I don’t think it’ll surprise anybody if I say that MariaDB or MySQL server knows a lot more about server system variables, then just their values. Indeed, every variable can be session or global only, read-only or writable, it has an associated help text (that is printed on mysqld --help --verbose), certain variables only accept values from a given set of strings (this set of allowed values is also printed in mysqld --help --verbose since MariaDB 10.1.0), numeric variables have lower and upper range boundaries of valid values (that are never printed anywhere), and so on. I always thought it’s kind of a waste that there is no way to query this information. That could’ve been very convenient, in particular for various GUI clients — they could show the help in tooltips, validate values and so on.

But recently we’ve got our users asking for it — precisely, for system variable metadata, whether a variable is read-only, how it was set and so on. You ask for it — you got it. Let me introduce the INFORMATION_SCHEMA.SYSTEM_VARIABLES table. For every system variable it shows both its global and session values, where the global value comes from (config file or a command line, assigned from SQL, auto-configured by the server, compile-time default), the type and the scope of the variable, its default value, range of values for numeric variables, set of allowed values for ENUM/SET variables, whether a variable is read-only, whether it can be set from the command line. And a help text, of course.
It is said that a picture is worth a thousand words, so here you are:

Note that READ_ONLY only means “cannot be changed run-time”, the variable might still be writable from the command line or a config file. Here’s the list of all truly read-only variables that can not be modified at all:

You might wonder what LOG_BIN is doing in this list. It’s because on the command line you can never specify this boolean read-only variable, what you actually specify is a command-line option --log-bin that takes a string. It happens that this new SYSTEM_VARIABLES table is good at highlighting dark corners in the MariaDB/MySQL internal implementation of system variables.

Introduction

When you e.g. delete rows, these rows are just marked as deleted not really physically deleted from indexes and free space introduced is not returned to operating system for later reuse. Purge thread will physically delete index keys and rows, but still free space introduced is not returned to operating system and this operation can lead holes on page. If you have variable length rows, this could lead to situation where this free space can’t be used for new rows (if these rows are larger than old ones). User may use OPTIMIZE TABLE or ALTER TABLE <table> ENGINE=InnoDB to reconstruct the table.

Unfortunately, running OPTIMIZE TABLE against an InnoDB table stored in the shared table-space file ibdata1 does two things:

  • Makes the table’s data and indexes contiguous inside ibdata1
  • Makes ibdata1 grow because the contiguous data and index pages are appended to ibdata1

New defragmentation

In MariaDB 10.1 we have merged Facebooks defragmentation code prepared for MariaDB by Matt, Seong Uck Lee from Kakao. Only major difference to Facebooks code and Matt’s patch is the fact that in MariaDB we decided not to introduce new literals to SQL and no changes to server code. Instead we use already existing OPTIMIZE TABLE and all code changes are inside InnoDB/XtraDB storage engines. To enable this new feature you need to add following to my.cnf (this requirement is to keep the original behavior of OPTIMIZE TABLE for those users that need it).

This new defragmentation feature works inplace, thus no new tables are created and there is no need to copy data from old table to new table. Instead this feature loads n pages and tries to move records so that pages would be full of records and frees pages that are fully empty after the operation.

New configuration variables

  • innodb_defragment: Enable/disable InnoDB defragmentation. When set to FALSE, all existing defragmentation will be paused. And new defragmentation command will fail. Paused defragmentation commands will resume when this variable is set to TRUE. Default value FALSE.
  • innodb_defragment_n_pages: Number of pages considered at once when merging multiple pages to defragment. Range of 2–32 and default is 7.
  • innodb_defragment_stats_accuracy: How many defragment stats changes there are before the stats are written to persistent storage. Set to 0 meaning disable defragment stats tracking. Default 0.
  • innodb_defragment_fill_factor_n_recs:  How many records of space defragmentation should leave on the page. This variable, together with innodb_defragment_fill_factor, is introduced so defragmentation won’t pack the page too full and cause page split on the next insert on every page. The variable indicating more defragmentation gain is the one effective. Range of 1–100 and default 20.
  • innodb_defragment_fill_factor: A number between [0.7, 1] that tells defragmentation how full it should fill a page. Default is 0.9. Number below 0.7 won’t make much sense. This variable, together with innodb_defragment_fill_factor_n_recs, is introduced so defragmentation won’t pack the page too full and cause page split on the next insert on every page. The variable indicating more defragmentation gain is the one effective.
  • innodb_defragment_frequency: Do not defragment a single index more than this number of time per second.This controls the number of time defragmentation thread can request X_LOCK on an index. Defragmentation thread will check whether 1/defragment_frequency (s) has passed since it worked on this index last time, and put the index back to the queue if not enough time has passed. The actual frequency can only be lower than this given number.

