Comment by pcr910303 | Hacker News Reader

Comment by pcr910303 | Hacker News Reader

Hmm... how is Overlayfs and Unionfs different? From the explanation I can't find any differences...

Unionfs: A Stackable Unification File System[0]:

> This project builds a stackable unification file system, which can appear to merge the contents of several directories (branches), while keeping their physical content separate.

> Unionfs allows any mix of read-only and read-write branches, as well as insertion and deletion of branches anywhere in the fan-out.

> To maintain unix semantics, Unionfs handles elimination of duplicates, partial-error conditions, and more.

If it is the same thing (but maybe more maintained or has more features...) , can we implement something like trip[1][2] package manager on top of Overlayfs?

(Porg is a package manager where all files that are installed by make install is tracked and mounted on a Unionfs layer.)

[0] http://unionfs.filesystems.org

[1] https://github.com/grencez/trip

[2] http://www.linuxfromscratch.org/hints/downloads/files/packag...

They're mostly the same, with different answers to tricky questions. e.g. if I stack filesystems A, B, C, and have the same file /foo/bar in all of the layers, and then do rm /foo/bar, what happens:

1. Does /foo/bar get removed from the topmost layer, exposing the one below?

2. Does /foo/bar get removed from all three layers?

3. Does /foo/bar get replaced with a "tombstone" record to pretend that it was deleted, while still appearing in some of A, B, or C on its own?

These semantics are tricky to get right, and during the process of upstreaming unionfs to the kernel, they made some incompatible changes to the model and chose different answers for these questions, and as a result, renamed it overlayfs.

It looks like overlayfs removes most of the tricky questions by only allowing the topmost filesystem to be read-write. In that case rm is pretty simple: remove it from the topmost filesystem if it exists there, and make a tombstone if it exists in a lower layer.

Unionfs is significantly more complicated.

Note that there is also a BSD UnionFS filesystem doing much the same thing. I don't know what relation it has to the Linux OverlayFS, only that they (superficially) do very similar things.

Interesting. It has apparently been around for decades.

>Union mounts have also been available in BSD since at least 1995.

See: https://en.wikipedia.org/wiki/UnionFS

They are very similar. Overlayfs is in the default kernel tree, which would be the biggest difference.

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