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SubjectRe: [PATCH 6/7 v3] overlay: hybrid overlay filesystem prototype
Date
On Monday 27 September 2010 20:47:47 Valerie Aurora wrote:
> Maybe I don't understand. It seems like directories created when the
> file system is *not* union mounted should definitely be merged with
> matching directories on the lower layer.
>
> Take the case of /etc/fstab. The first union mount never touches /etc
> and it doesn't exist on the topmost layer. Then we unmount the upper
> layer, mount it somewhere else as a plain mount, and create /etc/ and
> /etc/fstab. When we union mount it back over the lower layer again,
> we still want the lower layer /etc/ to be merged with the topmost
> /etc/, or else init.d will disappear.

I can't think of a reason why the upper layer would really *need* to be
modified separately as in this example though, and I'm sure that examples for
opaqueness by default can be constructed as well. Transparency comes at a
cost though (lookup, readdir, whiteouts), and defaulting to opaque directories
will be more efficient in some cases. This is why I think that opaqueness by
default is preferable.

> Again, maybe I'm misunderstanding, but this doesn't make much sense to
> me. Say I create:
>
> /upper/a_dir/upper_file
> /lower/a_dir/lower_file
>
> Then when I union mount them, I want a_dir/ to be transparent
> automatically and show both upper_file and lower_file, without marking
> it manually.

Why?

Thanks,
Andreas


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