New status variables

  • Innodb_defragment_compression_failures: Number of defragment re-compression failures
  • Innodb_defragment_failures: Number of defragment failures.
  • Innodb_defragment_count: Number of defragment operations.

Example

After CREATE TABLE and INSERT operations we can see following from INFORMATION_SCHEMA:

Now if we delete 3/4 of the records that will leave holes in pages and then we optimize table to execute defragmentation:

After this we can see that some pages are freed and some pages merged:

Links

WebScaleSQL Git repository https://github.com/webscalesql/webscalesql-5.6

Facebook Percona Live presentation: https://www.google.fi/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CCQQFjAB&url=https%3A%2F%2Fwww.percona.com%2Flive%2Fmysql-conference-2014%2Fsites%2Fdefault%2Ffiles%2Fslides%2Fdefragmentation.pdf&ei=UgNKVNnZMcHhywP7qwI&usg=AFQjCNGREUpen21jCcy0bchUa6Ro83ol_A&sig2=MDZU2Ue9sX1kB9OusvdiFA

Introduction

Online DDL is a new feature in MariaDB 10.0. Online DDL is processed through below 4 tasks in sequence.

  1. InnoDB::ha_prepare_inplace_alter_table(..)
  2. InnoDB::ha_inplace_alter_table(..)
  3. InnoDB::ha_commit_inplace_alter_table(..)
  4. mysql_rename_table(..)

InnoDB storage engine allocates temporal memory buffer for transaction logging in phase 1 where row changes during this phase are logged. Size of this buffer is at start sort_buffer_size and it can be grown up to innodb_online_alter_log_max size. During phase 2 thread processing the ALTER statement will copy old table’s rows to a new altered table. After this MariaDB will take exclusive lock for target table and applies row log buffer to the new altered table.

This introduces a new unpredictable failure case row log buffer overflow. MariaDB server will rollback ALTER statement if row log buffer overflows. Thus, there is following problems:

  • If row log buffer size is too small the ALTER statement is rolled back and you have wasted precious time and resources.
  • If row log buffer is too big, you have wasted precious main-memory that could be used e.g. for buffer pool.
  • Currently, there is no way to see how much row log buffer is used and how much there is free space.
  • Currently, there is not even estimate how much work has been done and how much there is till to be done.
  • Currently, merge sort phase could also take a long time and there is no progress information.

Improvements

There is two improvements in MariaDB 10.1: new status variables and progress information for online DDL.

New status variables and progress info

MariaDB Corporation would like to thank Matt, Seong Uck Lee from Kakao for contributing a patch that has now merged to MariaDB 10.1.

First of all there is three new global status variables.

  • Innodb_onlineddl_rowlog_rows: Shows how many rows is stored in the row log buffer.
  • Innodb_onlineddl_rowlog_pct_used: Shows row log buffer usage in 5-digit integer  (10000 means 100.00% ).
  • Innodb_onlineddl_pct_progress: Shows the progress of in-place alter table. It might be not so accurate because in-place alter is highly dependent on disk and buffer pool status.

Lets consider as an example where we have InnoDB table containing 150000 rows and we try to add a new column.

Concurrently, if we add new row, update some rows and delete few rows

This means that at the time of status statement there were 2003 rows on row log, 23\% of memory allocated for row log is used, and online alter table has completed 56.77\% of it’s work.

There is also additional output at error log, as example:

Merge sort progress

Additionally, show processlist statement will output estimate of index merge sort progress, e.g.

Links

http://seonguck.blogspot.kr/2014/09/what-is-problem-of-mysql-online-ddl.html
http://kakao-dbe.blogspot.kr/2014/09/mysql-status-variables-for-innodb.html