Docker镜像与容器

镜像是什么

镜像是一种轻量级、可执行的独立软件包，再来打包软件运行环境和基于运行环境开发的软件，它包含运行某个软件所需内容，包括代码、运行时库、环境变量和配置文件。

镜像加载原理

unionFS（联合文件系统）

是一种分层、轻量级并且高性能的文件系统，它支持对文件系统的修改，作为一次提交来一层一层的叠加，同时可以将不同目录挂载到同一个虚拟文件系统下(unite serveral directories into a single virtual filesystem)，union文件系统时docker镜像的基础，镜像可以通过分层来进行继承，基于基础镜像，可以制作各种具体的应用镜像。

docker加载镜像原理

docker镜像是由一层一层的文件系统组成，这种层级的文件系统，那么层级之间如何实现文件隔离（宿主机与容器隔离）

为了实现文件分层，rootfs诞生了，在介绍bootfs之前首先要了解一下bootfs！

bootfs（boot file system）主要包含bootloader和kernel，bootloader主要引导加载kernel，Linux刚启动时会加载bootfs文件系统，Docker镜像文件的最底层就是bootfs，这一层与linux\unix系统是一样的，包含boot加载器和内核，当boot加载完成之后整个内核就都在内存中了，此时内存的使用权已交由bootfs交给内核，同时会卸载bootfs。

rootfs（root file system）代表各种不同操作系统的发行版本，在bootfs之上，包含典型的Linux系统中的/dev,/proc,/etc等标准目录和文件。

理解分层

首先如下图可以看到，下载redis镜像包的时候，下载了多个分层

# 查看镜像元数据
root:~ $ docker image inspect redis:latest
[
    {
        "Id": "sha256:02c7f2054405dadaf295fac7281034e998646996e9768e65a78f90af62218be3",
        "RepoTags": [
            "redis:latest"
        ],
        "RepoDigests": [
            "redis@sha256:552acf4b9f1bdbb6eaee9d02ba533a8f996641c8dd453c7b4ef607525a76cd5a"
        ],
        "Parent": "",
        "Comment": "",
        "Created": "2021-09-03T13:26:48.395038582Z",
        "Container": "3627b30c1a4cf3ef703e0516beee6428efe1b08a5314f5f106b84250635809cb",
        "ContainerConfig": {
            "Hostname": "3627b30c1a4c",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "ExposedPorts": {
                "6379/tcp": {}
            },
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
                "GOSU_VERSION=1.12",
                "REDIS_VERSION=6.2.5",
                "REDIS_DOWNLOAD_URL=http://download.redis.io/releases/redis-6.2.5.tar.gz",
                "REDIS_DOWNLOAD_SHA=4b9a75709a1b74b3785e20a6c158cab94cf52298aa381eea947a678a60d551ae"
            ],
            "Cmd": [
                "/bin/sh",
                "-c",
                "#(nop) ",
                "CMD [\"redis-server\"]"
            ],
            "Image": "sha256:332cd8bceec776c2152877041d9e60cbaf4ba95504335e888311bcc1444155a7",
            "Volumes": {
                "/data": {}
            },
            "WorkingDir": "/data",
            "Entrypoint": [
                "docker-entrypoint.sh"
            ],
            "OnBuild": null,
            "Labels": {}
        },
        "DockerVersion": "20.10.7",
        "Author": "",
        "Config": {
            "Hostname": "",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "ExposedPorts": {
                "6379/tcp": {}
            },
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
                "GOSU_VERSION=1.12",
                "REDIS_VERSION=6.2.5",
                "REDIS_DOWNLOAD_URL=http://download.redis.io/releases/redis-6.2.5.tar.gz",
                "REDIS_DOWNLOAD_SHA=4b9a75709a1b74b3785e20a6c158cab94cf52298aa381eea947a678a60d551ae"
            ],
            "Cmd": [
                "redis-server"
            ],
            "Image": "sha256:332cd8bceec776c2152877041d9e60cbaf4ba95504335e888311bcc1444155a7",
            "Volumes": {
                "/data": {}
            },
            "WorkingDir": "/data",
            "Entrypoint": [
                "docker-entrypoint.sh"
            ],
            "OnBuild": null,
            "Labels": null
        },
        "Architecture": "amd64",
        "Os": "linux",
        "Size": 105408201,
        "VirtualSize": 105408201,
        "GraphDriver": {
            "Data": {
                "LowerDir": "/var/lib/docker/overlay2/52fc03c3b2ef6e64509de12f75f8e03f438e91e46a3b57848dadb901d2958e28/diff:/var/lib/docker/overlay2/e68b7712564163bd45a281a4cb70427eccc6809d5fe4b0fc7685df7000511527/diff:/var/lib/docker/overlay2/638d4e1035840a6fa95d50dc77c622d9ebbb91127a87b75e0f7e23dfe3f6574c/diff:/var/lib/docker/overlay2/ef858710684e26dfcbf497a47f82fc0bf01a6de07841119b6469c5beb768b73c/diff:/var/lib/docker/overlay2/7c548de738fe0a8c58f8962653ea083c9aa8c1e1b58c2b692245583ca4788326/diff",
                "MergedDir": "/var/lib/docker/overlay2/d03abe3f1d7ba89cbf16b3e631d2f850215286d01d4243b7045d48c77df7a4db/merged",
                "UpperDir": "/var/lib/docker/overlay2/d03abe3f1d7ba89cbf16b3e631d2f850215286d01d4243b7045d48c77df7a4db/diff",
                "WorkDir": "/var/lib/docker/overlay2/d03abe3f1d7ba89cbf16b3e631d2f850215286d01d4243b7045d48c77df7a4db/work"
            },
            "Name": "overlay2"
        },
        "RootFS": {
            "Type": "layers",
            "Layers": [
                "sha256:d000633a56813933cb0ac5ee3246cf7a4c0205db6290018a169d7cb096581046",
                "sha256:bdad86443e47c5665683ac41c1f24f28479d830d7e3cc47d0a337ee5166c7714",
                "sha256:6a7992ac480029d82b7dbb757d16fe5d023aa283ed32b52267cd1fe9e6b73c49",
                "sha256:be43d2475cf809c0f2ec31950e849d0f888f3121970fd99196a11a903f8c3820",
                "sha256:be5818ef2907adfe19be14bf66647b5fb5a2029143f9297f8ce1ff1fd1d35753",
                "sha256:c54e0c16ea22fa873c12f6a7f3fb4e021bb58d67c292156a6ce6e08637458b7c"
            ]
        },
        "Metadata": {
            "LastTagTime": "0001-01-01T00:00:00Z"
        }
    }
]