Re: FS Unions

Jan-Simon Pendry (jsp@ms.com)
Tue, 15 Jun 1999 05:51:45 +0100

Alexander Viro wrote:
>
> [ good explanation of create/delete process ... ]
>
> > To create a fully-realized implementation, would there have
> > to be modifications to commands or facilities to take into account
> > or exploit the fs unions? If so, this might be a way for some others
>
> mount(8), umount(8) and fsck(8). Mount should be taught to deal
> with the new flag, fsck should be taught to that whiteouts are valid
> entries. Other than that... Some utility to find and remove whiteouts.

just to summarise, the top level filesystem requires the following
(additional) functionality:

1. persistent representation of a whiteout node.
2. persistently mark a directory non-transparent.
3. return list of whiteouts in a directory.
4. create a named whiteout.
5. remove a named whiteout.

all apart from #1 must also be accessible via system call
interfaces.

underlying filesystems require no modifications.

user-level changes are:

1. change opendir et al. to deal with whiteouts.
2. change file-tree walker to deal with whiteouts.
3. add options to rm, ls and find to handle whiteouts.
4. teach fsck about specific storage implementation of whiteouts.
5. provide backup/recovery hooks.

note that the bsd implementation provides backup/recovery
using dump/restore. dump/restore correctly handle whiteout
nodes. tar, pax etc. have no knowledge of whiteouts.

jan-simon.

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