MySQL · myrocks · data dictionary 分析

data dictionary

rocksdb作为mysql的一个新的存储引擎，在存储引擎层，会维护自已的元数据信息。在innodb存储引擎中，我们通过information_schema下的INNODB_SYS_DATAFILES，INNODB_SYS_TABLES，INNODB_SYS_INDEXES等表，
可以窥视innodb的元数据信息。同样，rocksdb通过information_schema下的ROCKSDB_INDEX_FILE_MAP，ROCKSDB_DDL，ROCKSDB_GLOBAL_INFO等表可以查看原数据信息。

show create table ROCKSDB_INDEX_FILE_MAP\G
*************************** 1. row ***************************
       Table: ROCKSDB_INDEX_FILE_MAP
Create Table: CREATE TEMPORARY TABLE `ROCKSDB_INDEX_FILE_MAP` (
  `COLUMN_FAMILY` int(4) NOT NULL DEFAULT '0',
  `INDEX_NUMBER` int(4) NOT NULL DEFAULT '0',
  `SST_NAME` varchar(193) NOT NULL DEFAULT '',
  `NUM_ROWS` bigint(8) NOT NULL DEFAULT '0',
  `DATA_SIZE` bigint(8) NOT NULL DEFAULT '0',
  `ENTRY_DELETES` bigint(8) NOT NULL DEFAULT '0',
  `ENTRY_SINGLEDELETES` bigint(8) NOT NULL DEFAULT '0',
  `ENTRY_MERGES` bigint(8) NOT NULL DEFAULT '0',
  `ENTRY_OTHERS` bigint(8) NOT NULL DEFAULT '0'
) ENGINE=MEMORY DEFAULT CHARSET=utf8

show create table ROCKSDB_DDL\G
*************************** 1. row ***************************
       Table: ROCKSDB_DDL
Create Table: CREATE TEMPORARY TABLE `ROCKSDB_DDL` (
  `TABLE_SCHEMA` varchar(193) NOT NULL DEFAULT '',
  `TABLE_NAME` varchar(193) NOT NULL DEFAULT '',
  `PARTITION_NAME` varchar(193) DEFAULT NULL,
  `INDEX_NAME` varchar(193) NOT NULL DEFAULT '',
  `COLUMN_FAMILY` int(4) NOT NULL DEFAULT '0',
  `INDEX_NUMBER` int(4) NOT NULL DEFAULT '0',
  `INDEX_TYPE` smallint(2) NOT NULL DEFAULT '0',
  `KV_FORMAT_VERSION` smallint(2) NOT NULL DEFAULT '0',
  `CF` varchar(193) NOT NULL DEFAULT ''
) ENGINE=MEMORY DEFAULT CHARSET=utf8

show create table ROCKSDB_GLOBAL_INFO\G
*************************** 1. row ***************************
       Table: ROCKSDB_GLOBAL_INFO
Create Table: CREATE TEMPORARY TABLE `ROCKSDB_GLOBAL_INFO` (
  `TYPE` varchar(513) NOT NULL DEFAULT '',
  `NAME` varchar(513) NOT NULL DEFAULT '',
  `VALUE` varchar(513) NOT NULL DEFAULT ''
) ENGINE=MEMORY DEFAULT CHARSET=utf8

元数据详情

下面我们来具体看看rocksdb维护了哪些元数据信息,从源码中看定义了以下类型，这些数据都以KV的形式存储在名叫__system__的系统column family中。

  // Data dictionary types
  enum DATA_DICT_TYPE {
    DDL_ENTRY_INDEX_START_NUMBER= 1,
    INDEX_INFO=                   2,
    CF_DEFINITION=                3,
    BINLOG_INFO_INDEX_NUMBER=     4,      
    DDL_DROP_INDEX_ONGOING=       5,
    INDEX_STATISTICS=             6,      
    MAX_INDEX_ID=                 7,
    DDL_CREATE_INDEX_ONGOING=     8,
    END_DICT_INDEX_ID=          255
  };
  

• DDL_ENTRY_INDEX_START_NUMBER
  表和索引之间的映射关系
  key: Rdb_key_def::DDL_ENTRY_INDEX_START_NUMBER(0x1) + dbname.tablename
  value: version + {global_index_id}*n_indexes_of_the_table

• INDEX_INFO
  索引id和索引属性的关系
  key: Rdb_key_def::INDEX_INFO(0x2) + global_index_id
  value: version, index_type, key_value_format_version
  index_type:主键/二级索引/隐式主键
  key_value_format_version: 记录存储格式的版本

• CF_DEFINITION
  column family属性
  key: Rdb_key_def::CF_DEFINITION(0x3) + cf_id
  value: version, {is_reverse_cf, is_auto_cf}
  is_reverse_cf： 是否是reverse column family
  is_auto_cf: column family名字是否是$per_index_cf，名字自动由table.indexname组成

• BINLOG_INFO_INDEX_NUMBER
  binlog位点及gtid信息，binlog_commit更新此信息
  key: Rdb_key_def::BINLOG_INFO_INDEX_NUMBER (0x4)
  value: version, {binlog_name,binlog_pos,binlog_gtid}

• DDL_DROP_INDEX_ONGOING
  等待删除的索引信息
  key: Rdb_key_def::DDL_DROP_INDEX_ONGOING(0x5) + global_index_id
  value: version

• INDEX_STATISTICS
  索引统计信息
  key: Rdb_key_def::INDEX_STATISTICS(0x6) + global_index_id
  value: version, {materialized PropertiesCollector::IndexStats}

