Optimisations Mysql serveur dédié

Salut à tous,

On trouve régulièrement des astuces pour optimiser son serveur Mysql mais souvent les conseils sont données par rapport à de tout petit serveur (1-2Go de ram + petit CPU).

Mon serveur dédié à 24Go de Ram et un Intel Xeon.
Il a une charge assez forte avec une trentaine de site (nginx/apache/php/mysql) + environ 1000 boites emails (postifx en POP/IMAP).

Au niveau du CPU il supporte la charge et la bande passante n'est pas saturée.

J'essaye d'optimiser les performances mysql mais je n'arrive pas à faire le bon réglage.

Voici le résultat d'un tuning-primer.sh:

        -- MYSQL PERFORMANCE TUNING PRIMER --
             - By: Matthew Montgomery -

MySQL Version 5.1.61-log x86_64

Uptime = 6 days 16 hrs 41 min 59 sec
Avg. qps = 36
Total Questions = 20904053
Threads Connected = 18

Server has been running for over 48hrs.
It should be safe to follow these recommendations

To find out more information on how each of these
runtime variables effects performance visit:
http://dev.mysql.com/doc/refman/5.1/en/server-system-variables.html
Visit http://www.mysql.com/products/enterprise/advisors.html
for info about MySQL's Enterprise Monitoring and Advisory Service

SLOW QUERIES
The slow query log is enabled.
Current long_query_time = 5.000000 sec.
You have 171514 out of 20904081 that take longer than 5.000000 sec. to complete
Your long_query_time seems to be fine

BINARY UPDATE LOG
The binary update log is NOT enabled.
You will not be able to do point in time recovery
See http://dev.mysql.com/doc/refman/5.1/en/point-in-time-recovery.html

WORKER THREADS
Current thread_cache_size = 132
Current threads_cached = 49
Current threads_per_sec = 0
Historic threads_per_sec = 0
Your thread_cache_size is fine

MAX CONNECTIONS
Current max_connections = 400
Current threads_connected = 19
Historic max_used_connections = 68
The number of used connections is 17% of the configured maximum.
Your max_connections variable seems to be fine.

MEMORY USAGE
Max Memory Ever Allocated : 2.71 G
Configured Max Per-thread Buffers : 1.22 G
Configured Max Global Buffers : 2.50 G
Configured Max Memory Limit : 3.73 G
Physical Memory : 23.59 G
Max memory limit seem to be within acceptable norms

KEY BUFFER
1705419 * 1024 / 2147483648 * 100
Current MyISAM index space = 581 M
Current key_buffer_size = 2.00 G
Key cache miss rate is 1 : 272
Key buffer free ratio = 0 %
Your key_buffer_size seems to be too high.
Perhaps you can use these resources elsewhere

QUERY CACHE
Query cache is enabled
Current query_cache_size = 512 M
Current query_cache_used = 443 M
Current query_cache_limit = 1.00 G
Current Query cache Memory fill ratio = 86.64 %
Current query_cache_min_res_unit = 4 K
However, 713741 queries have been removed from the query cache due to lack of memory
Perhaps you should raise query_cache_size
MySQL won't cache query results that are larger than query_cache_limit in size

SORT OPERATIONS
Current sort_buffer_size = 1 M
Current read_rnd_buffer_size = 768 K
Sort buffer seems to be fine

JOINS
Current join_buffer_size = 132.00 K
You have had 1875 queries where a join could not use an index properly
You have had 335 joins without keys that check for key usage after each row
You should enable "log-queries-not-using-indexes"
Then look for non indexed joins in the slow query log.
If you are unable to optimize your queries you may want to increase your
join_buffer_size to accommodate larger joins in one pass.

Note! This script will still suggest raising the join_buffer_size when
ANY joins not using indexes are found.

OPEN FILES LIMIT
Current open_files_limit = 10410 files
The open_files_limit should typically be set to at least 2x-3x
that of table_cache if you have heavy MyISAM usage.
Your open_files_limit value seems to be fine

TABLE CACHE
Current table_open_cache = 5000 tables
Current table_definition_cache = 4500 tables
You have a total of 5124 tables
You have 5000 open tables.
Current table_cache hit rate is 28%, while 100% of your table cache is in use
You should probably increase your table_cache
You should probably increase your table_definition_cache value.

TEMP TABLES
Current max_heap_table_size = 1.00 G
Current tmp_table_size = 1.00 G
Of 271837 temp tables, 46% were created on disk
Perhaps you should increase your tmp_table_size and/or max_heap_table_size
to reduce the number of disk-based temporary tables
Note! BLOB and TEXT columns are not allow in memory tables.
If you are using these columns raising these values might not impact your 
ratio of on disk temp tables.

TABLE SCANS
Current read_buffer_size = 1 M
Current table scan ratio = 406 : 1
read_buffer_size seems to be fine

TABLE LOCKING
Current Lock Wait ratio = 1 : 3813
You may benefit from selective use of InnoDB.
If you have long running SELECT's against MyISAM tables and perform
frequent updates consider setting 'low_priority_updates=1'
If you have a high concurrency of inserts on Dynamic row-length tables
consider setting 'concurrent_insert=2'.

C'est surtout la partie TABLE CACHE et TEMP CACHE que je n'arrive pas à optimiser.

Merci par avance