运行docker system df 命令：

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[root@VM_100_4_centos ~]# docker system df 
TYPE                TOTAL                       SIZE                RECLAIMABLE
Images              955                         77.06GB             74.21GB (96%)
Containers          65                          3.28MB              0B (0%)
Local Volumes       0                           0B                  0B
Build Cache         0                           0B                  0B

最后一列RECLAIMABLE字段标明了可回收的磁盘空间大小，此处表示有74.21GB的docker镜像可以收回，而containers、volumes等没有可回收的空间。

可以回收的镜像分为两类：dangling和其他未被container关联的镜像。 dangling可以理解为TAG空的某些镜像，使用命令docker images -f dangling=true可以找出来，如下是在某节点上：

[root@VM_100_4_centos ~]# docker images -f dangling=true
REPOSITORY                                                    TAG                 IMAGE ID            CREATED             SIZE
ccr.ccs.tencentyun.com/middleware/skywalking-agent        <none>              1054bfb1c79a        3 weeks ago         689MB
ccr.ccs.tencentyun.com/middleware/logstash-logs           <none>              a0e0f37728bf        2 months ago        805MB
ccr.ccs.tencentyun.com/middleware/log-clean               <none>              b4fe723fcd28        3 months ago        747MB
ago        747MB
ccr.ccs.tencentyun.com/middleware/clean-job               <none>              53aac26b0560        3 months ago        747MB

要删除上面这些dangling镜像，执行命令docker image prune即可。

[root@VM_100_4_centos ~]# docker image prune
WARNING! This will remove all dangling images.
Are you sure you want to continue? [y/N] y
Deleted Images:
untagged: ccr.ccs.tencentyun.com/gwy-middleware/clean-job@sha256:2633cc3ad0abc3dd632befecb5dcf90465021dde27e3cc0dcec2a7c37bfa1a50
deleted: sha256:77c50813e2090e2a45c4988eece1bdb79672c0dec070fad7eec1179e743c06e2
deleted: sha256:cdd7d4f6cc290f6220c3ee07061f64227646e1efdf9382204a150c735bd5a028
deleted: sha256:5c603aa4622920d7b242ab6e106538a4a5d72d72febef6e1e60d5d2c515f2311
untagged: ccr.ccs.tencentyun.com/gwy-middleware/filebeat-logs@sha256:e9ed0280250d74eb3ff9d506fd275cad280e3e781afd745aab5f0226e28d111e
deleted: sha256:45db79ccd3b44940c85c4979c3b307907d723bc7e4379d77abfecb5ca006e030
deleted: sha256:eb93a48f29bba12835ade4123e2bd1538d80b755f621369704675a9c19b18f65
......

Total reclaimed space: 1.478GB

如果要删除所有未被使用的镜像，包括dangling镜像，可以用docker image prune -a，这里可以加上时间参数，比如删除30天（720小时）前创建的所有不再使用的镜像。

[root@VM_100_4_centos ~]# docker image prune -a --force --filter "until=720h"
Deleted Images:
untagged: ccr.ccs.tencentyun.com/gwy-cloud/guowy-cloud-zuul:202103041715
untagged: ccr.ccs.tencentyun.com/gwy-cloud/guowy-cloud-zuul@sha256:9e2d18d259582ca6a04b27d1609c8201a648f8feae37ff984f237dc4e907da6b
deleted: sha256:95f7d7107c50e77f0aa4a93ace02f031ae29edd13b56531757ed86fd63b9cc04
deleted: sha256:68d9605658fa91174c1867a5f91d6b6cc063f3ace1fbb061c72abbc84ea5de5b

Total reclaimed space: 63.23GB

对比docker system df可以看到效果，释放了60多GB的空间。该节点的磁盘空间使用率也从原来的83%的降到了18%。

[root@VM_100_4_centos ~]# docker system df
TYPE                TOTAL               ACTIVE              SIZE                RECLAIMABLE
Images              131                 33                  12.36GB             8.979GB (72%)
Containers          65                  59                  3.287MB             0B (0%)
Local Volumes       0                   0                   0B                  0B
Build Cache         0                   0                   0B                  0B

同理，回收containers、volumes磁盘空间的方法是一样的。 而docker提供了一个删除所有未使用的资源的命令docker system prune -a，这包括了container状态是exited的情况。