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KatriceMart
5年前发布

MySQL 如何存储大数据

   <p>最近,在工作中遇到了MySQL中如何存储长度较长的字段类型问题,于是花了一周多的时间抽空学习了一下,并且记录下来。</p>    <p>MySQL大致的逻辑存储结构在这篇文章中有介绍,做为基本概念: InnoDB 逻辑存储结构</p>    <p>注:文中所指的大数据指的是长度较长的数据字段,包括varchar/varbinay/text/blob。</p>    <h3>Compact行格式</h3>    <p>我们首先来看一下行格式为Compact是如何存储大数据的:</p>    <pre>  <code class="language-sql">mysql> select version();  +-----------+  | version() |  +-----------+  | 5.1.73    |  +-----------+  1 row in set (0.01 sec)    mysql> show table status like 'row'\G;  *************************** 1. row ***************************             Name: row           Engine: InnoDB          Version: 10       Row_format: Compact             Rows: 1   Avg_row_length: 81920      Data_length: 81920  Max_data_length: 0     Index_length: 0        Data_free: 0   Auto_increment: NULL      Create_time: 2017-01-04 21:46:02      Update_time: NULL       Check_time: NULL        Collation: latin1_swedish_ci         Checksum: NULL   Create_options:           Comment:   1 row in set (0.00 sec)</code></pre>    <p>我们建立一张测试表,插入数据:</p>    <pre>  <code class="language-sql">CREATE TABLE `row` (    `content` varchar(65532) NOT NULL DEFAULT ''  ) ENGINE=InnoDB DEFAULT CHARSET=latin1    mysql> insert into row(content) select repeat('a',65532);  Query OK, 1 row affected (0.03 sec)  Records: 1  Duplicates: 0  Warnings: 0</code></pre>    <p>我们使用 py_innodb_page_info.py 工具来查看表中的页分布:</p>    <pre>  <code class="language-sql">[root@localhost mysql]# python py_innodb_page_info.py -v com/row.ibd   page offset 00000000, page type <File Space Header>  page offset 00000001, page type <Insert Buffer Bitmap>  page offset 00000002, page type <File Segment inode>  page offset 00000003, page type <B-tree Node>, page level <0000>  page offset 00000004, page type <Uncompressed BLOB Page>  page offset 00000005, page type <Uncompressed BLOB Page>  page offset 00000006, page type <Uncompressed BLOB Page>  page offset 00000007, page type <Uncompressed BLOB Page>  Total number of page: 8:  Insert Buffer Bitmap: 1  Uncompressed BLOB Page: 4  File Space Header: 1  B-tree Node: 1  File Segment inode: 1</code></pre>    <p>可以看出,第4页的 <B-tree Node>, page level <0000> 格式为数据页,存放着MySQL的行数据。 <Uncompressed BLOB Page> 可以理解为MySQL存放大数据的地方,暂且叫作外部存储页。Compact格式没有将大数据全部放在数据页中,而是将一部分数据放在了外部存储页中。那么,是全部数据在外部存储页中,还是一部分数据。假如是一部分数据,这一部分是多少呢?</p>    <p>我们使用 hexdump -Cv row.ibd 查看一下数据页 <B-tree Node>, page level <0000> ,也就是第4页:</p>    <pre>  <code class="language-sql">3073 0000c000  8c 25 17 57 00 00 00 03  ff ff ff ff ff ff ff ff  |.%.W....????????|  3074 0000c010  00 00 00 00 00 07 3a b8  45 bf 00 00 00 00 00 00  |......:?E?......|  3075 0000c020  00 00 00 00 00 02 00 02  03 a6 80 03 00 00 00 00  |.........?......|  3076 0000c030  00 7f 00 05 00 00 00 01  00 00 00 00 00 00 00 00  |................|  3077 0000c040  00 00 00 00 00 00 00 00  00 13 00 00 00 02 00 00  |................|  3078 0000c050  00 02 00 f2 00 00 00 02  00 00 00 02 00 32 01 00  |...?.........2..|  3079 0000c060  02 00 1c 69 6e 66 69 6d  75 6d 00 02 00 0b 00 00  |...infimum......|  3080 0000c070  73 75 70 72 65 6d 75 6d  14 c3 00 00 10 ff f1 00  |supremum.?...??.|  3081 0000c080  00 00 00 04 03 00 00 00  00 13 12 80 00 00 00 2d  |...............