1. 问题

Linux的/tmp目录想必都用过

The /tmp directory in Linux based systems contains necessary files that are temporarily required by the system as well as other software and applications running on the machine.

使用tomcat的人可能遇到, IOException: The temporary upload location xxx is not valid
使用hadoop的人可能遇到, /tmp/hadoop-xxx.pid, No such file
那么文件去哪儿呢?

2. 分析

为了防止系统磁盘被/tmp耗尽, 需要定时清理, 于是出现了误删操作

3. 原理

centos7通过systemd-tmpfiles实现, 清理的动作来自systemd-tmpfiles-clean

[root@localhost ~]# systemctl list-unit-files |grep tmpfile
systemd-tmpfiles-clean.service                static  
systemd-tmpfiles-setup-dev.service            static  
systemd-tmpfiles-setup.service                static  
systemd-tmpfiles-clean.timer                  static  

3.1. 本质

本质是执行/usr/bin/systemd-tmpfiles --clean

man systemd-tmpfiles

--clean
If this option is passed, all files and directories with an age parameter configured will be cleaned up.

[root@localhost ~]# systemctl cat systemd-tmpfiles-clean
[Unit]
Description=Cleanup of Temporary Directories
Documentation=man:tmpfiles.d(5) man:systemd-tmpfiles(8)
DefaultDependencies=no
Conflicts=shutdown.target
After=systemd-readahead-collect.service systemd-readahead-replay.service local-fs.target time-sync.target
Before=shutdown.target

[Service]
Type=oneshot
ExecStart=/usr/bin/systemd-tmpfiles --clean
IOSchedulingClass=idle

3.2. 清理的对象

既然是清理, 那么哪些应该清理, 哪些应该保留呢?

配置文件路径/usr/lib/tmpfiles.d, /etc/tmpfiles.d, 其中/etc/tmpfiles.d优先级更高, 用户通过修改此处文件实现覆盖

[root@localhost ~]# ll /usr/lib/tmpfiles.d/
total 68
-rw-r--r--. 1 root root  464 Nov  6  2016 etc.conf
-rw-r--r--. 1 root root   39 Nov  5  2016 initscripts.conf
-rw-r--r--. 1 root root 1181 Nov  6  2016 legacy.conf
-rw-r--r--. 1 root root   34 Mar 31  2020 libselinux.conf
-r--r--r--. 1 root root   61 Nov 11  2016 lvm2.conf
-rw-r--r--. 1 root root  110 Nov  5  2016 pam.conf
-rw-r--r--. 1 root root   61 Jan 26  2014 ppp.conf
-rw-r--r--. 1 root root   16 Nov  5  2016 python.conf
-rw-r--r--. 1 root root   22 Nov  5  2016 rpm.conf
-rw-r--r--. 1 root root  228 Nov  6  2016 sap.conf
-rw-r--r--. 1 root root  137 Nov 16  2020 selinux-policy.conf
-rw-r--r--. 1 root root 1393 Nov  6  2016 systemd.conf
-rw-r--r--. 1 root root  496 Nov  6  2016 systemd-nologin.conf
-rw-r--r--. 1 root root  638 Nov  6  2016 tmp.conf
-rw-r--r--. 1 root root   56 Jun 16  2016 tuned.conf
-rw-r--r--. 1 root root  563 Nov  6  2016 var.conf
-rw-r--r--. 1 root root  623 Nov  6  2016 x11.conf

3.3. 清理策略

知道清理的对象后, 下一步应当如何清理呢, 清理的规则是什么?

我们看下/tmp的规则
/tmp保留10天, /var/tmp目录保留30天 (起始时间取文件/目录的创建/访问/修改的最新时间)
规则的具体含义, 请查阅man tmpfiles.d

[root@localhost ~]# cat /usr/lib/tmpfiles.d/tmp.conf 
v /tmp 1777 root root 10d
v /var/tmp 1777 root root 30d

# Exclude namespace mountpoints created with PrivateTmp=yes
x /tmp/systemd-private-%b-*
X /tmp/systemd-private-%b-*/tmp
x /var/tmp/systemd-private-%b-*
X /var/tmp/systemd-private-%b-*/tmp

3.4. 何时执行

执行周期由systemd-tmpfiles-clean.timer决定
默认是从服务器启动后15分钟作起点, 每隔1天执行一次

[root@localhost ~]# systemctl cat systemd-tmpfiles-clean.timer
# /usr/lib/systemd/system/systemd-tmpfiles-clean.timer
#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.

[Unit]
Description=Daily Cleanup of Temporary Directories
Documentation=man:tmpfiles.d(5) man:systemd-tmpfiles(8)

[Timer]
OnBootSec=15min
OnUnitActiveSec=1d