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overlay-read-write-splitting-snapshot

一个支持读写分离的 OverlayFS snapshotter，作为 containerd 外部 gRPC 插件存在。

An OverlayFS snapshotter that splits readonly and writable layers, as an external gRPC plugin for containerd.

使用

适用场景：容器镜像较大，但又期望容器可写层性能更好。若将快照都存放在高性能磁盘上则十分浪费储存资源，读写分离可以很好地降低成本。

使用 Go 1.21 以上编译，将程序作为 daemon 启动后，自动创建 /var/lib/containerd/tech.imvictor.containerd.overlay-read-write-splitting-snapshotter。

将可写层文件系统挂载于 /var/lib/containerd/tech.imvictor.containerd.overlay-read-write-splitting-snapshotter/writable，即可享受读写分离！

在 /etc/containerd/config.toml 需要配置：

[proxy_plugins]
  [proxy_plugins.overlay-read-write-splitting-snapshotter]
    type = "snapshot"
    address = "/var/lib/containerd/tech.imvictor.containerd.overlay-read-write-splitting-snapshotter/grpc.socks"

若使用 Kubernetes，还需要按以下 TOML 路径配置：

[plugins]
  [plugins."io.containerd.grpc.v1.cri"]
    [plugins."io.containerd.grpc.v1.cri".containerd]
      snapshotter = "overlay-read-write-splitting-snapshotter"

若从其他 snapshotter 迁移，请确保将 containerd 目录完全清空，否则可能出现不可预料的问题。

效果

这是 Kubernetes 宿主机上的展示效果：

root@NAS:/var/lib/containerd # mount | grep /var/lib/containerd/tech.imvictor.containerd.overlay-read-write-splitting-snapshotter | egrep -v '^overlay'
/dev/zd512 on /var/lib/containerd/tech.imvictor.containerd.overlay-read-write-splitting-snapshotter type ext4 (rw,relatime,stripe=2)
/dev/zd528 on /var/lib/containerd/tech.imvictor.containerd.overlay-read-write-splitting-snapshotter/writable type ext4 (rw,relatime,stripe=2)
root@NAS:/var/lib/containerd # df -h /var/lib/containerd/tech.imvictor.containerd.overlay-read-write-splitting-snapshotter/writable
Filesystem      Size  Used Avail Use% Mounted on
/dev/zd528       69G  226M   65G   1% /var/lib/containerd/tech.imvictor.containerd.overlay-read-write-splitting-snapshotter/writable
root@NAS:/var/lib/containerd # df -h /var/lib/containerd/tech.imvictor.containerd.overlay-read-write-splitting-snapshotter
Filesystem      Size  Used Avail Use% Mounted on
/dev/zd512       98G   26G   68G  28% /var/lib/containerd/tech.imvictor.containerd.overlay-read-write-splitting-snapshotter
root@NAS:/var/lib/containerd # kubectl exec -it busybox -- sh
/ # df -h /
Filesystem                Size      Used Available Use% Mounted on
overlay                  68.4G    225.2M     64.6G   0% /
/ # mount
overlay on / type overlay (rw,relatime,lowerdir=/var/lib/containerd/tech.imvictor.containerd.overlay-read-write-splitting-snapshotter/snapshots/131/fs,upperdir=/var/lib/containerd/tech.imvictor.containerd.overlay-read-write-splitting-snapshotter/writable/snapshots/141/fs,workdir=/var/lib/containerd/tech.imvictor.containerd.overlay-read-write-splitting-snapshotter/writable/snapshots/141/work)

原理和参考

该 snapshotter 大致继承于 overlayfs snapshotter，通过劫持 mount 列表和特化回收逻辑，并对 containerd 操作进行判断，从而在容器启动时将可写层和只读层分离。

特别感谢 rectcircle/overlay-custom-add-lower-snapshotter，为本项目工程框架提供复用能力，且作者的博文提供了很多思路。