-|  3082 0000c090  01 10 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |..aaaaaaaaaaaaaa|  3083 0000c0a0  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|  3084 0000c0b0  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|  3085 0000c0c0  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|  ....  ....  3128 0000c370  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|  3129 0000c380  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|  3130 0000c390  61 61 00 00 00 02 00 00  00 04 00 00 00 26 00 00  |aa...........&..|  3131 0000c3a0  00 00 00 00 fc fc 00 00  00 00 00 00 00 00 00 00  |....??..........|  3132 0000c3b0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|  3133 0000c3c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|  3134 0000c3d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|  ...  ...  4093 0000ffc0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|  4094 0000ffd0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|  4095 0000ffe0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|  4096 0000fff0  00 00 00 00 00 70 00 63  01 a1 6c 2b 00 07 3a b8  |.....p.c.?l+..:?|</code></pre>    <p>我们可以看出,数据页中存储了一部分数据,算下来一共是768字节,然后剩余部分存储在外部存储页中。那么数据页与外部存储页、外部存储页与外部存储页是如何连接在一起的呢?</p>    <p>我们观察这一行:</p>    <pre>  <code class="language-sql">3130 0000c390  61 61 00 00 00 02 00 00  00 04 00 00 00 26 00 00  |aa...........&..|  3131 0000c3a0  00 00 00 00 fc fc 00 00  00 00 00 00 00 00 00 00  |................|</code></pre>    <p>这一行是前缀768字节的结尾。注意最后的20个字节:</p>    <ul>     <li>00 00 00 02:4字节,代表外部存储页所在的space id</li>     <li>00 00 00 04:4字节,代表第一个外部页的Page no</li>     <li>00 00 00 26:4字节,值为38,指向blob页的header</li>     <li>00 00 00 00 00 00 fc fc:8字节,代表该列存在外部存储页的总长度。此处的值为64764,加上前缀768正好是65532。(注意一点,虽然表示BLOB长度的是8字节,实际只有4个字节能使用,所有对于BLOB字段,存储数据的最大长度为4GB。)</li>    </ul>    <p>验证下第一个外部存储页的头部信息:</p>    <pre>  <code class="language-sql">4097 00010000  cd c3 b6 8e 00 00 00 04  00 00 00 00 00 00 00 00  |?ö.............|  4098 00010010  00 00 00 00 00 06 b8 a2  00 0a 00 00 00 00 00 00  |......??........|  4099 00010020  00 00 00 00 00 02 00 00  3f ca 00 00 00 05 61 61  |........??....aa|  4100 00010030  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|  ...  ...</code></pre>    <p>前38个字节为File Header(关于InnoDB数据页的详细结构请参见《MySQL技术内幕 InnoDB存储引擎》4.4),这个简单提一下:</p>    <ul>     <li>cd c3 b6 8e:4字节,该页的checksum。</li>     <li>00 00 00 04:4字节,页偏移,此页为表空间中的第5个页。</li>     <li>00 00 00 00:4字节,当前页的上一个页。此页为 <Uncompressed BLOB Page> ,所以没有上一页。</li>     <li>00 00 00 00:4字节,当前页的下一个页。此页为 <Uncompressed BLOB Page> ,所以没有下一页。</li>     <li>00 00 00 00 00 06 b8 a2:8字节,该页最后被修改的日志序列位置LSN。</li>     <li>00 0a:2字节,页类型,0x000A代表BLOB页。</li>     <li>00 00 00 00 00 00 00 00:8字节,略过。</li>     <li>00 00 00 02:页属于哪个表空间,此处指表空间的ID为2。</li>    </ul>    <p>之后是4字节的 00 00 3f ca ,这里的值为16330,代表此BLOB页的有效数据的字节数。 00 00 00 05 代表下一个BLOB页的page number。</p>    <p>我们看最后一个 <Uncompressed BLOB Page> ,第8个页:</p>    <pre>  <code class="language-sql">7169 0001c000  fa 78 9b 27 00 00 00 07  00 00 00 00 00 00 00 00  |?x.'............|  7170 0001c010  00 00 00 00 00 07 3a b8  00 0a 00 00 00 00 00 00  |......:?........|  7171 0001c020  00 00 00 00 00 02 00 00  3d 9e ff ff ff ff 61 61  |........=.????aa|  7172 0001c030  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|  7173 0001c040  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|  ...  ...