破除MySQL開啟的檔案描述子限制-資料庫Too many open files問題

由於挖站否免費CN2空間用戶增多，近期總是出現半夜Mysql進程掛掉的情況，查詢了一下Mysql的錯誤日誌，提示有：[ERROR] Error in accept: Too many open files，原來是因為open files不足導致資料庫掛掉了，尤其是晚上DirectAdmin系統備份時更是如此。

Linux系統預設的opens file是1024，可以使用ulimit -a 檢視,這個是系統級的限制，另外在mysql中預設的open files也是1024，很多時候這個數值無法滿足我們的建站需要，尤其是一個大型的網站和巨大型的資料庫中心，很容易就會出現Too many open files問題。

這篇文章就是來分享如何解決錯誤：[ERROR] Error in accept: Too many open files的問題，更多的關於伺服器最佳化以及問題解決方法，參考：

讓圖片飛一會兒！網站圖片WebP格式批次轉換設定及加速效果體驗
巧用又拍雲端FTP和堅果雲WebDAV-打造個人檔案備份與資料雲端存儲
VPS主機與伺服器安全防護:SSH修改連接埠,新增白名單,僅限金鑰登入

一、資料庫問題詳情

查看資料庫錯誤日誌，如果用的是DirectAdmin面板，相關的日誌路徑如下：

#DirectAdmin:
/var/log/directadmin/error.log
/var/log/directadmin/errortaskq.log
/var/log/directadmin/system.log
/var/log/directadmin/security.log


#Apache:
/var/log/httpd/error_log
/var/log/httpd/access_log
/var/log/httpd/suexec_log
/var/log/httpd/fpexec_log
/var/log/httpd/domains/domain.com.error.log
/var/log/httpd/domains/domain.com.log
/var/log/messages (generic errors)


#Proftpd:
/var/log/proftpd/access.log
/var/log/proftpd/auth.log
/var/log/messages (generic errors)


#PureFTPd:
/var/log/pureftpd.log


#Dovecot and vm-pop3d:
/var/log/maillog
/var/log/messages


#named (bind):
/var/log/messages


#exim:
/var/log/exim/mainlog
/var/log/exim/paniclog
/var/log/exim/processlog
/var/log/exim/rejectlog

#(on FreeBSD, they have "exim_" in front of the filenames)

#mysqld:
#RedHat:
/var/lib/mysql/server.hostname.com.err

#FreeBSD and Debian:
/usr/local/mysql/data/server.hostname.com.err


#crond:
/var/log/cron


#To view a log file, run:
less /var/log/filename

#Where /var/log/filename is the path of the log you wish to view.  If the log is too large you can use the "tail" command:
tail -n 30 /var/log/filename

#Where 30 is the number of lines from the end you wish to view.

開啟Mysql錯誤日誌，一般會提示如下錯誤：

190902  3:16:52 [ERROR] Error in accept: Too many open files
190902  3:21:08 [ERROR] Error in accept: Too many open files
190902  3:25:24 [ERROR] Error in accept: Too many open files
190902  3:29:40 [ERROR] Error in accept: Too many open files
190902  3:33:56 [ERROR] Error in accept: Too many open files
190902  3:38:12 [ERROR] Error in accept: Too many open files
190902  3:42:28 [ERROR] Error in accept: Too many open files
190902  3:46:44 [ERROR] Error in accept: Too many open files
190902  3:51:00 [ERROR] Error in accept: Too many open files
190902  3:55:16 [ERROR] Error in accept: Too many open files
190902  3:59:32 [ERROR] Error in accept: Too many open files
190902  4:03:48 [ERROR] Error in accept: Too many open files
190902  4:08:04 [ERROR] Error in accept: Too many open files
190902  4:12:20 [ERROR] Error in accept: Too many open files
190902  4:16:37 [ERROR] Error in accept: Too many open files
190902  4:20:53 [ERROR] Error in accept: Too many open files
190902  4:25:09 [ERROR] Error in accept: Too many open files
190902  4:29:25 [ERROR] Error in accept: Too many open files
190902  4:33:41 [ERROR] Error in accept: Too many open files
190902  4:37:57 [ERROR] Error in accept: Too many open files
190902  4:42:13 [ERROR] Error in accept: Too many open files
190902  4:46:29 [ERROR] Error in accept: Too many open files
190902  4:50:45 [ERROR] Error in accept: Too many open files

