過了這麼久，discuz論壇的問題還是困擾著很多網友，其實從各論壇裡看到的問題總結出來，很關鍵的一點都是因為沒有將數據表引擎轉成InnoDB導致的，discuz在並發稍微高一點的環境下就表現的非常糟糕，產生大量的鎖等待，這時候如果把數據表引擎改成InnoDB的話，我相信會好很多。這次就寫個掃盲貼吧。
1. 啟用innodb引擎，並配置相關參數
#skip-innodb
innodb_additional_mem_pool_size = 16M #一般16M也夠了，可以適當調整下 innodb_buffer_pool_size = 6G #如果是專用db的話，一般是內存總量的80% innodb_data_file_path = ibdata1:1024M:autoextend innodb_file_io_threads = 4 innodb_thread_concurrency = 20 innodb_flush_log_at_trx_commit = 1 innodb_log_buffer_size = 16M innodb_log_file_size = 256M innodb_log_files_in_group = 3 innodb_max_dirty_pages_pct = 50 innodb_lock_wait_timeout = 120 innodb_file_per_table
修改表引擎為innodb：
mysql> alter table cdb_access engine = innodb;
其他表類似上面，把表名換一下即可...
將表存儲引擎改成innodb後，不僅可以避免大量的鎖等待，還可以提升查詢的效率，因為innodb會把data和index都放在buffer pool中，效率更高。
2.緩存優化
在 my.cnf 中添加/修改以下選項:
#取消文件系統的外部鎖 skip-locking #不進行域名反解析,注意由此帶來的權限/授權問題 skip-name-resolve #索引緩存,根據內存大小而定,如果是獨立的db服務器,可以設置高達80%的內存總量 key_buffer = 512M #連接排隊列表總數 back_log = 200 max_allowed_packet = 2M #打開表緩存總數,可以避免頻繁的打開數據表產生的開銷 table_cache = 512 #每個線程排序所需的緩沖 sort_buffer_size = 4M #每個線程讀取索引所需的緩沖 read_buffer_size = 4M #MyISAM表發生變化時重新排序所需的緩沖 myisam_sort_buffer_size = 64M #緩存可重用的線程數 thread_cache = 128 #查詢結果緩存 query_cache_size = 128M #設置超時時間,能避免長連接 set-variable = wait_timeout=60 #最大並發線程數,cpu數量*2 thread_concurrency = 4 #記錄慢查詢,然後對慢查詢一一優化 log-slow-queries = slow.log long_query_time = 1 #關閉不需要的表類型,如果你需要,就不要加上這個 skip-bdb
以上參數根據各自服務器的配置差異進行調整,僅作為參考.
3.索引優化
上面提到了,已經開啟了慢查詢,那麼接下來就要對慢查詢進行逐個優化了.
搜索的查詢SQL大致如下:
SELECT t.* FROM cdb_posts p, cdb_threads t WHERE
t.fid IN ('37', '45', '4', '6', '17', '41', '28', '32', '31', '1', '42')
AND p.tid=t.tid AND p.author LIKE 'JoansWin'
GROUP BY t.tid ORDER BY lastpost DESC LIMIT 0, 80;
用 EXPLAIN 分析的結果如下:
mysql>EXPLAIN SELECT t.* FROM cdb_posts p, cdb_threads t WHERE
t.fid IN ('37', '45', '4', '6', '17', '41', '28', '32', '31', '1', '42')
AND p.tid=t.tid AND p.author LIKE 'JoansWin'
GROUP BY t.tid ORDER BY lastpost DESC LIMIT 0, 80;
+-----------+------------+----------+--------------+-------------+-----------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra +-----------+------------+----------+--------------+-------------+-----------+-------------+ | 1 | SIMPLE | t | range | PRIMARY,fid | fid | 2 | NULL | 66160 | Using where; Using temporary; Using filesort | | 1 | SIMPLE | p | ref | tid | tid | 3 | Forum.t.tid | 10 | Using where | +----+-------------+-------+-------+---------------+------+---------+-------------+-------+ ---------
只用到了 t.fid 和 p.tid,而 p.author 則沒有索引可用,總共需要掃描
66160*10 = 661600 次索引,夠誇張吧 :(
再分析 cdb_threads 和 cdb_posts 的索引情況:
mysql>show index from cdb_posts;
+-----------+------------+----------+--------------+-------------+-----------+---------- ---+----------+--------+------+--+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | +-----------+------------+----------+--------------+---- ---------+-----------+-------------+----------+--------+------+--+ | cdb_posts | 0 | PRIMARY | 1 | pid | A | 680114 | NULL | NULL | | BTREE | | | cdb_posts | 1 | fid | 1 | fid | A | 10 | NULL | NULL | | BTREE | | | cdb_posts | 1 | tid | 1 | tid | A | 68011 | NULL | NULL | | BTREE | | | cdb_posts | 1 | tid | 2 | dateline | A | 680114 | NULL | NULL | | BTREE | | | cdb_posts | 1 | dateline | 1 | dateline | A | 680114 | NULL | NULL | | BTREE | | +-----------+------------+----------+--------------+-------------+-----------+---
以及
mysql>show index from cdb_threads;
+-----------+------------+----------+--------------+-------------+-----------+-------------+ ----------+--------+------+-----+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | +-----------+------------+----------+--------------+----- --------+-----------+-------------+----------+--------+------+-----+ | cdb_threads | 0 | PRIMARY | 1 | tid | A | 68480 | NULL | NULL | | BTREE | | | cdb_threads | 1 | lastpost | 1 | topped | A | 4 | NULL | NULL | | BTREE | | | cdb_threads | 1 | lastpost | 2 | lastpost | A | 68480 | NULL | NULL | | BTREE | | | cdb_threads | 1 | lastpost | 3 | fid | A | 68480 | NULL | NULL | | BTREE | | | cdb_threads | 1 | replies | 1 | replies | A | 233 | NULL | NULL | | BTREE | | | cdb_threads | 1 | dateline | 1 | dateline | A | 68480 | NULL | NULL | | BTREE | | | cdb_threads | 1 | fid | 1 | fid | A | 10 | NULL | NULL | | BTREE | | | cdb_threads | 1 | enablehot | 1 | enablehot | A | 2 | NULL | NULL | | BTREE | | +-------------+------------+-----------+--------------+-------------+------
看到索引 fid 和 enablehot 基數太小,看來該索引完全沒必要,不過,對於fid基數較大的情況,則可能需要保留>該索引.
所做修改如下:
ALTER TABLE `cdb_threads` DROP INDEX `enablehot`, DROP INDEX `fid`, ADD INDEX (`fid`, `lastpost`); ALTER TABLE `cdb_posts` DROP INDEX `fid`, ADD INDEX (`author`(10)); OPTIMIZE TABLE `cdb_posts`; OPTIMIZE TABLE `cdb_threads`;
在這裡, p.author 字段我設定的部分索引長度是 10, 是我經過分析後得出來的結果,不同的系統,這裡的長度也不同,最好自己先取一下平均值,然後再適當調整.
現在,再來執行一次上面的慢查詢,發現時間已經從 6s 變成 0.19s,提高了 30 倍.
