On Thu, Mar 24, 2011 at 2:43 PM, Ric Wheeler [off-list ref] 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