Comment
-------------------- -------
58 4c 4f 47 0a "XLOG\n"
30 2e 31 33 0a "0.13\n" = version
53 65 72 76 65 72 3a 20 "Server: "
38 62 66 32 32 33 65 30 2d [Server UUID]\n
36 39 31 34 2d 34 62 35 35
2d 39 34 64 32 2d 64 32 62
36 64 30 39 62 30 31 39 36
0a
56 43 6c 6f 63 6b 3a 20 "Vclock: "
7b 7d "{}" = vclock value, initially blank
... (not shown = tuples for system spaces)
d5 ba 0b ab Magic row marker always = 0xab0bbad5
19 Length, not including length of header, = 25 bytes
00 Record header: previous crc32
ce 8c 3e d6 70 Record header: current crc32
a7 cc 73 7f 00 00 66 39 Record header: padding
84 msgpack code meaning "Map of 4 elements" follows
00 02 element#1: tag=request type, value=0x02=IPROTO_INSERT
02 01 element#2: tag=server id, value=0x01
03 04 element#3: tag=lsn, value=0x04
04 cb 41 d4 e2 2f 62 fd d5 d4 element#4: tag=timestamp, value=an 8-byte "Float64"
82 msgpack code meaning "map of 2 elements" follows
10 cd 02 00 element#1: tag=space id, value=512, big byte first
21 91 01 element#2: tag=tuple, value=1-element fixed array={1}
Tarantool processes requests atomically: a change is either accepted and recorded in the WAL, or discarded completely. To clarify how this happens, see the example with the REPLACE request below:
- The server instance attempts to locate the original tuple by primary key. If found, a reference to the tuple is retained for later use.
- The new tuple is validated. If for example it does not contain an indexed field, or it has an indexed field whose type does not match the type according to the index definition, the change is aborted.
- The new tuple replaces the old tuple in all existing indexes.
- A message is sent to the WAL writer running in a separate thread, requesting that the change be recorded in the WAL. The instance switches to work on the next request until the write is acknowledged.
- On success, a confirmation is sent to the client. On failure, a rollback
procedure begins. During the rollback procedure, the transaction processor
rolls back all changes to the database which occurred after the first failed
change, from latest to oldest, up to the first failed change. All rolled back
requests are aborted with
ER_WAL_IOerror. No new change is applied while rollback is in progress. When the rollback procedure is finished, the server restarts the processing pipeline.
One advantage of the described algorithm is that complete request pipelining is achieved, even for requests on the same value of the primary key. As a result, database performance doesn’t degrade even if all requests refer to the same key in the same space.
The transaction processor thread communicates with the WAL writer thread using asynchronous (yet reliable) messaging. The transaction processor thread, not being blocked on WAL tasks, continues to handle requests quickly even at high volumes of disk I/O. A response to a request is sent as soon as it is ready, even if there were earlier incomplete requests on the same connection. In particular, SELECT performance, even for SELECTs running on a connection packed with UPDATEs and DELETEs, remains unaffected by disk load.
The WAL writer employs a number of durability modes, as defined in configuration
variable wal.mode.
It is possible to turn the write-ahead log completely off, by setting the wal_mode option to none.
Even without the write-ahead log it’s still possible to take a persistent copy of the
entire data set with the box.snapshot() request.
An .xlog file always contains changes based on the primary key.
Even if the client requested an update or delete using
a secondary key, the record in the .xlog file contains the primary key.
The format of a snapshot (.snap) file is the following:
- The snapshot header contains the instance’s global unique identifier and the snapshot file’s position in history, relative to earlier snapshot files.
- The snapshot content contains the records of inserts to memtx spaces.
That differs from the content of an
.xlogfile that may contain records for any data-change requests (inserts, updates, upserts, and deletes).
Primarily, the records in the snapshot file have the following order:
- System spaces (id >= 256 && id <= 511), ordered by ID.
- Non-system spaces, ordered by ID.
Secondarily, the .snap file’s records are ordered by primary key within space ID.
The header of a .snap or .xlog file might look in the following way:
<type>\n SNAP\n or XLOG\n
<version>\n currently 0.13\n
Server: <server_uuid>\n where UUID is a 36-byte string
VClock: <vclock_map>\n e.g. {1: 0}\n
\n
After the file header come the data tuples.
Tuples begin with a row marker 0xd5ba0bab and
the last tuple may be followed by an EOF marker
0xd510aded.
Thus, between the file header and the EOF marker, there
may be data tuples that have this form:
0 3 4 17
+-------------+========+============+===========+=========+
| | | | | |
| 0xd5ba0bab | LENGTH | CRC32 PREV | CRC32 CUR | PADDING |
| | | | | |
+-------------+========+============+===========+=========+
MP_FIXEXT2 MP_INT MP_INT MP_INT ---
+============+ +===================================+
| | | |
| HEADER | | BODY |
| | | |
+============+ +===================================+
MP_MAP MP_MAP