docker映象之快取特性
阿新 • • 發佈：2020-11-16
1、docker映象的快取特性是怎樣的？如何說明？
Docker 會快取已有映象的映象層，構建新映象時，如果某映象層已經存在，就直接使用，無需重新建立。
root@richardo-docker01:~# docker images REPOSITORY TAG IMAGE ID CREATED SIZE centos-with-vim-dockerfile-2 latest fa86e8214b98 2 days ago 272MB centos-with-vim-dockerfile latest e99684eeca8a 2 days ago 272MB centos-with-vim latest 0ec55bc57a3d 2 days ago 272MB httpd latest 3dd970e6b110 4 weeks ago 138MB centos latest 0d120b6ccaa8 3 months ago 215MB hello-world latest bf756fb1ae65 10 months ago 13.3kB root@richardo-docker01:~# ls 0d120b6ccaa8 Dockerfile Running root@richardo-docker01:~# touch testfile root@richardo-docker01:~# vim Dockerfile root@richardo-docker01:~# docker build -t centos-with-vim-dockerfile-2 . Sending build context to Docker daemon 14.85kB Step 1/3 : FROM centos ---> 0d120b6ccaa8 Step 2/3 : RUN yum makecache && yum install -y vim ---> Using cache ---> e99684eeca8a Step 3/3 : COPY testfile / ---> fa86e8214b98 Successfully built fa86e8214b98 Successfully tagged centos-with-vim-dockerfile-2:latest root@richardo-docker01:~#
2、如果希望構建映象的時候不使用快取？應該怎麼做？
如果我們希望在構建映象時不使用快取，可以在 docker build 命令中加上 --no-cache 引數。
3、什麼情況下綁在會失效？
Dockerfile 中每一個指令都會建立一個映象層，上層是依賴於下層的。無論什麼時候，只要某一層發生變化，其上面所有層的快取都會失效。
也就是說，如果我們改變 Dockerfile 指令的執行順序，或者修改或新增指令，都會使快取失效。
eg:交換前面 RUN 和 COPY 的順序
雖然在邏輯上這種改動對映象的內容沒有影響，但由於分層的結構特性，Docker 必須重建受影響的映象層。
root@richardo-docker01:~# cat Dockerfile FROM centos COPY testfile / RUN yum makecache && yum install -y vim root@richardo-docker01:~# docker build -t centos-with-vim-dockerfile-3 . Sending build context to Docker daemon 15.36kB Step 1/3 : FROM centos ---> 0d120b6ccaa8 Step 2/3 : COPY testfile / ---> 32568ab9551d Step 3/3 : RUN yum makecache && yum install -y vim ---> Running in 3d10325475a8 CentOS-8 - AppStream 1.8 MB/s | 5.8 MB 00:03 CentOS-8 - Base 1.9 MB/s | 2.2 MB 00:01 CentOS-8 - Extras 13 kB/s | 8.6 kB 00:00 Last metadata expiration check: 0:00:01 ago on Mon Nov 16 01:26:25 2020. Metadata cache created. Last metadata expiration check: 0:00:04 ago on Mon Nov 16 01:26:25 2020. Dependencies resolved. ================================================================================ Package Arch Version Repository Size ================================================================================ Installing: vim-enhanced x86_64 2:8.0.1763-13.el8 AppStream 1.4 M Installing dependencies: gpm-libs x86_64 1.20.7-15.el8 AppStream 39 k vim-common x86_64 2:8.0.1763-13.el8 AppStream 6.3 M vim-filesystem noarch 2:8.0.1763-13.el8 AppStream 48 k which x86_64 2.21-12.el8 BaseOS 49 k Transaction Summary ================================================================================ Install 5 Packages Total download size: 7.8 M Installed size: 31 M Downloading Packages: (1/5): gpm-libs-1.20.7-15.el8.x86_64.rpm 238 kB/s | 39 kB 00:00 (2/5): vim-filesystem-8.0.1763-13.el8.noarch.rp 616 kB/s | 48 kB 00:00 (3/5): which-2.21-12.el8.x86_64.rpm 433 kB/s | 49 kB 00:00 (4/5): vim-enhanced-8.0.1763-13.el8.x86_64.rpm 1.5 MB/s | 1.4 MB 00:00 (5/5): vim-common-8.0.1763-13.el8.x86_64.rpm 1.7 MB/s | 6.3 MB 00:03 -------------------------------------------------------------------------------- Total 1.4 MB/s | 7.8 MB 00:05 warning: /var/cache/dnf/AppStream-02e86d1c976ab532/packages/gpm-libs-1.20.7-15.el8.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID 8483c65d: NOKEY CentOS-8 - AppStream 66 kB/s | 1.6 kB 00:00 Importing GPG key 0x8483C65D: Userid : "CentOS (CentOS Official Signing Key) <[email protected]>" Fingerprint: 99DB 70FA E1D7 CE22 7FB6 4882 05B5 55B3 8483 C65D From : /etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial Key imported successfully Running transaction check Transaction check succeeded. Running transaction test Transaction test succeeded. Running transaction Preparing : 1/1 Installing : which-2.21-12.el8.x86_64 1/5 Installing : vim-filesystem-2:8.0.1763-13.el8.noarch 2/5 Installing : vim-common-2:8.0.1763-13.el8.x86_64 3/5 Installing : gpm-libs-1.20.7-15.el8.x86_64 4/5 Running scriptlet: gpm-libs-1.20.7-15.el8.x86_64 4/5 Installing : vim-enhanced-2:8.0.1763-13.el8.x86_64 5/5 Running scriptlet: vim-enhanced-2:8.0.1763-13.el8.x86_64 5/5 Running scriptlet: vim-common-2:8.0.1763-13.el8.x86_64 5/5 Verifying : gpm-libs-1.20.7-15.el8.x86_64 1/5 Verifying : vim-common-2:8.0.1763-13.el8.x86_64 2/5 Verifying : vim-enhanced-2:8.0.1763-13.el8.x86_64 3/5 Verifying : vim-filesystem-2:8.0.1763-13.el8.noarch 4/5 Verifying : which-2.21-12.el8.x86_64 5/5 Installed: gpm-libs-1.20.7-15.el8.x86_64 vim-common-2:8.0.1763-13.el8.x86_64 vim-enhanced-2:8.0.1763-13.el8.x86_64 vim-filesystem-2:8.0.1763-13.el8.noarch which-2.21-12.el8.x86_64 Complete! Removing intermediate container 3d10325475a8 ---> f41e704ce73b Successfully built f41e704ce73b Successfully tagged centos-with-vim-dockerfile-3:latest
4、除了構建的時候用快取還有什麼時候會用快取？
除了構建時使用快取，Docker 在下載映象時也會使用。例如我們下載 httpd 映象。
由 Dockerfile 可知 httpd 的 base 映象為 debian，正好之前已經下載過 debian 映象，所以有快取可用。通過 docker history 可以進一步驗證。
https://mp.weixin.qq.com/s/aEi_-nm49EuFBecae4-Baw