Re: Union mounts comparison with overlay file system prototype?

[Szeredi Miklos]

On Thu, Mar 24, 2011 at 2:43 PM, Ric Wheeler <rwheeler@xxxxxxxxxx> wrote:
> Can one or both of you summarize what we union mounts and overlay do better
> or worse? Do we need both or just one?

The semantics are very similar, the differences are in the implementation.

Union mounts:

 - whiteout/opaque/fallthrough support in filesystems
 - whiteout operation is atomic
 - no dentry and inode duplication
 - copy up on lookup and readdir
 - does not support union of two read-only trees
 - merged directory stored in upper tree

Overlayfs

- whiteout/opaque as xattrs
- whiteout operation is not atomic
- dentry and inode duplication(*)
- only copy up on modification
- supports union of two read-only trees
- merged directory not cached(**)

(*) it's possible to eliminate inode duplication of non-directories
with some VFS modifications
(**) caching should be possible to do
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