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SubjectRe: [PATCH 0/7] overlay filesystem: request for inclusion

Erez Zadok:
> ... Asking =
> overlayfs or other stackable file systems to solve this multi-layer =
> coherency perfectly is somewhat ridiculous: we don't expect file systems =
> like ext3 to detect and correctly handle changes to lower devices =97 =
> i.e., if someone hand-edited direct blocks in /dev/sda1, do we?

I agree with you if we discuss about union-type-mount, which handles a
block device as its member. As long as the layered-fs handles a
directory (mounted filesystem) as its member, it is obviously right that
users expect the modification on the member fs (by-passing a union) is

Of course I agree it brings complication to us, and I'd suggest three
level options to support this issue.
- detect the direct changes and reflect it to union (hardest option)
- skip the detection, but verify the parent-child relationship or more
at least. (this is something like overlayfs is trying to do)
- skip both of the detection and verification (lowest option)
this option depends how user sets up the union and its member. if user
hides the members totally by over-mounting an empty dir on the member
(or something), then he can specify this option. otherwise, this
option is dangerous. also some symlinks may not work.
# mkdir /hide
# mount -o upper=/rw,lower=/ro none /union
# mount -o bind /hide /rw
# mount -o bind /hide /ro

J. R. Okajima

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