Messages in this thread | | | Date | Tue, 31 Jul 2007 13:11:59 -0400 | From | Josef Sipek <> | Subject | Re: [RFC 12/26] ext2 white-out support |
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On Tue, Jul 31, 2007 at 07:00:12PM +0200, Jan Blunck wrote: > On Tue, Jul 31, Josef Sipek wrote: > > > On Mon, Jul 30, 2007 at 06:13:35PM +0200, Jan Blunck wrote: > > > Introduce white-out support to ext2. > > > > I think storing whiteouts on the branches is wrong. It creates all sort of > > nasty cases when people actually try to use unioning. Imagine a (no-so > > unlikely) scenario where you have 2 unions, and they share a branch. If you > > create a whiteout in one union on that shared branch, the whiteout magically > > affects the other union as well! Whiteouts are a union-level construct, and > > therefore storing them at the branch level is wrong. > > So you think that just because you mounted the filesystem somewhere else it > should look different? This is what sharing is all about. If you share a > filesystem you also share the removal of objects.
The removal happens at the union level, not the branch level. Say you have:
/a/ /b/foo /c/foo
And you mount /u1 as a union of {a,b}, and /u2 as union of {a,c}.
$ find /u* /u1 /u1/foo /u2 /u2/foo $ rm /u1/foo # this creates whiteout for "foo" in /a $ find /u* /u1 /u2
Is that what you'd expect as a user? I don't think so.
... > > Really the only sane way of keeping track of whiteouts seems some external > > store. We did an experiment with Unionfs, and moving the whiteout handling > > to effectively a "library" that did all the dirty work cleaned up the code > > considerably [2,3]. > > Haven't checked if you could use ODF for a generic store for filesystems that > couldn't support whiteouts. This might be an interesting idea. Yes, since the ODF is completely separate, you can use _any_ filesystem and regardless of whether or not they support whiteouts.
Josef 'Jeff' Sipek.
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