0. 前言

由于 transmission 与 samba 等服务在启动时不考虑启动顺序，可能会出现硬盘还未挂载但服务已经启动的状况，造成找不到路径等问题。
通过设置 systemd 的 After 项，可以实现在硬盘挂载后再启动相应的服务。

1. 列出由 systemd 生成的挂载服务

执行 systemctl list-units --type=mount ，列出所有与 mount 相关的服务：

$ systemctl list-units --type=mount
  UNIT                          LOAD   ACTIVE SUB     DESCRIPTION                     
  -.mount                       loaded active mounted Root Mount                      
  dev-mqueue.mount              loaded active mounted POSIX Message Queue File System 
  mnt-backup.mount              loaded active mounted /mnt/backup                     
  mnt-storage.mount             loaded active mounted /mnt/storage                    
  run-user-1000.mount           loaded active mounted /run/user/1000                  
  sys-fs-fuse-connections.mount loaded active mounted FUSE Control File System        
  sys-kernel-config.mount       loaded active mounted Kernel Configuration File System
  sys-kernel-debug.mount        loaded active mounted Kernel Debug File System        
  sys-kernel-tracing.mount      loaded active mounted Kernel Trace File System        

LOAD   = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB    = The low-level unit activation state, values depend on unit type.

9 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.

其中，以 mnt-*.mount 命名的服务，即为硬盘挂载服务

2. 找到相应的 systemd 配置文件的路径

以 transmission-daemon 为例，
执行 systemctl status transmission-daemon 命令：

$ systemctl status transmission-daemon
● transmission-daemon.service - Transmission BitTorrent Daemon
     Loaded: loaded (/lib/systemd/system/transmission-daemon.service; enabled; vendor preset: enabled)
     Active: active (running) since Tue 2023-01-24 08:37:49 UTC; 1h 26min ago
   Main PID: 636 (transmission-da)
     Status: "Uploading 0.11 KBps, Downloading 0.00 KBps."
      Tasks: 4 (limit: 8904)
     Memory: 2.4G
     CGroup: /system.slice/transmission-daemon.service
             └─636 /usr/bin/transmission-daemon -f --log-error

Warning: some journal files were not opened due to insufficient permissions.

可以看到，transmission 的 systemd 文件的路径为 /lib/systemd/system/transmission-daemon.service

3. 修改 systemd 配置

打开 systemd 配置文件，并填写 After 项：

sudo vim /lib/systemd/system/transmission-daemon.service

在 After= 后，添加查询到的硬盘挂载服务，如：

After=network.target mnt-storage.mount

保存文件并退出。

4. 保存 systemd 配置

执行 sudo systemctl daemon-reload 命令即可

$ sudo systemctl daemon-reload