二、資料庫問題分析

2.1 查看限制

使用指令：ulimit -a可以檢視到本機的open_files_limit：

[root@sc3 ~]# ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 15675
max locked memory       (kbytes, -l) 64
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) 15675
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

或使用以下指令僅查看某一項進程的open_files_limit：

#直接查看 open files限制数字
ulimit -n

#仅查看soft 和 hard open files limits ：
ulimit -Hn
ulimit -Sn

#仅查看Mysql的open files limits:

su mysql ulimit -a

2.2 修改limits.conf

方法一：

你可以使用以下指令來修改/etc/security/limits.conf文件，指令如下：

#To set the ceiling of the limits available (the max a soft or hard limit can be set by each user).
#NOTE: this is set by a PAM during authentication, and NOT at boot. 
#编辑文件

vi /etc/security/limits.conf

#Add the following to bottom of file to set for everything *:
#添加以下谷贱伤农

* soft nofile 1024000
* hard nofile 1024000
* soft nproc 10240
* hard nproc 10240

#To set only a specific user, like mysql then put in:
#或者指定Mysql

mysql hard nofile 1024000
mysql soft nofile 1024000

#Repeat for /etc/security/limits.d/90-nproc.conf
#同样对90-nproc.conf也是一样的操作

vi /etc/security/limits.d/90-nproc.conf

#Add the following:

* soft nofile 1024000
* hard nofile 1024000
* soft nproc 10240
* hard nproc 10240

#root soft nproc unlimited
#I selected '1024000', which is fairly high; you can surely set this lower to something like '102400'，请根据自己的实际情况来设定数字

或使用root帳號登錄，然後使用下列指令暫時修改/etc/security/limits.conf：

ulimit -Hn 1024000


#Edit /etc/init.d/mysqld and add this to the top, after #!/bin/sh
#或者编辑修改/etc/init.d/mysqld，加入以下代码：
ulimit -HSn 1024
ulimit -HSn 32768
ulimit -HSn 1024000

方法二：

打開vi /etc/security/limits.conf文件，在文件裡添加一行：* - nofile 32768，這樣就把open files調整到了32768。

2.3 修改my.cnf文件

編輯：vi /etc/my.cnf，新增以下程式碼在 [mysqld]下方：

[mysqld]
open_files_limit = 1024000

然後重啟Mysql，如下：

/etc/init.d/mysqld restart
#或者
service mysqld restart

2.4 查看是否生效

執行以下命令：

mysql -u root -p 
show global variables like 'open%';
#如果是DirectAdmin面板
mysql -u da_admin -p 
show global variables like 'open%';

可以查看open_files_limit已經修改成功了：

三、重啟後失效問題

如果你發現系統重開機後open files又恢復到了1024，依照下列指令修改設定 :

#One fix: simply restart mysqld after the system boots up.
#最简单的方法再次重启Mysql

service mysqld restart

#另一种方法
#Another fix is to set these values at boot time before everything else (permanent):
vi /etc/init.d/mysqld

#Add the following:
#添加以下内容：
ulimit -S -n ${DAEMON_FILES_LIMIT:-102400} >/dev/null 2>&1

#然后在 /etc/my.cnf 中添加以下内容：
[myqld_safe]
open_files_limit = 102400
#或者是
[myqld]
open_files_limit = 102400

四、總結

MySQL開啟的檔案描述子限制都是Linux作業系統的檔案描述子限制，也就是預設為1024，和Mysql設定檔中open_files_limit的設定沒有關係。所以要修改open_files_limit，必須先修改作業系統的open_files_limit。

當然，如果你不想修改系統設置，也可以使用root帳號，執行mysqld_safe腳本啟動MySQL（或使用mysql.server啟動），加上指令：–open-files-limit是可以成功設定的，因為mysqld_safe啟動MySQL時，其實是在啟動mysqld程式之前，先呼叫了ulimit -n $open_files來實作檔案描述符的限制。

./mysqld_safe --open-files-limit=25000 &
root@(none) 02:50:54>show variables like "%open_files_limit%";
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| open_files_limit | 25000 |
+------------------+-------+