</code></pre>    <p>最后一页的有效数据大小为0x00003d9e=15774,768+16330*3+15774 = 65532字节,符合初始插入数据的大小。</p>    <p>由于这是最后一个 <Uncompressed BLOB Page> ,所以指向下一个 <Uncompressed BLOB Page> 的指针为ff ff ff ff。</p>    <p>由此我们可以很清晰的看出数据页与BLOB页的连接关系(引用淘宝数据库月报上的一张图):</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/a37d6fcc7e78730b65b931e03cb1181c.png"></p>    <p>我们来再看一个比较有意思的例子。:</p>    <pre>  <code class="language-sql">CREATE TABLE `testblob` (    `blob1` blob NOT NULL,    `blob2` blob NOT NULL,    `blob3` blob NOT NULL,    `blob4` blob NOT NULL,    `blob5` blob NOT NULL,    `blob6` blob NOT NULL,    `blob7` blob NOT NULL,    `blob8` blob NOT NULL,    `blob9` blob NOT NULL,    `blob10` blob NOT NULL,    `blob11` blob NOT NULL  ) ENGINE=InnoDB DEFAULT CHARSET=latin1;    mysql> insert into testblob select repeat('a',1000),repeat('b',1000),repeat('c',1000),repeat('d',1000),repeat('e',1000),repeat('f',1000),repeat('g',1000),repeat('h',1000),repeat('i',1000),repeat('j',1000),repeat('k',1000);  ERROR 1030 (HY000): Got error 139 from storage engine</code></pre>    <p>我们建立一张新表,有11个blob字段。然后向每个字段插入1000字节的数据,MySQL会提示 ERROR 1030 (HY000): Got error 139 from storage engine ,什么意思呢?</p>    <p>InnoDB是以B+树来组织数据的,假如每一行数据都占据一整个Page页,那么B+树将退化为单链表,所以InnoDB规定了一个Page必须包含两行数据。也就是一行数据存储在Page上的大小大概为8000字节。</p>    <p>而上面的例子,一行数据有11个1000字节的数据,Page层肯定放不下,所以在Page层留下768*11=8448字节,已经超过了8000字节,所以MySQL会提示 ERROR 1030 (HY000): Got error 139 from storage engine 。我们很轻松的定义一个字段,来存储11000个字节,但是却无法将他们分成11个字段来存储,有点意思!</p>    <p>那么如何解决上面的问题呢?</p>    <ul>     <li>将行格式转为接下来要说的Dynamic格式。此种格式只用20字节指向外部存储空间。</li>     <li>将多个blob字段转为一个blob字段。多个字段可以用数组存储,然后json_encode打包进blob。</li>    </ul>    <p>我们向表中插入一条有效记录:</p>    <pre>  <code class="language-sql">mysql>  insert into testblob(blob1,blob2,blob3,blob4,blob5,blob6,blob7,blob8,blob9) select repeat('a',8000),repeat('b',8000),repeat('c',8000),repeat('d',8000),repeat('e',8000),repeat('f',8000),repeat('g',8000),repeat('h',8000),repeat('i',8000);  Query OK, 1 row affected (0.12 sec)  Records: 1  Duplicates: 0  Warnings: 0</code></pre>    <pre>  <code class="language-sql">[root@localhost mysql]# python py_innodb_page_info.py -v com/testblob.ibd  page offset 00000000, page type <File Space Header>  page offset 00000001, page type <Insert Buffer Bitmap>  page offset 00000002, page type <File Segment inode>  page offset 00000003, page type <B-tree Node>, page level <0000>  page offset 00000004, page type <Uncompressed BLOB Page>  page offset 00000005, page type <Uncompressed BLOB Page>  page offset 00000006, page type <Uncompressed BLOB Page>  page offset 00000007, page type <Uncompressed BLOB Page>  page offset 00000008, page type <Uncompressed BLOB Page>  page offset 00000009, page type <Uncompressed BLOB Page>  page offset 0000000a, page type <Uncompressed BLOB Page>  page offset 0000000b, page type <Uncompressed BLOB Page>  page offset 0000000c, page type <Uncompressed BLOB Page>  Total number of page: 13:  Insert Buffer Bitmap: 1  Uncompressed BLOB Page: 9  File Space Header: 1  B-tree Node: 1  File Segment inode: 1</code></pre>    <p>我们可以看出这一行数据有9个外部存储页,而我们一共就插入了9列数据,是不是当每一列的数据在page页放不下,都单独申请一个外部存储页,而互相之前不共享外部存储页。我们看一下page页的结构就知道了:</p>    <pre>  <code class="language-sql">3130 0000c390  61 61 61 61 61 61 61 61  61 61 61 61 61 61 61 61  |aaaaaaaaaaaaaaaa|   3131 0000c3a0  61 61 61 61 00 00 00 05  00 00 00 04 00 00 00 26  |aaaa...........