我有一个大而重的 mysql 数据库，它有时执行得非常快，但有时会变得非常慢。所有表都是InnoDB，服务器有32GB 的 RAM，数据库大小约为 40GB。
我的前 20 个查询slow_query_log是和查询update，我不明白为什么它们这么慢（有时长达 120 秒！）insertdelete
这是最常见的查询：
UPDATE comment_fallows set comment_cnt_new = 0 WHERE user_id = 1;
分析结果：
mysql> set profiling = 1;
Query OK, 0 rows affected (0.00 sec)
mysql> update comment_fallows set comment_cnt_new = 0 where user_id = 1;
Query OK, 0 rows affected (2.77 sec)
Rows matched: 18 Changed: 0 Warnings: 0
mysql> show profile for query 1;
+---------------------------+----------+
| Status | Duration |
+---------------------------+----------+
| starting | 0.000021 |
| checking permissions | 0.000004 |
| Opening tables | 0.000010 |
| System lock | 0.000004 |
| init | 0.000041 |
| Searching rows for update | 0.000084 |
| Updating | 0.000055 |
| end | 0.000010 |
| query end | 2.766245 |
| closing tables | 0.000007 |
| freeing items | 0.000013 |
| logging slow query | 0.000003 |
| cleaning up | 0.000002 |
+---------------------------+----------+
13 rows in set (0.00 sec)
我正在使用主/服务器复制，因此启用了二进制日志。我已经放弃了我在互联网上找到的一个建议并设置flush_log_at_trx_commit为0但它没有任何区别：
mysql> show variables like '%trx%';
+-------------------------------------------+-------+
| Variable_name | Value |
+-------------------------------------------+-------+
| innodb_flush_log_at_trx_commit | 0 |
| innodb_use_global_flush_log_at_trx_commit | ON |
+-------------------------------------------+-------+
表结构：
CREATE TABLE `comment_fallows` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user_id` int(11) NOT NULL,
`part_id` int(11) DEFAULT NULL,
`article_id` int(11) DEFAULT NULL,
`request_id` int(11) DEFAULT NULL,
`comment_cnt` int(10) unsigned NOT NULL,
`comment_cnt_new` int(10) unsigned NOT NULL DEFAULT '0',
`last_comment_date` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`,`last_comment_date`),
KEY `part_id` (`part_id`),
KEY `last_comment_date` (`last_comment_date`),
KEY `request_id` (`request_id`),
CONSTRAINT `comment_fallows_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE CASCADE,
CONSTRAINT `comment_fallows_ibfk_2` FOREIGN KEY (`part_id`) REFERENCES `fanfic_parts` (`id`) ON DELETE CASCADE,
CONSTRAINT `comment_fallows_ibfk_3` FOREIGN KEY (`request_id`) REFERENCES `requests` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=2239419 DEFAULT CHARSET=utf8
以及所有的 innodb 设置（服务器有 32 GB 的 RAM）：
mysql> show variables like '%innodb%';
+-------------------------------------------+------------------------+
| Variable_name | Value |
+-------------------------------------------+------------------------+
| have_innodb | YES |
| ignore_builtin_innodb | OFF |
| innodb_adaptive_flushing | ON |
| innodb_adaptive_flushing_method | estimate |
| innodb_adaptive_hash_index | ON |
| innodb_adaptive_hash_index_partitions | 1 |
| innodb_additional_mem_pool_size | 16777216 |
| innodb_autoextend_increment | 8 |
| innodb_autoinc_lock_mode | 1 |
| innodb_blocking_buffer_pool_restore | OFF |
| innodb_buffer_pool_instances | 1 |
| innodb_buffer_pool_restore_at_startup | 0 |
| innodb_buffer_pool_shm_checksum | ON |
| innodb_buffer_pool_shm_key | 0 |
| innodb_buffer_pool_size | 21474836480 |
| innodb_change_buffering | all |
| innodb_checkpoint_age_target | 0 |
| innodb_checksums | ON |
| innodb_commit_concurrency | 0 |
| innodb_concurrency_tickets | 500 |
| innodb_corrupt_table_action | assert |
| innodb_data_file_path | ibdata1:10M:autoextend |
| innodb_data_home_dir | |
| innodb_dict_size_limit | 0 |
