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I have come across various articles as well as an explanation on Wikipedia however am having a hard time understanding what it means. So far I understand that UnionFS is a module that provides a union view of directories. It resides a a layer above the directories which are mounted. UnionFS does not have a mount point. What confuses me is how does UnionFS know what should be shown in a unified manner? |
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UnionFS works at a directory level, as opposed to a device level, so it dosen't have a mount point - it sits over existing mount points each of which might be a branch - for example, having a base layer (or to use proper terminology - a low precedence layer with the root filesystem) on a read only iso9660 cd rom file system, and a branch on a ramdisk. Each branch is assigned a precedence and a branch with a higher precedence overrides one of a lower precedence. If a directory exists in two underlying branches, the contents and attributes of the Unionfs directory are the combination of the two lower directories. If a file exists in two branches, the contents and attributes of the Unionfs file are the same as the file in the higher-priority branch, and the file in the lower-priority branch is ignored. Finally if there's a duplicate, the duplicate directory is hidden to simplify things. Linuxjournal has a fairly comprehensive writeup on how unionfs works, if you want something more than a simplified explanation of it |
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Its VFS operations are small stubs that call back into the VFS layer of the underlying filesystems. So when you e.g. read a directory, it reads the directories of the underlying filesystems and merges the file lists. |
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