Docker的Native Overlay Diff选项相关随记

核心原因是因为overlay module 在load的时候参数值的差异

• 与overlay模块参数redirect_dir有关
• redirect_dir 参数值为off 的时候, Native Overlay Diff为true
• 此参数会对docker build带来性能等差异

当native overlay diff 为false带来的影响

(注意此下结论为引用自他人结论，未经我实际测试)

如果返回是 False。意味这 kernel 不会处理 Diff，而是通过“上层”的方法处理，性能很低。另外可能会有bug，这些问题可以通过 docker 的日志看到。

具体bug可以参考  https://github.com/moby/moby/blob/ed07e1152879a4d156dff2e86abca3c4c811e743/daemon/graphdriver/overlay2/check.go

doesSupportNativeDiff checks whether the filesystem has a bug which copies up the opaque flag when copying up an opaque directory or the kernel enable CONFIG_OVERLAY_FS_REDIRECT_DIR.When these exist naive diff should be used.

Not using native diff for overlay2, this may cause degraded performance for building images

排障

core command:

/sys/module/$module/parameters/$parameter

script: dump all loaded module parameter values

#/bin/env bash
cat /proc/modules | cut -f 1 -d " " | while read module; do \
 echo "Module: $module"; \
 if [ -d "/sys/module/$module/parameters" ]; then \
  ls /sys/module/$module/parameters/ | while read parameter; do \
   echo -n "Parameter: $parameter --> "; \
   cat /sys/module/$module/parameters/$parameter; \
  done; \
 fi; \
 echo; \
done

解决方法/ workaround

echo 'options overlay redirect_always_follow=off redirect_dir=off' > /etc/modprobe.d/disable_overlay_redirect_dir.conf