| innodb_doublewrite | ON |
| innodb_doublewrite_file | |
| innodb_fake_changes | OFF |
| innodb_fast_checksum | OFF |
| innodb_fast_shutdown | 1 |
| innodb_file_format | Antelope |
| innodb_file_format_check | ON |
| innodb_file_format_max | Antelope |
| innodb_file_per_table | ON |
| innodb_flush_log_at_trx_commit | 0 |
| innodb_flush_method | |
| innodb_flush_neighbor_pages | area |
| innodb_force_load_corrupted | OFF |
| innodb_force_recovery | 0 |
| innodb_ibuf_accel_rate | 100 |
| innodb_ibuf_active_contract | 1 |
| innodb_ibuf_max_size | 10737401856 |
| innodb_import_table_from_xtrabackup | 0 |
| innodb_io_capacity | 10000 |
| innodb_kill_idle_transaction | 0 |
| innodb_large_prefix | OFF |
| innodb_lazy_drop_table | 0 |
| innodb_lock_wait_timeout | 120 |
| innodb_locks_unsafe_for_binlog | OFF |
| innodb_log_block_size | 512 |
| innodb_log_buffer_size | 8388608 |
| innodb_log_file_size | 268435456 |
| innodb_log_files_in_group | 3 |
| innodb_log_group_home_dir | ./ |
| innodb_max_dirty_pages_pct | 90 |
| innodb_max_purge_lag | 0 |
| innodb_mirrored_log_groups | 1 |
| innodb_old_blocks_pct | 37 |
| innodb_old_blocks_time | 0 |
| innodb_open_files | 300 |
| innodb_page_size | 16384 |
| innodb_purge_batch_size | 20 |
| innodb_purge_threads | 1 |
| innodb_random_read_ahead | OFF |
| innodb_read_ahead | linear |
| innodb_read_ahead_threshold | 56 |
| innodb_read_io_threads | 8 |
| innodb_recovery_stats | OFF |
| innodb_recovery_update_relay_log | OFF |
| innodb_replication_delay | 0 |
| innodb_rollback_on_timeout | OFF |
| innodb_rollback_segments | 128 |
| innodb_show_locks_held | 10 |
| innodb_show_verbose_locks | 0 |
| innodb_spin_wait_delay | 6 |
| innodb_stats_auto_update | 1 |
| innodb_stats_method | nulls_equal |
| innodb_stats_on_metadata | ON |
| innodb_stats_sample_pages | 8 |
| innodb_stats_update_need_lock | 1 |
| innodb_strict_mode | OFF |
| innodb_support_xa | ON |
| innodb_sync_spin_loops | 30 |
| innodb_table_locks | ON |
| innodb_thread_concurrency | 16 |
| innodb_thread_concurrency_timer_based | OFF |
| innodb_thread_sleep_delay | 10000 |
| innodb_use_global_flush_log_at_trx_commit | ON |
| innodb_use_native_aio | ON |
| innodb_use_sys_malloc | ON |
| innodb_use_sys_stats_table | OFF |
| innodb_version | 1.1.8-rel25.1 |
| innodb_write_io_threads | 8 |
+-------------------------------------------+------------------------+
92 rows in set (0.00 sec)
我已经为这个问题苦苦挣扎了好几个星期，如果有任何关于如何解决这个问题的建议，我都会非常高兴。
为什么我的update,insert和delete查询一步上来这么慢query end？
更新
我禁用了查询缓存，但是，update查询仍然非常慢（没有改变）insertdelete
show variables like '%cache%';
+------------------------------+----------------------+
| Variable_name | Value |
+------------------------------+----------------------+
| binlog_cache_size | 4194304 |
| binlog_stmt_cache_size | 32768 |
| have_query_cache | YES |
| key_cache_age_threshold | 300 |
| key_cache_block_size | 1024 |
| key_cache_division_limit | 100 |
| max_binlog_cache_size | 18446744073709547520 |
| max_binlog_stmt_cache_size | 18446744073709547520 |
| metadata_locks_cache_size | 1024 |
| query_cache_limit | 16777216 |
| query_cache_min_res_unit | 4096 |
| query_cache_size | 0 |
| query_cache_strip_comments | OFF |
| query_cache_type | ON |
| query_cache_wlock_invalidate | OFF |
| stored_program_cache | 256 |
| table_definition_cache | 400 |
| table_open_cache | 2048 |
| thread_cache_size | 8 |
+------------------------------+----------------------+