使用 systemd 作为问题定位工具

虽然 systemd 并非真正的故障定位工具，但其输出中的信息为解决问题指明了方向。

没有人会认为 systemd 是一个故障定位工具，但当我的 web 服务器遇到问题时，我对 systemd 和它的一些功能的不断了解帮助我找到并规避了问题。

我遇到的问题是这样，我的服务器 yorktown 为我的家庭办公网络提供名称服务 、DHCP、NTP、HTTPD 和 SendMail 邮件服务，它在正常启动时未能启动 Apache HTTPD 守护程序。在我意识到它没有运行之后，我不得不手动启动它。这个问题已经持续了一段时间，我最近才开始尝试去解决它。

你们中的一些人会说，systemd 本身就是这个问题的原因，根据我现在了解的情况，我同意你们的看法。然而，我在使用 SystemV 时也遇到了类似的问题。（在本系列文章的第一篇中，我探讨了围绕 systemd 作为旧有 SystemV 启动程序和启动脚本的替代品所产生的争议。如果你有兴趣了解更多关于 systemd 的信息，也可以阅读第二篇和第三篇文章。）没有完美的软件，systemd 和 SystemV 也不例外，但 systemd 为解决问题提供的信息远远多于 SystemV。

确定问题所在

找到这个问题根源的第一步是确定 httpd 服务的状态：

[root@yorktown ~]# systemctl status httpd
● httpd.service - The Apache HTTP Server
 Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
 Active: failed (Result: exit-code) since Thu 2020-04-16 11:54:37 EDT; 15min ago
  Docs: man:httpd.service(8)
 Process: 1101 ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND (code=exited, status=1/FAILURE)
Main PID: 1101 (code=exited, status=1/FAILURE)
 Status: "Reading configuration..."
   CPU: 60ms

Apr 16 11:54:35 yorktown.both.org systemd[1]: Starting The Apache HTTP Server...
Apr 16 11:54:37 yorktown.both.org httpd[1101]: (99)Cannot assign requested address: AH00072: make_sock: could not bind to address 192.168.0.52:80
Apr 16 11:54:37 yorktown.both.org httpd[1101]: no listening sockets available, shutting down
Apr 16 11:54:37 yorktown.both.org httpd[1101]: AH00015: Unable to open logs
Apr 16 11:54:37 yorktown.both.org systemd[1]: httpd.service: Main process exited, code=exited, status=1/FAILURE
Apr 16 11:54:37 yorktown.both.org systemd[1]: httpd.service: Failed with result 'exit-code'.
Apr 16 11:54:37 yorktown.both.org systemd[1]: Failed to start The Apache HTTP Server.
[root@yorktown ~]#

# Modifying this file in-place is not recommended, because changes 
# will be overwritten during package upgrades.  To customize the 
# behaviour, run "systemctl edit httpd" to create an override unit.

# For example, to pass additional options (such as -D definitions) to 
# the httpd binary at startup, create an override unit (as is done by                             
# systemctl edit) and enter the following:                                           

#    [Service]
#    Environment=OPTIONS=-DMY_DEFINE             

[Unit]                                               
Description=The Apache HTTP Server
Wants=httpd-init.service
After=network.target remote-fs.target nss-lookup.target httpd-init.service
Documentation=man:httpd.service(8)

[Service]
Type=notify
Environment=LANG=C

ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND
ExecReload=/usr/sbin/httpd $OPTIONS -k graceful
# Send SIGWINCH for graceful stop
KillSignal=SIGWINCH
KillMode=mixed
PrivateTmp=true

[Install]
WantedBy=multi-user.target

[root@yorktown ~]# cd /etc/systemd/system/httpd.service.d/
[root@yorktown httpd.service.d]# ll
total 4
-rw-r--r-- 1 root root 243 Apr 16 11:43 override.conf
[root@yorktown httpd.service.d]# cat override.conf
# Trying to delay the startup of httpd so that the network is
# fully up and running so that httpd can bind to the correct
# IP address
#
# By David Both, 2020-04-16

[Service]
ExecStartPre=/bin/sleep 30

[root@yorktown ~]# systemctl status httpd
● httpd.service - The Apache HTTP Server
 Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
 Drop-In: /etc/systemd/system/httpd.service.d
     └─override.conf
     /usr/lib/systemd/system/httpd.service.d
     └─php-fpm.conf
 Active: activating (start-pre) since Thu 2020-04-16 12:14:29 EDT; 28s ago
  Docs: man:httpd.service(8)
Cntrl PID: 1102 (sleep)
  Tasks: 1 (limit: 38363)
 Memory: 260.0K
   CPU: 2ms
 CGroup: /system.slice/httpd.service
     └─1102 /bin/sleep 30

Apr 16 12:14:29 yorktown.both.org systemd[1]: Starting The Apache HTTP Server...
Apr 16 12:15:01 yorktown.both.org systemd[1]: Started The Apache HTTP Server.
[root@yorktown ~]#

[root@yorktown ~]# systemctl status httpd
● httpd.service - The Apache HTTP Server
 Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
 Drop-In: /etc/systemd/system/httpd.service.d
     └─override.conf
     /usr/lib/systemd/system/httpd.service.d
     └─php-fpm.conf
 Active: active (running) since Thu 2020-04-16 12:15:01 EDT; 1min 18s ago
  Docs: man:httpd.service(8)
 Process: 1102 ExecStartPre=/bin/sleep 30 (code=exited, status=0/SUCCESS)
Main PID: 1567 (httpd)
 Status: "Total requests: 0; Idle/Busy workers 100/0;Requests/sec: 0; Bytes served/sec:  0 B/sec"
  Tasks: 213 (limit: 38363)
 Memory: 21.8M
   CPU: 82ms
 CGroup: /system.slice/httpd.service
     ├─1567 /usr/sbin/httpd -DFOREGROUND
     ├─1569 /usr/sbin/httpd -DFOREGROUND
     ├─1570 /usr/sbin/httpd -DFOREGROUND
     ├─1571 /usr/sbin/httpd -DFOREGROUND
     └─1572 /usr/sbin/httpd -DFOREGROUND

Apr 16 12:14:29 yorktown.both.org systemd[1]: Starting The Apache HTTP Server...
Apr 16 12:15:01 yorktown.both.org systemd[1]: Started The Apache HTTP Server.