Re: Union mounts comparison with overlay file system prototype?

[Ric Wheeler <rwheeler@redhat.com>]

On 03/25/2011 07:38 AM, Szeredi Miklos wrote:
> On Thu, Mar 24, 2011 at 2:43 PM, Ric Wheeler<rwheeler@redhat.com>  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

Thanks for the high level overview!

Ric