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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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Isn't that covered by the wikipedia article? When mounting branches, the priority of one branch over the other is specified. So when both branches contain a file with the same name, one gets priority over the other. – Caspar Aug 22 '11 at 3:10
@Caspar - Yes it is however I would like to understand how it unifies directories and gives precedence to one over another – PeanutsMonkey Aug 22 '11 at 3:17
up vote 4 down vote accepted

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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Thanks. I did read the article on LinuxJournal but what confused me was how does this work with say a Linux LiveCD and a base installation of say Windows. Also what do you mean by a base layer and a branch on ramdisk? – PeanutsMonkey Aug 22 '11 at 3:19
clarified base layer. A ramdisk is simply a disk image or a special filesystem entirely run on ram. – Journeyman Geek Aug 22 '11 at 3:21
@Journerman Geek - When you say a low precedence layer with the root filesystem do you mean any directory below \? – PeanutsMonkey Aug 22 '11 at 3:27
@Journerman Geek - I came across this article at LWN and from what I understand a UnionFS is managed manually not automatically i.e. an administrator would have to create it as opposed to it being dynamically created. – PeanutsMonkey Aug 22 '11 at 3:29

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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Thanks. I did finally figure it out but will have a read of what it is because I have no idea of its function. – PeanutsMonkey Aug 22 '11 at 3:33

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