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What is an overlay in Linux?
An overlay filesystem combines two filesystems – an ‘upper’ filesystem and a ‘lower’ filesystem. When a name exists in both filesystems, the object in the ‘upper’ filesystem is visible while the object in the ‘lower’ filesystem is either hidden or, in the case of directories, merged with the ‘upper’ object.
What is overlay filesystem Docker?
OverlayFS is a modern union filesystem that is similar to AUFS, but faster and with a simpler implementation. Docker provides two storage drivers for OverlayFS: the original overlay , and the newer and more stable overlay2 .
What is overlay in DF output?
Brought into the Linux kernel mainline with version 3.18, OverlayFS allows you to overlay the contents (both files and directories) of one directory onto another. We can think of them as “upper” and “lower,” with the rest of Linux and applications positioned above that. The “lower” directory is read-only.
What is Ubuntu Squashfs tools?
Squashfs is a highly compressed read-only filesystem for Linux. It uses zlib compression to compress both files, inodes and directories. Inodes in the system are very small and all blocks are packed to minimize data overhead.
What is overlay folder?
Posted by: Thom Denholm. Union file systems are a creative solution to allow a virtual merge of multiple folders, while keeping their actual contents separate. The Overlay file system (OverlayFS) is one example of these, though it is more of a mounting mechanism than a file system.
What are the 3 different directories in var lib Docker AUFS?
The container layer
- diff/ : Differences introduced in the writable container layer, such as new or modified files.
- layers/ : Metadata about the writable container layer’s parent layers.
- mnt/ : A mount point for each running container’s unified filesystem, exactly as it appears from within the container.
When to use metadata only copy up in OverlayFS?
When metadata only copy up feature is enabled, overlayfs will only copy up metadata (as opposed to whole file), when a metadata specific operation like chown/chmod is performed. Full file will be copied up later when file is opened for WRITE operation.
How does the overlay filesystem work in Linux?
In case the underlying inode number does overflow into the high xino bits, overlay filesystem will fall back to the non xino behavior for that inode. The “xino” feature can be enabled with the “-o xino=on” overlay mount option.
How does an overlay filesystem support RM and rmdir?
In order to support rm and rmdir without changing the lower filesystem, an overlay filesystem needs to record in the upper filesystem that files have been removed. This is done using whiteouts and opaque directories (non-directories are always opaque). A whiteout is created as a character device with 0/0 device number.
When to use overlayfs to rename a directory?
When renaming a directory that is on the lower layer or merged (i.e. the directory was not created on the upper layer to start with) overlayfs can handle it in two different ways: return EXDEV error: this error is returned by rename (2) when trying to move a file or directory across filesystem boundaries.