On Wed, 2008-01-16 at 12:13 -0500, Erez Zadok wrote: > In message <[EMAIL PROTECTED]>, Paul Albrecht writes: > > Hi, > > > > I have a question about how unionfs handles file deletion when a write > > enabled file system is union mounted over a read only file system. For > > example, I do something like the following: > > > > mount -t unionfs -o dirs=/cow=rw:/rofs=ro unionfs /mnt > > > > If I create and delete a file in /mnt which is not present in /rofs it > > persists as whiteout in the cow file system which is not what I would > > have expected. > > Paul, the alternative is to scan all branches (there could be many) to > ensure that the file may not exist by that name anywhere else, and if so, > try to delete all instances of it. This was deemed too expensive and > complex. >
I'm not sure we're talking about the same problem. What I do is union mount a write enabled file system like tmpfs over a read only file system like squashfs. There's no way to create, modify, or delete files in a squashed file system; they can be copied up when they're modified; or, they can be whited out when they're deleted. Whenever a file is created in the union mount, it necessarily gets created in tmpfs. When that file gets deleted, it gets whited out which doesn't make sense because it doesn't exist in the other layer. This is a problem because over time as files are created, modified, and deleted whiteout cruft accumulates in the cow layer of the union mount. Fixing the problem doesn't seem that complex and shouldn't require searching all the layers of the union mount. If the union file system simply took note of whether a file was created in the cow layer because it's new or because it's been modified and copied up from the read only file system, then it would simply delete the file in the former case and and use whiteout in the latter. > Another possible problem is that if you choose to insert a new branch in the > middle, and you didn't have the whiteout, you may re-expose the file name > unintentionally. > I don't see how the a "deleted" file in a read only file system could be re-exposed unless its whiteout in the cow layer was deleted, but that's really not the issue. What I'm objecting to is creating the whiteout in the cow layer when the file didn't get there via a copy up from a read only file system. In this case there's no worry about re-exposing the deleted file because it's really deleted. > You might want to take a look at our unionfs-odf version: it places > whiteouts in a separate persistent store outside the branches, not as .wh.* > files in individual branches. > > > Why does the deleted file persist as whiteout in the /cow file system of > > the union mount? > > > > Please cc me in your response as I'm not subscribed to the lkml. > > > > -- > > > > Paul Albrecht > > Cheers, > Erez. -- Paul Albrecht -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
