[Unionfs] whiteouts and object removal inconsistency

[Yiannos Pericleous]

On 22/08/07, Bharata B Rao <bharata.rao at gmail.com> wrote:
> Hi Josef/Erez,
> In Unionfs with ODF, since you have the concept of per-union whiteout,
> the removal of objects from unions is not shared IIUC. But when trying
> out the ODF patch, I hit upon a few cases which appear inconsistent to
> me.
>
> Consider 2 directories a/ and b/ with a file in each of them.
> #ls a/
> top
>
> #ls b/
> bottom
>
> Let these two be union mounted at union1/ and union2/ separately with
> a/ being the topmost branch in both. Here a/ is mounted rw and b/ is
> mounted ro.
> (mount -t unionfs -o odf=/odf/odf1,dirs=a=rw:b=ro none union1
> mount -t unionfs -o odf=/odf/odf2,dirs=a=rw:b=ro none union2)
>
> Case 1: removal of  'bottom' in b/ from union1/
> In this case a whiteout gets created for 'bottom' under union1(in ODF
> i,e.,) and subsequently it becomes invisible from union1/ (as
> expected).
> But the removed file continues to be visible from union2/ as the
> whiteout is per-union and union2/ doesn't know about the whiteout that
> got created in union1/

This is expected since: as b is ro, the whiteout is created only for union1
and gets recorded in odf1. Now odf2 knows nothing about this wh and since
the file was not physically removed it appears in union2


> Case 2: removal of 'top' in a/ from union1/
> In this case, 'top' gets removed from the underlying filesystem itself
> as a/ is a rw topmost layer. This makes 'top' to go away from union2/
> also.

In this case since the file exists only in a rw branch no whiteout is
necessary and thus is not created and the file is physically removed.
Thus the file is completely removed and appears in none of the unions.

> So in the first case, the removal of an object from a union didn't
> affect the other union. But in the second case, it did. Isn't this
> behavior inconsistent if we look at 'top' and 'bottom' just as files
> in the union instead of thinking them as files lying in different
> layers of the union ? So is this the expected behavior ? If so how do
> you explain this seemingly inconsistent behavior ?

I would say this is the expected behavior since whiteouts are
created only when necessary.

> Case 3: removal from rw lower layer
> Consider the same scenario as above, but the lower layer(b) is mounted
> rw instead of ro in both the unions. Now the removal of file 'bottom'
> from union1/ gets effected on the underlying filesystem itself and
> hence gets reflected in union2 also. So again this is not consistent
> with the behavior in Case 1.

Same as case 2, since the branch is rw and the file can be removed
it is removed and no whiteout is necessary

> Have you tried to define all such use cases and the expected behavior
> somewhere ?

I believe we had the expected behaviour in the odf documentation
(somewhere in Documentation/filesystems/unionfs?)
The basic approach for creating whiteouts on ODF we decided on was:
try removing on all branches that contain the file, and on the first
failure (or ro branch)
stop trying and create a wh


> Regards,
> Bharata.

yiannis