• MAX_INDEX_ID
  当前的index id，每次创建索引index id都从这个获取和更新
  key: Rdb_key_def::CURRENT_MAX_INDEX_ID(0x7)
  value: version, current max index id

• DDL_CREATE_INDEX_ONGOING
  等待创建的索引信息
  key: Rdb_key_def::DDL_CREATE_INDEX_ONGOING(0x8) + global_index_id
  value: version

rocksdb DDL 实现

这里以建表和删表来举例

• create table

CREATE TABLE t1 (a INT, b CHAR(8), pk INT AUTO_INCREMENT ,PRIMARY KEY(pk), key idx1(b) comment 'cf_1') ENGINE=rocksdb;

通过以下步骤建表

1. 创建column family (get_or_create_cf)
   primary key 存在default column family中，idx1存在cf_1中，需增加一条cf_1的，CF_DEFINITION的记录
   {CF_DEFINITION(4)+cf_id(4)} —> {CF_DEFINITION_VERSION(2)+cf_flags(4)}

2. 创建索引
   两条索引
   {INDEX_INFO(4)+cf_id(0)+index_id(260)—> { INDEX_INFO_VERSION_VERIFY_KV_FORMAT(1)+index_type(1)+kv_version(11)
   {INDEX_INFO(4)+cf_id(2)+index_id(261)—> { INDEX_INFO_VERSION_VERIFY_KV_FORMAT(2)+index_type(2)+kv_version(11)

3. 建立表和索引的映射
   {DDL_ENTRY_INDEX_START_NUMBER(4)+dbname(test)+tablename(t1) } –> { DDL_ENTRY_INDEX_VERSION+cf_id(0)+index_id(260)+cf_id(2)+index_id(261}

以上信息通过同一batch一起存入rocksdb中。

另外，建索引时，会更新MAX_INDEX_ID信息，使用单独的batch写入，参考(Rdb_seq_generator::get_and_update_next_number)

select * from INFORMATION_SCHEMA.ROCKSDB_DDL where table_name='t1';
+--------------+------------+----------------+------------+---------------+--------------+------------+-------------------+---------+
| TABLE_SCHEMA | TABLE_NAME | PARTITION_NAME | INDEX_NAME | COLUMN_FAMILY | INDEX_NUMBER | INDEX_TYPE | KV_FORMAT_VERSION | CF      |
+--------------+------------+----------------+------------+---------------+--------------+------------+-------------------+---------+
| test         | t1         | NULL           | PRIMARY    |             0 |          260 |          1 |                11 | default |
| test         | t1         | NULL           | idx1       |             2 |          261 |          2 |                11 | cf_1    |
+--------------+------------+----------------+------------+---------------+--------------+------------+-------------------+---------+

select d.*,i.* from INFORMATION_SCHEMA.ROCKSDB_INDEX_FILE_MAP i,INFORMATION_SCHEMA.ROCKSDB_DDL d where i.INDEX_NUMBER=d.INDEX_NUMBER;
+--------------+------------+----------------+------------+---------------+--------------+------------+-------------------+---------+---------------+--------------+------------+----------+-----------+---------------+---------------------+--------------+--------------+
| TABLE_SCHEMA | TABLE_NAME | PARTITION_NAME | INDEX_NAME | COLUMN_FAMILY | INDEX_NUMBER | INDEX_TYPE | KV_FORMAT_VERSION | CF      | COLUMN_FAMILY | INDEX_NUMBER | SST_NAME   | NUM_ROWS | DATA_SIZE | ENTRY_DELETES | ENTRY_SINGLEDELETES | ENTRY_MERGES | ENTRY_OTHERS |
+--------------+------------+----------------+------------+---------------+--------------+------------+-------------------+---------+---------------+--------------+------------+----------+-----------+---------------+---------------------+--------------+--------------+
| test         | t1         | NULL           | PRIMARY    |             0 |          260 |          1 |                11 | default |             0 |          260 | 000025.sst |        2 |        42 |             0 |                   0 |            0 |            0 |
| test         | t1         | NULL           | idx1       |             2 |          261 |          2 |                11 | cf_1    |             2 |          261 | 000027.sst |        2 |        42 |             0 |                   0 |            0 |            0 |
+--------------+------------+----------------+------------+---------------+--------------+------------+-------------------+---------+---------------+--------------+------------+----------+-----------+---------------+---------------------+--------------+--------------+
2 rows in set (0.00 sec)

实际数据分布如下图：

元数据分布在系统column family __system__中
primary key 分布在column family default中
idx1 分布在column family cf_1中
黄线之间代表数据分布的范围

• drop table

drop table t1;

batch->Put 将索引加入到待删的kv队列中
{DDL_DROP_INDEX_ONGOING(4)+cf_id(0)+index_id(260)} –> {DDL_DROP_INDEX_ONGOING_VERSION(2)}
{DDL_DROP_INDEX_ONGOING(4)+cf_id(2)+index_id(261)} –> {DDL_DROP_INDEX_ONGOING_VERSION(2)}
batch->Delete 删除表的映射关系
表和索引的映射关系

后台线程再从待删的kv队列取出待删的索引，通过 DeleteFilesInRange, CompactRange 删除索引数据。
后台线程确定索引数据删除完成后，batch删除相应的DDL_DROP_INDEX_ONGOING和INDEX_INFO的索引信息。