今天啓動 Redis 時阻塞很長時間，之後顯示啓動失敗，啓動狀態如下。
systemd[1]: redis.service start operation timed out. Terminating.
systemd[1]: Failed to start A persistent key-value database.
systemd[1]: Unit redis.service entered failed state.
看了下 service 文件，發現 Systemd 啓動命令如下
ExecStart=/usr/sbin/redis-server /etc/redis.conf
手動運行這條命令，發現是正常的，所以猜想是 service 文件的問題，後來發現只需要把 [Service] 部分的 Type=forking 註釋掉就行了。
[Service]
# Type=forking
# PIDFile=/var/run/redis/redis.pid
ExecStart=/usr/sbin/redis-server /etc/redis.conf
User=redis
Group=redis
之後重新加載 Service 文件並啓動 Redis 服務
sudo systemctl daemon-reload
sudo systemctl start redis
Man pages 對 Systemd 服務啓動類型 Type 的解釋如下
If set to forking, it is expected that the process configured with ExecStart= will call fork() as part of its start-up. The parent process is expected to exit when start-up is complete and all communication channels are set up. The child continues to run as the main daemon process. This is the behavior of traditional UNIX daemons. If this setting is used, it is recommended to also use the PIDFile= option, so that systemd can identify the main process of the daemon. systemd will proceed with starting follow-up units as soon as the parent process exits.
因爲 Redis 配置文件裏配置的是daemonize off