What should I do if docker start cannot start?

docker start起不来的解决办法：1、检查运行状态，然后通过“echo 3 > /proc/sys/vm/drop_caches”命令释放被占用的内存；2、通过“$netstat -nltp|grep ...”命令检查端口是否已经被占用，如果上线后发现占有，则改为可用端口再重启即可。

本教程操作环境：linux7.3系统、docker19.03版、Dell G3电脑。

docker start起不来怎么办？

Docker无法正常启动的原因及解决办法

概述

Docker启动异常表现

状态反复restaring，用命令查看

$docker ps -a
CONTAINER ID    IMAGE  COMMAND       CREATED      STATUS       PORTS    NAMES
21c09be88c11     docker.xxxx.cn:5000/xxx-tes/xxx_tes:1.0.6   "/usr/local/tomcat..."   9 days ago          Restarting (1) Less than a second ago                      xxx10

Docker日志有明显问题：

$docker logs [容器名/容器ID]
比如：
docker logs openresty

Docker启动异常的可能原因

内存不够

Docker 启动至少需要2G内存，首先执行free -mh命令查看剩余内存是否足够

直接查看内存

$free -mh
            total        used        free      shared  buff/cache   available
Mem:         15G         14G          627M        195M        636M     726M
Swap:         0B          0B          0B

分析日志

有时候一瞬间内存过载溢出，导致部分进程被杀死，看起来内存也是够用的，事实上docker还是会反复重启，就需要通过docker日志和系统日志信的息来进一步分析：

分析docker日志

查看docker日志看到内存溢出的信息，要仔细翻阅才能找到信息，并不是在最下面

$docker logs [容器名/容器ID]|less 
Java HotSpot(TM) 64-Bit Server VM warning: INFO: os::commit_memory(0x0000000769990000, 1449590784, 0) failed; error='Cannot allocate memory' (errno=12)
#
# There is insufficient memory for the Java Runtime Environment to continue.
# Native memory allocation (malloc) failed to allocate 1449590784 bytes for committing reserved memory.
# An error report file with more information is saved as:
# //hs_err_pid1.log
Java HotSpot(TM) 64-Bit Server VM warning: INFO: os::commit_memory(0x0000000769990000, 1449590784, 0) failed; error='Cannot allocate memory' (errno=12)
#
# There is insufficient memory for the Java Runtime Environment to continue.
# Native memory allocation (malloc) failed to allocate 1449590784 bytes for committing reserved memory.
# An error report file with more information is saved as:
# /tmp/hs_err_pid1.log
Java HotSpot(TM) 64-Bit Server VM warning: INFO: os::commit_memory(0x0000000769990000, 1449590784, 0) failed; error='Cannot allocate memory' (errno=12)
#
# There is insufficient memory for the Java Runtime Environment to continue.
# Native memory allocation (malloc) failed to allocate 1449590784 bytes for committing reserved memory.
# Can not save log file, dump to screen..
#
# There is insufficient memory for the Java Runtime Environment to continue.
# Native memory allocation (malloc) failed to allocate 1449590784 bytes for committing reserved memory.
# Possible reasons:
#   The system is out of physical RAM or swap space
#   In 32 bit mode, the process size limit was hit
# Possible solutions:
#   Reduce memory load on the system
#   Increase physical memory or swap space
#   Check if swap backing store is full
#   Use 64 bit Java on a 64 bit OS
#   Decrease Java heap size (-Xmx/-Xms)
#   Decrease number of Java threads
#   Decrease Java thread stack sizes (-Xss)
#   Set larger code cache with -XX:ReservedCodeCacheSize=
# This output file may be truncated or incomplete.
#
#  Out of Memory Error (os_linux.cpp:2756), pid=1, tid=140325689620224
#
# JRE version:  (7.0_79-b15) (build )
# Java VM: Java HotSpot(TM) 64-Bit Server VM (24.79-b02 mixed mode linux-amd64 compressed oops)
# Core dump written. Default location: //core or core.1
#

分析系统日志

查看系统日志，发现有大量由于内存溢出，进程被杀死的记录

$grep -i 'Out of Memory' /var/log/messages
Apr  7 10:04:02 centos106 kernel: Out of memory: Kill process 1192 (java) score 54 or sacrifice child
Apr  7 10:08:00 centos106 kernel: Out of memory: Kill process 2301 (java) score 54 or sacrifice child
Apr  7 10:09:59 centos106 kernel: Out of memory: Kill process 28145 (java) score 52 or sacrifice child
Apr  7 10:20:40 centos106 kernel: Out of memory: Kill process 2976 (java) score 54 or sacrifice child
Apr  7 10:21:08 centos106 kernel: Out of memory: Kill process 3577 (java) score 47 or sacrifice child
Apr  7 10:21:08 centos106 kernel: Out of memory: Kill process 3631 (java) score 47 or sacrifice child
Apr  7 10:21:08 centos106 kernel: Out of memory: Kill process 3634 (java) score 47 or sacrifice child
Apr  7 10:21:08 centos106 kernel: Out of memory: Kill process 3640 (java) score 47 or sacrifice child
Apr  7 10:21:08 centos106 kernel: Out of memory: Kill process 3654 (java) score 47 or sacrifice child
Apr  7 10:27:27 centos106 kernel: Out of memory: Kill process 6998 (java) score 51 or sacrifice child
Apr  7 10:27:28 centos106 kernel: Out of memory: Kill process 7027 (java) score 52 or sacrifice child
Apr  7 10:28:10 centos106 kernel: Out of memory: Kill process 7571 (java) score 42 or sacrifice child
Apr  7 10:28:10 centos106 kernel: Out of memory: Kill process 7586 (java) score 42 or sacrifice child

端口冲突

该docker监听端口已经被其他进程占用，一般此种问题容易出现在新部署的服务，或在原有机器上部署新的后台服务，所以在部署之前应该执行命令检查端口是否已经被占用，如果上线后发现占有则应改为可用端口再重启之。

检查命令：    netstat -nltp|grep [规划的端口号]

解决方案

内存不够的方案

进程在运行一段时间后占用大量内存，需要将其重启。重启命令可能有时并不起作用。主要检查运行状态，如果未成功停止，则重新重启；

ELK日志收集程序或者其他java进程占用过高，用top和ps命令排查，谨慎确定进程的作用，在确保不影响业务的情况下，停止相关进程；

释放被占用的内存（buff/cache）

 [root]# sync  #将内存数据写入磁盘
 [root]# echo 3 > /proc/sys/vm/drop_caches  #释放被占用的内存

有时候并不是buff/cache过高导致内存不够用，确实是被很多必要的进程消耗掉了内存，那就需要从机器资源分配使用的层面去考虑和解决了。

端口冲突的对策

一般此种问题容易出现在新部署的服务，或在原有机器上部署新的后台服务，所以在部署之前应该执行命令检查端口是否已经被占用，如果上线后发现占有则应改为可用端口再重启之。

检查命令：    $netstat -nltp|grep [规划的端口号]

推荐学习：《docker视频教程》

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