Cache pre-loading on mysqld startup

The following quirky dynamic SQL will scan each index of each table so that they’re loaded into the key_buffer (MyISAM) or innodb_buffer_pool (InnoDB). If you also use the PBXT engine which does have a row cache but no clustered primary key, you could also incorporate some full table scans.

To make mysqld execute this on startup, create /var/lib/mysql/initfile.sql and make it be owned by mysql:mysql

SET SESSION group_concat_max_len=100*1024*1024;
SELECT GROUP_CONCAT(CONCAT('SELECT COUNT(`',column_name,'`) FROM `',table_schema,'`.`',table_name,'` FORCE INDEX (`',index_name,'`)') SEPARATOR ' UNION ALL ') INTO @sql FROM information_schema.statistics WHERE table_schema NOT IN ('information_schema','mysql') AND seq_in_index = 1;
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET SESSION group_concat_max_len=@@group_concat_max_len;

and in my.cnf add a line in the [mysqld] block

init-file = /var/lib/mysql/initfile.sql

That’s all. mysql reads that file on startup and executes each line. Since we can do the whole select in a single (admittedly quirky) query and then use dynamic SQL to execute the result, we don’t need to create a stored procedure.

Of course this kind of simplistic “get everything” only really makes sense if the entire dataset+indexes fit in memory, otherwise you’ll want to be more selective. Still, you could use the above as a basis, perhaps using another table to provide a list of tables/indexes to be excluded – or if the schema is really stable, simply have a list of tables/indexes to be included instead of dynamically using information_schema.

Practical (albeit niche) application:

In a system with multiple slaves, adding in a new slave makes it start with cold caches, but since with loadbalancing it will pick up only some of the load it often works out ok. However, some environments have dual masters but the application is not able to do read/write splits to utilise slaves. In that case all the reads also go to the active master. Consequentially, the passive master will have relatively cold caches (only rows/indexes that have been updated will be in memory) so in case of a failover the amount of disk reads for the many concurrent SELECT queries will go through the roof – temporarily slowing the effective performance to a dismal crawl: each query takes longer with the required additional disk access so depending on the setup the server may even run out of connections which in turn upsets the application servers. It’d sort itself out but a) it looks very bad on the frontend and b) it may take a number of minutes.

The above construct prevents that scenario, and as mentioned it can be used as a basis to deal with other situations. Not many people know about the init-file option, so this is a nice example.

If you want to know how the SQL works, read on. The original line is very long so I’ll reprint it below with some reformatting:

SELECT GROUP_CONCAT(CONCAT(
  'SELECT COUNT(`',column_name,'`)
          FROM `',table_schema,'`.`',table_name,
          '` FORCE INDEX (`',index_name,'`)'
       ) SEPARATOR ' UNION ALL ')
  INTO @sql
  FROM information_schema.statistics
  WHERE table_schema NOT IN ('information_schema','mysql')
  AND seq_in_index = 1;

The outer query grabs each regular db/table/index/firstcol name that exists in the server, writing out a SELECT query that counts all not-NULL values of the indexed column (so it must scan the index), forcing that specific index. We then abuse the versatile and flexible GROUP_CONCAT() function to glue all those SELECTs together, with “UNION ALL” inbetween. The result is a single very long string, so we need to tweak the maximum allowed group_concat output beforehand to prevent truncation.

2 Responses to “Cache pre-loading on mysqld startup”

  1. Have you tested it with innodb plugin?

    Presumably these queries only load the data into the old sublist of the LRU:
    http://dev.mysql.com/doc/refman/5.1/en/innodb-parameters.html#sysvar_innodb_old_blocks_pct / http://dev.mysql.com/doc/refman/5.1/en/innodb-buffer-pool.html

  2. Hey Morgan!
    The two clients we developed this for are still on 5.0 (OurDelta builds) and also have the special case that all their data/indexes can fit in memory (in one case due to the fact that the masters run on very serious boxes).
    Since they were our priority we haven’t yet tested it on a recent 5.1, but yes I would expect all this stuff to end up in the old sublist since it only reads them in once and then doesn’t touch until the regular db operations happen to use some. So the innodb_old_blocks_pct might need to be adjusted otherwise they’ll fall straight out again even with a large buffer pool. But if the entire dataset fits in RAM, things should just be tuned to take that into account.

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