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What does "native overlay diff" mean in the overlay2 storage driver?

I experienced that the bottom layer (diff) of the image that is associated with the running container has been removed. (Therefore, some files in the container are deleted)

I think the option “Native Overlay Diff” from docker info is pretty suspicious.

My docker info as below:

$ docker info ... Storage Driver: overlay2 Backing Filesystem: xfs Supports d_type: true Native Overlay Diff: false ...

Do you guys know the exact meaning of the word "Native Overlay Diff"?

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This is similar to the kernel parameter OVERLAY_FS_REDIRECT_DIR , which is described in Kconfig as:

config OVERLAY_FS_REDIRECT_DIR
bool "Overlayfs: enable dir redirection function by default"
depends on OVERLAY_FS
help

If this configuration option is enabled, then overlaid file systems will use redirects when renaming default directories. In this case, it is still possible to disable redirection worldwide with the option "redirect_dir = off" or based on an instance of the file system with the option "redirect_dir = off".

Please note that redirects do not support backward compatibility. That is, mounting an overlay that redirects to a kernel that does not support this feature will have unexpected results.

If you are not sure, say N.

Some discussion on mobility issues 34342 and 34320 indicates that if all of the following values ​​are true:

  • OVERLAY_FS_REDIRECT_DIR kernel option OVERLAY_FS_REDIRECT_DIR
  • storage engine - overlay2 (default in most cases)
  • docker stores images on a file system that is not mounted using redirect_dir=off
  • uses the native diff driver.

  • a non-empty directory is renamed as part of a docker assembly, for example. in a Docker file, for example:

     FROM busybox RUN mkdir /dir1 RUN touch /dir1/newfile RUN mv /dir1 /dir2

Then the resulting image will not correctly record the contents of the renamed directory (i.e. dir2 will not contain the new file), because the renaming of the directory was implemented as a redirect using the extended file attribute (xattr), which is not understood by the docker archiving process. To solve this problem, when the first three conditions above are met, then the docker will use the “naive” diff driver, which creates the correct images, but slower than the “native” diff driver.

It seems like you can ignore the warning, but if you notice slow builds, try reinstalling the volume servicing /var/lib/docker with the redirect_dir=off option.

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Source: https://habr.com/ru/post/1272687/

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