Fault analysis | from data_ Free exception

author ： Yang Qilong

Net name “ The north is in the south ”, Senior DBA, Mainly responsible for database architecture design and operation and maintenance platform development , Good at database performance tuning 、 Troubleshooting .

In this paper, the source ： Original contribution

* Produced by aikesheng open source community , Original content is not allowed to be used without authorization , For reprint, please contact the editor and indicate the source .

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One Preface

A customer feedback query database found information_schema.tables Of data_free Value burst exception , achieve 13G about . Pictured ：

It is necessary to find out what causes , This article combs the process of troubleshooting and the methods to solve problems .

Two screening

2.1 analysis

First data_free The meaning is Table space ibd After the file is written and deleted , The amount of space left without recycling .

Let the students check the primary and standby databases at the same time , Compare the file size and configuration . Find the main library of data_free The value is 13G about , The standby database is normal .

From the result, we can guess that it is related to some request actions on the main database , Void is MySQL because sql There is no result of automatic reclaiming of the space requested to be allocated for writing . Based on the information given by the front line , No other ideas , Look at the screenshot sent by the front line :

Accident from Screenshot's ibtmp1 File size found some clues , Screenshots show ibtmp1 The file size is also 13G , The standby database is the initial value .

Ignore the red arrow , see ibtmp1 File size is 13G , There seems to be some clue ,data_free Whether and ibtmp1 of .

2.2 Verify the conjecture

Use sysbench Create test table sbtest1 , structure 2w Bar record , Then create sbtest2 , take sbtest1 The data of Import to sbtest2 . Why do you do this , It will be explained later .

mysql > show  variables like 'innodb_temp_data_file_path';
+----------------------------+-----------------------+
| Variable_name              | Value                 |
+----------------------------+-----------------------+
| innodb_temp_data_file_path | ibtmp1:12M:autoextend |
+----------------------------+-----------------------+
1 row in set (0.00 sec)

View physics ibtmp1 file size :

[[email protected] data]# du -sm ibtmp1
12	ibtmp1

Communicate test cases , Let the system automatically generate temporary tables

mysql  > create table sbtest2 like sbtest1;
Query OK, 0 rows affected (0.01 sec)
mysql > insert into sbtest2(k,c,pad) select k,c ,pad from sbtest1;
Query OK, 200000 rows affected (1.18 sec)
Records: 200000  Duplicates: 0  Warnings: 0
mysql > insert into sbtest2(k,c,pad) select k,c ,pad from sbtest1;
Query OK, 200000 rows affected (1.06 sec)
mysql > insert into sbtest2(k,c,pad) select k,c ,pad from sbtest2;
Query OK, 400000 rows affected (2.49 sec)
Records: 400000  Duplicates: 0  Warnings: 0
mysql > insert into sbtest2(k,c,pad) select k,c ,pad from sbtest2;
Query OK, 800000 rows affected (6.18 sec)
Records: 800000  Duplicates: 0  Warnings: 0

Check again ibtmp1 file size 204MB

[[email protected] data]# du -sm ibtmp1
204	ibtmp1

mysql > SELECT FILE_NAME, TABLESPACE_NAME, ENGINE, INITIAL_SIZE, TOTAL_EXTENTS*EXTENT_SIZE
    ->        AS TotalSizeBytes, DATA_FREE, MAXIMUM_SIZE FROM INFORMATION_SCHEMA.FILES
    ->        WHERE TABLESPACE_NAME = 'innodb_temporary'\G
*************************** 1. row ***************************
      FILE_NAME: ./ibtmp1
TABLESPACE_NAME: innodb_temporary
         ENGINE: InnoDB
   INITIAL_SIZE: 12582912
 TotalSizeBytes: 213909504
      DATA_FREE: 207618048  ##  Corresponding to the physical file size 
   MAXIMUM_SIZE: NULL
1 row in set (0.00 sec)

see I_S.tables Of data_free Value :

see insert select from table Temporary tables are indeed used during execution .

mysql > explain insert into sbtest2(k,c,pad) select k,c ,pad from sbtest2\G
*************************** 1. row ***************************
..
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: sbtest2
   partitions: NULL
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 1578168
     filtered: 100.00
        Extra: Using temporary  ## 
2 rows in set (0.00 sec)
Records: 200000  Duplicates: 0  Warnings: 0

thus , An instance of a customer can be identified because some SQL The system temporary table space is occupied during the process , After use, the temporary table space is not recycled, resulting in the initial problem . Now let's learn more about MySQL Knowledge of temporary tables .

3、 ... and Temporary table space

3.1 Introduce

ibtmp1 Is uncompressed innodb Separate tablespaces for temporary tables , adopt innodb_temp_data_file_path Parameter specifies the path to the file , File name and size , Default configuration is ibtmp1:12M:autoextend, If no location is specified , The temporary table space will be created to innodb_data_home_dir Specified path .

It should be noted that : By default , The size of this file can grow indefinitely . and 5.7 The version will not follow SQL At the end of the statement, the temporary table space is actively reclaimed , Security risks resulting in insufficient space resources .

3.2 When will temporary tables be used

When explain View the results of the execution plan extra In the column , If you include Using Temporary It means that temporary tables will be used , For example, the following common situations will usually be used ：

1. insert into tab1 select … from tab2 .
2. group by No indexed fields or group by order by The fields are different .
3. distinct The value of and group by The values are different , Cannot take advantage of sparse indexes .

Others are welcome to add .

3.3 Parameters and metadata related to temporary tables

5.7 edition :

innodb_temp_data_file_path
default_tmp_storage_engine 
internal_tmp_disk_storage_engine 

8.0 Versions are divided into session level and global level temporary table spaces

innodb_temp_tablespaces_dir # Specify session level creation of temporary tables to BASEDIR/data/#innodb_temp
innodb_temp_data_file_path #  Global variables 
internal_tmp_disk_storage_engine 

Temporary tables created by users can be queried INFORMATION_SCHEMA.INNODB_TEMP_TABLE_INFO

mysql > CREATE TEMPORARY TABLE t1 (c1 INT PRIMARY KEY) ENGINE=INNODB;
Query OK, 0 rows affected (0.00 sec)

mysql  > SELECT * FROM INFORMATION_SCHEMA.INNODB_TEMP_TABLE_INFO\G
*************************** 1. row ***************************
            TABLE_ID: 54
                NAME: #sqlfd5_b_0
              N_COLS: 4
               SPACE: 36
PER_TABLE_TABLESPACE: FALSE
       IS_COMPRESSED: FALSE
1 row in set (0.00 sec)

MySQL In execution sql Tables created by the optimizer in the process , They can't get through INFORMATION_SCHEMA.INNODB_TEMP_TABLE_INFO Direct view . For example, the case of this article .

3.4 How to solve ibtmp1 The problem of file space occupation

1. Universal restart method , Find the right time , Switch database , Restart the old main database .

2. By configuring innodb_temp_data_file_path control ibtmp1 Maximum value of file , Avoid unlimited increase in table space size .

   innodb_temp_data_file_path = ibtmp1:12M:autoextend:max:10G

   12M Is the initial size of the file ,10G Is the maximum value of the file , If the maximum value is exceeded, the system will prompt an error

   ERROR 1114 (HY000): The table ‘/data/msb_5_7_31/tmp#sql_xxxxx_0’ is full

Reference article

https://dev.mysql.com/doc/refman/5.7/en/innodb-temporary-tablespace.html

https://dev.mysql.com/doc/refman/8.0/en/innodb-temporary-tablespace.html