AUFS: Details on Modifying and Deleting Files/Directories

layer where it is found. - **The file only exists in the container layer**: If a container opens a file for read access and the file exists in the container layer, it is read from there. - **The file exists in both the container layer and the image layer**: If a container opens a file for read access and the file exists in the container layer and one or more image layers, the file is read from the container layer. Files in the container layer obscure files with the same name in the image layers. ### Modifying files or directories Consider some scenarios where files in a container are modified. - **Writing to a file for the first time**: The first time a container writes to an existing file, that file does not exist in the container (`upperdir`). The `aufs` driver performs a *copy_up* operation to copy the file from the image layer where it exists to the writable container layer. The container then writes the changes to the new copy of the file in the container layer. However, AUFS works at the file level rather than the block level. This means that all copy_up operations copy the entire file, even if the file is very large and only a small part of it is being modified. This can have a noticeable impact on container write performance. AUFS can suffer noticeable latencies when searching for files in images with many layers. However, it is worth noting that the copy_up operation only occurs the first time a given file is written to. Subsequent writes to the same file operate against the copy of the file already copied up to the container. - **Deleting files and directories**: - When a _file_ is deleted within a container, a *whiteout* file is created in the container layer. The version of the file in the image layer is not deleted (because the image layers are read-only). However, the whiteout file prevents it from being available to the container. - When a _directory_ is deleted within a container, an _opaque file_ is created in the container layer. This works in the same way as a whiteout file and effectively prevents the directory from being accessed, even though it still exists in the image layer.

When a container modifies an existing file for the first time, AUFS performs a `copy_up` operation, copying the entire file to the container layer, which can impact write performance. Subsequent writes modify the copy in the container layer. Deleting a file creates a 'whiteout' file in the container layer, preventing access to the original in the image layer. Deleting a directory creates an 'opaque' file, effectively blocking access even though the directory still exists in the image layer.