&|  ...  ...   3180 0000c6b0  62 62 62 62 62 62 62 62  00 00 00 05 00 00 00 05  |bbbbbbbb........|   3181 0000c6c0  00 00 00 26 00 00 00 00  00 00 1c 40 63 63 63 63  |...&.......@cccc|  ...  ...   3229 0000c9c0  63 63 63 63 63 63 63 63  63 63 63 63 00 00 00 05  |cccccccccccc....|   3230 0000c9d0  00 00 00 06 00 00 00 26  00 00 00 00 00 00 1c 40  |.......&.......@|  ...  ...</code></pre>    <p>根据前面的分析,我们现在可以看出,外部存储页是不共享的,即使一个列的数据多出一个字节,这一个字节也是独占一个16KB空间的大小,这很浪费存储空间。(当然,这对现代计算机可能不是问题,呵呵)。</p>    <p>说了这么多,总结下Compact格式存储大数据的缺点:</p>    <ul>     <li>由于存在768字节的前缀在Page页,所以会存在能定义一个字段,存储11000字节,但是不能定义11个字段,每个字段存储1000字节的"bug"。</li>     <li>外部存储页不共享,即使多余一个字节也是独享16KB的页面。</li>    </ul>    <h3>Dynamic行格式</h3>    <p>接着我们首先看一下行格式为Dynamic是如何存储大数据的:</p>    <pre>  <code class="language-sql">mysql> select version();  +-----------+  | version() |  +-----------+  | 5.7.14    |  +-----------+  1 row in set (0.00 sec)    mysql> show table status like 'row'\G;  *************************** 1. row ***************************             Name: row           Engine: InnoDB          Version: 10       Row_format: Dynamic             Rows: 0   Avg_row_length: 0      Data_length: 16384  Max_data_length: 0     Index_length: 0        Data_free: 0   Auto_increment: NULL      Create_time: 2017-01-03 22:45:16      Update_time: NULL       Check_time: NULL        Collation: latin1_swedish_ci         Checksum: NULL   Create_options:          Comment:  1 row in set (0.00 sec)</code></pre>    <p>创建和compact格式一样的表:</p>    <pre>  <code class="language-sql">CREATE TABLE `row` (    `content` varchar(65532) NOT NULL DEFAULT ''  ) ENGINE=InnoDB DEFAULT CHARSET=latin1    insert into row(content) select repeat('a',65532);  Query OK, 1 row affected (0.03 sec)  Records: 1  Duplicates: 0  Warnings: 0</code></pre>    <p>看下页分布:</p>    <pre>  <code class="language-sql">[root@localhost mysql]# python py_innodb_page_info.py -v row.ibd   page offset 00000000, page type <File Space Header>  page offset 00000001, page type <Insert Buffer Bitmap>  page offset 00000002, page type <File Segment inode>  page offset 00000003, page type <B-tree Node>, page level <0000>  page offset 00000004, page type <Uncompressed BLOB Page>  page offset 00000005, page type <Uncompressed BLOB Page>  page offset 00000006, page type <Uncompressed BLOB Page>  page offset 00000007, page type <Uncompressed BLOB Page>  page offset 00000008, page type <Uncompressed BLOB Page>  Total number of page: 9:  Insert Buffer Bitmap: 1  Uncompressed BLOB Page: 5  File Space Header: 1  B-tree Node: 1  File Segment inode: 1</code></pre>    <p>第4页是数据页,第5-9页是二进制页。我们直接看磁盘中第4页的数据:</p>    <pre>  <code class="language-sql">3073 0000c000  dc 2d b0 f5 00 00 00 03  ff ff ff ff ff ff ff ff  |.-..............|  3074 0000c010  00 00 00 00 00 a3 4b 59  45 bf 00 00 00 00 00 00  |......KYE.......|  3075 0000c020  00 00 00 00 00 36 00 02  00 a6 80 03 00 00 00 00  |.....6..........|  3076 0000c030  00 7f 00 05 00 00 00 01  00 00 00 00 00 00 00 00  |................|  3077 0000c040  00 00 00 00 00 00 00 00  00 64 00 00 00 36 00 00  |.........d...6..|  3078 0000c050  00 02 00 f2 00 00 00 36  00 00 00 02 00 32 01 00  |.......6.....2..|  3079 0000c060  02 00 1c 69 6e 66 69 6d  75 6d 00 02 00 0b 00 00  |...infimum......|  3080 0000c070  73 75 70 72 65 6d 75 6d  14 c0 00 00 10 ff f1 00  |supremum........