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6.4.6.6 Restructuring a Key Cache - MySQL

While optimization is possible with limited knowledge of your system or application, the more you know about your system, the better your optimization will be. This article, the second of two parts, covers some of the different points you will need to know for optimizing MySQL. It is excerpted from chapter six of the book MySQL Administrator's Guide, by MySQL AB (Sams, 2004; ISBN: 0672326345).

TABLE OF CONTENTS:
  1. MySQL Optimization, part 2
  2. 6.4 Optimizing Database Structure
  3. 6.4.5 How MySQL Uses Indexes
  4. 6.4.6.1 Shared Key Cache Access
  5. 6.4.6.6 Restructuring a Key Cache
  6. 6.5.2 Tuning Server Parameters
  7. 6.5.4 How MySQL Uses Memory
  8. 6.6.1.1 Using Symbolic Links for Databases on Unix
By: Sams Publishing
Rating: starstarstarstarstar / 32
April 20, 2005

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A key cache can be restructured at any time by updating its parameter values. For example:

mysql> SET GLOBAL cold_cache.key_buffer_size=4*1024*1024;

If you assign to either the key_buffer_size or key_cache_block_size key cache component a value that differs from the component's current value, the server destroys the cache's old structure and creates a new one based on the new values. If the cache contains any dirty blocks, the server saves them to disk before destroying and re-creating the cache. Restructuring does not occur if you set other key cache parameters.

When restructuring a key cache, the server first flushes the contents of any dirty buffers to disk. After that, the cache contents become unavailable. However, restructuring does not block queries that need to use indexes assigned to the cache. Instead, the server directly accesses the table indexes using native filesystem caching. Filesystem caching is not as efficient as using a key cache, so although queries will execute, a slowdown can be anticipated. Once the cache has been restructured, it becomes available again for caching indexes assigned to it, and the use of filesystem caching for the indexes ceases.

6.4.7 How MySQL Counts Open Tables

When you execute a mysqladmin status command, you'll see something like this:

Uptime: 426 Running threads: 1 Questions: 11082
Reloads: 1 Open tables: 12

The Open tables value of 12 can be somewhat puzzling if you have only six tables.

MySQL is multi-threaded, so there may be many clients issuing queries for a given table simultaneously. To minimize the problem with multiple client threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory but normally increases performance. With MyISAM tables, one extra file descriptor is required for the data file for each client that has the table open. (By contrast, the index file descriptor is shared between all threads.) The ISAM storage engine shares this behavior.

You can read more about this topic in the next section. See Section 6.4.8, "How MySQL Opens and Closes Tables."

6.4.8 How MySQL Opens and Closes Tables

The table_cache, max_connections, and max_tmp_tables system variables affect the maximum number of files the server keeps open. If you increase one or more of these values, you may run up against a limit imposed by your operating system on the per-process number of open file descriptors. Many operating systems allow you to increase the open-files limit, although the method varies widely from system to system. Consult your operating system documentation to determine whether it is possible to increase the limit and how to do so.

table_cache is related to max_connections. For example, for 200 concurrent running connections, you should have a table cache size of at least 200 * N, where N is the maximum number of tables in a join. You also need to reserve some extra file descriptors for temporary tables and files.

Make sure that your operating system can handle the number of open file descriptors implied by the table_cache setting. If table_cache is set too high, MySQL may run out of file descriptors and refuse connections, fail to perform queries, and be very unreliable. You also have to take into account that the MyISAM storage engine needs two file descriptors for each unique open table. You can increase the number of file descriptors available for MySQL with the --open-files-limit startup option to mysqld_safe. See Section A.2.17, "File Not Found."

The cache of open tables will be kept at a level of table_cache entries. The default value is 64; this can be changed with the --table_cache option to mysqld. Note that MySQL may temporarily open even more tables to be able to execute queries.

An unused table is closed and removed from the table cache under the following circumstances:

  • When the cache is full and a thread tries to open a table that is not in the cache.

  • When the cache contains more than table_cache entries and a thread is no longer using a table.

  • When a table flushing operation occurs. This happens when someone issues a FLUSH TABLES statement or executes a mysqladmin flush-tables or mysqladmin refresh command.

When the table cache fills up, the server uses the following procedure to locate a cache entry to use:

  • Tables that are not currently in use are released, in least recently used order.

  • If a new table needs to be opened, but the cache is full and no tables can be released, the cache is temporarily extended as necessary.

When the cache is in a temporarily extended state and a table goes from a used to unused state, the table is closed and released from the cache.

A table is opened for each concurrent access. This means the table needs to be opened twice if two threads access the same table or if a thread accesses the table twice in the same query (for example, by joining the table to itself). Each concurrent open requires an entry in the table cache. The first open of any table takes two file descriptors: one for the data file and one for the index file. Each additional use of the table takes only one file descriptor, for the data file. The index file descriptor is shared among all threads.

If you are opening a table with the HANDLER tbl_name OPEN statement, a dedicated table object is allocated for the thread. This table object is not shared by other threads and is not closed until the thread calls HANDLER tbl_name CLOSE or the thread terminates. When this happens, the table is put back in the table cache (if the cache isn't full).

You can determine whether your table cache is too small by checking the mysqld status variable Opened_tables:

mysql> SHOW STATUS LIKE 'Opened_tables';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Opened_tables | 2741  |
+---------------+-------+

If the value is quite big, even when you haven't issued a lot of FLUSH TABLES statements, you should increase your table cache size. See Section 4.2.3, "Server System Variables," and Section 4.2.4, "Server Status Variables."

6.4.9 Drawbacks to Creating Many Tables in the Same Database

If you have many MyISAM or ISAM tables in a database directory, open, close, and create operations will be slow. If you execute SELECT statements on many different tables, there will be a little overhead when the table cache is full, because for every table that has to be opened, another must be closed. You can reduce this overhead by making the table cache larger.

6.5 Optimizing the MySQL Server

6.5.1 System Factors and Startup Parameter Tuning

We start with system-level factors, because some of these decisions must be made very early to achieve large performance gains. In other cases, a quick look at this section may suffice. However, it is always nice to have a sense of how much can be gained by changing things at this level.

The default operating system to use is very important! To get the best use of multiple-CPU machines, you should use Solaris (because its threads implementation works really well) or Linux (because the 2.2 kernel has really good SMP support). Note that older Linux kernels have a 2GB filesize limit by default. If you have such a kernel and a desperate need for files larger than 2GB, you should get the Large File Support (LFS) patch for the ext2 filesystem. Other filesystems such as ReiserFS and XFS do not have this 2GB limitation.

Before using MySQL in production, we advise you to test it on your intended platform.

Other tips:

  • If you have enough RAM, you could remove all swap devices. Some operating systems will use a swap device in some contexts even if you have free memory.

  • Use the --skip-external-locking MySQL option to avoid external locking. This option is on by default as of MySQL 4.0. Before that, it is on by default when compiling with MIT-pthreads, because flock() isn't fully supported by MIT-pthreads on all platforms. It's also on by default for Linux because Linux file locking is not yet safe.

  • Note that the --skip-external-locking option will not affect MySQL's functionality as long as you run only one server. Just remember to take down the server (or lock and flush the relevant tables) before you run myisamchk. On some systems this option is mandatory, because the external locking does not work in any case.

  • The only case when you can't use --skip-external-locking is if you run multiple MySQL servers (not clients) on the same data, or if you run myisamchk to check (not repair) a table without telling the server to flush and lock the tables first.

  • You can still use LOCK TABLES/UNLOCK TABLES even if you are using --skip-external-locking.



 
 
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