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    pascal Guest

    [HowTo] Optimising MYSQL


    I’d like to share with the interworx-cp community my knowledge about tuning / tweaking mysql.

    First of all, sorry if my English is not perfect. Maybe some sentences may be difficult to understand. So do not hesitate to ask me for explanations :-p

    Secondly, the default INTEWORX-CP mysql setup works very fine and is setup for all common usage.
    DO NOT CHANGE your setting without understanding what you do.
    Make a backup of your my.cnf before editing the /etc/my.cnf

    My thread is only to help you to more well understand how tuning mysql.

    Optimising mysql is very well commented on the net, and you’ll find huge information on how to do this. There is never “best parameters”, the best parameters is those fits your needs, box hardware, mysql usage…
    So, I’ll not give the best parameters but rather how to define these ones. Make some tests, and you’ll quickly find your own parameters.

    I’ll give you at the end of this post some web pointers which may help you.

    There a lot of available parameters but only few one are very important to tweak your mysql box.

    The most important variables are (for me, and it is not exhaustive)
    - max_connections
    - wait_timeout
    - thread_cache_size
    - table_cache
    - key_buffer_size
    - query_cache_size
    - tmp_table_size

    First of all, how to find your variable, and the mysql usage ?

    from mysql :
    show variables;

    or from command line :
    mysqladmin variables
    from Mysql :
    show status;

    or from command line
    mysqladmin –i10 processlist extended-status

    >ps –axfu

    >vmstat 1


    To obtain the stat of your mysql server since it has been loaded, run mysqladmin processlist extended-status as mentionned above.

    1 - The two most important variables : Table_cache and Key_buffer_size

    * If Opened_tables is big, then your table_cache variable is probably
    too small.

    table_cache 64
    Open_tables 64
    Opened_tables 544468

    This is the first serious problem. "The table_cache is the number of open
    tables for all threads. MySQL, being multi-threaded, may be running many
    queries on the table at one time, and each of these will open a table."
    Therefore, even though we only have a few tables, we will need many more

    The Opened_tables value is high and shows the number of
    cache misses. Getting the table_cache size correct is one of the two best
    things you can do to improve performance.

    * If Key_reads is big, then your key_buffer_size variable is probably
    too small. The cache hit rate can be calculated with

    key_buffer_size 16M
    Key_read_requests 2973620399
    Key_reads 8490571
    (cache hit rate = 0.0028)

    “The key_buffer_size affects the size of the index buffers and the speed
    of index handling, particularly reading." The MySQL manual (and other sources) say that
    "Key_reads/Key_read_request ratio should normally be < 0.01." This is the
    other most important thing to get correct. Here the value seems to be correct (< 0.01)

    Also check key_write_requests and key_writes.
    The key_writes/key_writes_request should normally be < 1 (near 0.5 seems to be fine)

    Here is a very interesting web pointer :

    2 - Others important settings are : Wait_timeout, max_connexion, thread_cache

    A little explanation :
    Generaly you have a lot of mysql process that are sleeping because wait_timeout are not set low. So I make sure that the wait_timeout is set to a very low value: 15 seconds (for me) . That means MySQL would close any connection that was idle for more than 15 seconds.

    The problem is you also have to increment your max_connexion (mine is set to 300) to be sure there is not a lot of idle clients holding connections and blocking out new clients from connecting and getting real work done.
    The pbm is that the box has to create new threads (MySQL is a multi-threaded server) at a very high rate. That may sucks up a measurable amount of CPU time.

    So the solution is to use the Thread_cache (from mysql doc) :
    “How many threads we should keep in a cache for reuse. When a client disconnects, the client's threads are put in the cache if there aren't more than thread_cache_size threads from before. All new threads are first taken from the cache, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. (Normally this doesn't give a notable performance improvement if you have a good thread implementation.) By examing the difference between the Connections and Threads_created you can see how efficient the current thread cache is for you.”

    * If Threads_created is big, you may want to increase the
    thread_cache_size variable. The cache hit rate can be calculated with

    thread_cache_size 0
    Threads_created 150022
    Connections 150023

    This is the second problem that should be fixed. A cache size of zero is the default for my-medium.cnf but the recommended size in my-large.cnf is 8.

    you may try this formula : table_cache = opened table / max_used_connection

    3 - Finally, you may also have a look at : tmp_table_size and Handler_read_rnd / Handler_read_rnd_next

    * If Created_tmp_disk_tables is big, you may want to increase the
    tmp_table_size variable to get the temporary tables memory-based instead
    of disk based.

    tmp_table_size 32M
    Created_tmp_disk_tables 3227
    Created_tmp_tables 159832
    Created_tmp_files 4444

    Created_tmp_disk_tables are the "number of implicit temporary tables on
    disk created while executing statements" and Created_tmp_tables are
    memory-based. Obviously it is bad if you have to go to disk instead of
    memory. About 2% of temp tables go to disk, which doesn't seem too bad
    but increasing the tmp_table_size probably couldn't hurt either.

    * If Handler_read_rnd is big, then you probably have a lot of queries
    that require MySQL to scan whole tables or you have joins that don't use
    keys properly.

    Handler_read_rnd 27712353
    Handler_read_rnd_next 283536234

    These values are high, that we could probably stand to improve
    the indexes and queries.

    I hope this will help some of you to more understand how it is possible to optimise MYSQL to fit your needs, hardaware box, or mysql current usage.

    Maybe there is others tweaks to perform, but I know well only these ones. I did setup using these ones on differents mysql box, and generally it did help us to increase performance without have to change hardware (our boxes have 2GB ram)

    Last edited by pascal; 01-21-2005 at 10:11 PM.

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