|  3081 0000c080  00 00 00 02 00 00 00 00  00 07 07 a7 00 00 01 1b  |................|  3082 0000c090  01 10 00 00 00 36 00 00  00 04 00 00 00 26 00 00  |.....6.......&..|  3083 0000c0a0  00 00 00 00 ff fc 00 00  00 00 00 00 00 00 00 00  |................|  3084 0000c0b0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|  3085 0000c0c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|  3086 0000c0d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|  3087 0000c0e0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|  ...  ...  ...</code></pre>    <p>和Compact格式有着明显的不同,当大数据在Page页存放不下时,Dynamic行格式不会留768字节在Page页,并且将全部大数据都放在外部存储页。具体的数据页和外部存储页的连接关系同Compact格式一样。</p>    <p>我们再看看Dynamic格式的外部存储页是不是每一个列独享外部存储空间,还是同Compact格式实验过程一样:</p>    <pre>  <code class="language-sql">CREATE TABLE `testblob` (    `blob1` blob NOT NULL,    `blob2` blob NOT NULL,    `blob3` blob NOT NULL,    `blob4` blob NOT NULL,    `blob5` blob NOT NULL,    `blob6` blob NOT NULL,    `blob7` blob NOT NULL,    `blob8` blob NOT NULL,    `blob9` blob NOT NULL,    `blob10` blob NOT NULL,    `blob11` blob NOT NULL  ) ENGINE=InnoDB DEFAULT CHARSET=latin1;    mysql>   insert into testblob(blob1,blob2,blob3,blob4,blob5,blob6,blob7,blob8,blob9,blob10,blob11) select repeat('a',8000),repeat('b',8000),repeat('c',8000),repeat('d',8000),repeat('e',8000),repeat('f',8000),repeat('g',8000),repeat('h',8000),repeat('i',8000),repeat('j',8000),repeat('k',8000);  Query OK, 1 row affected (0.10 sec)  Records: 1  Duplicates: 0  Warnings: 0</code></pre>    <p>看一下外部存储页数据:</p>    <pre>  <code class="language-sql">4599 00011f60  61 61 61 61 61 61 61 61  61 61 61 61 61 61 00 00  |aaaaaaaaaaaaaa..|   4600 00011f70  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|</code></pre>    <p>好的,可以不用向下看其他列的了,Dynamic的外部存储页也不是共享的。</p>    <p>但是MySQL为什么要这么设计呢?可能是为了实现简单吧,沿着链表通过有效数据大小就能读取blob的全部数据。假如多个字段的blob混在一起,可能设计更复杂,要更新每个字段的偏移量之类的,更新的话页数据管理也比较麻烦。我的个人猜测,呵呵。</p>    <p>总结下Dynamic格式存储大数据的特点:</p>    <ul>     <li>当数据页放不下时,MySQL会将大数据全部放在外部存储页,数据页只留指向外部存储页的指针。</li>     <li>外部存储页不共享,即使多余一个字节也是独享16KB的页面。</li>    </ul>    <h3>将列放入外部存储页的标准</h3>    <p>当一行中的数据不能在数据页中放下,需要申请外部存储页时,MySQL需要决定将哪一列的数据放到外部存储页,遵循的规则如下:</p>    <ul>     <li>长度固定的字段不会被放到外部存储页(int、char(N)等)</li>     <li>长度小于20字节的字段不会被放到外部存储页。(假如放到外部存储页,不仅会单独占据16KB,还要额外的20字节指针,没有必要)</li>     <li>对于Compact和REDUNDANT格式的行数据,长度小于768字节的字段不会被放到外部存储页。(这个原因很显然,本来就不够768字节的前缀,总不能生搬硬凑吧)。</li>    </ul>    <p>当有多个大数据字段满足上面条件,需要被放到外部存储页时,MySQL会优先选择大的字段放到外部存储页,因为这样可以最大限度的省下数据页的空间,使得更多的字段能够被放到数据页。</p>    <p>由于有较多的实验过程,所以显得比较乱,建议看到这篇文章人自己实践一遍,毕竟自己动手会思考更多的问题与细节,理解的也比较深刻,哈哈哈。</p>    <p>参考资料: <a href="/misc/goto?guid=4959736347158162765" rel="nofollow,noindex">http://dev.mysql.com/doc/refman/5.7/en/column-count-limit.html</a></p>    <p><a href="/misc/goto?guid=4959736347245761665" rel="nofollow,noindex">http://mysqlserverteam.com/externally-stored-fields-in-innodb/</a></p>    <p><a href="/misc/goto?guid=4959736347341615212" rel="nofollow,noindex">https://www.percona.com/blog/2010/02/09/blob-storage-in-innodb/</a></p>    <p><a href="/misc/goto?guid=4958987955502305550" rel="nofollow,noindex">http://mysql.taobao.org/monthly/2016/02/01/</a></p>    <p> </p>    <p>来自:https://github.com/zhangyachen/zhangyachen.github.io/issues/96</p>    <p> </p>    
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MySQL 大数据 数据库服务器