linux-kernel - Re: [RFC][PATCH 0/15] VFS based Union Mount

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Date:	Wed, 18 Apr 2007 12:49:47 +0530
From:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:	Shaya Potter <spotter@...columbia.edu>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Jan Blunck <j.blunck@...harburg.de>
Subject: Re: [RFC][PATCH  0/15] VFS based Union Mount

On Tue, Apr 17, 2007 at 12:56:24PM -0400, Shaya Potter wrote:
> Bharata B Rao wrote:
> 
> >No. foo is not visible. While looking for a file in a union mounted
> >directory, the lookup starts from the topmost directory and proceeds
> >downwards if the file isn't present the top layers. If a whiteout is
> >found in any of the top layers, the lookup is abondoned and -ENOENT
> >is removed. Thus until a whiteout exists in any upper layer for
> >a corresponding file in the lower layer, the lower layer file remains
> >hidden until the whiteout is removed.
> >
> >However in the case of dir-c containing foo, the foo(from dir-c) will 
> >become
> >visible after union mounting dir-c on top of dir-b and dir-a.
> 
> ok, so the major limitation of this approach is that the top most layer 
> has to either be, ext2, ext3 or tmpfs (in patch), and most likely not 
> NFS (assumption is that NFS has no conception of the whiteout type of 
> file).

I haven't played with union mounts with NFS. Hence would let Jan answer
this. However note that union mount provides a writable union only
if the filesystem supports the notion of whiteouts.

> One thing the unionfs people are doing w/ their ODF approach, is 
> within the ODF fs, they have a special inode that is the "whiteout" 
> inode, and when they create a whiteout, they just create a hardlink from 
> the dentry they want to whiteout to the "whiteout inode".  could that be 
> a worthwhile approach instead of the whiteout file type?  (i.e. many 
> file systems support the concept of a hard link).

We we thinking something on similar lines as noted in our documentation.
Right now we maintain one inode for every whiteout. We were planning to
have a single whiteout inode and have all whiteout dentries point to this.
But here again we were thinking of having every filesystem support
this whiteout inode type.

Anyway I will have a look at ODF from unionfs to see how this is done.

>
> I ask, because using union in a diskless environment.  Imagine pxe 
> booting a kernel/initramfs and then using union to create a real root fs 
>  (shared lower layer, private rw upper layer, ala live cds).  Which 
> brings up a different point, with unionfs, one can pivot_root into it, 
> can one do the same for these "union mounts"?  Don't know enough about 
> the VFS to know if this should "just work" or might be a problem.

I would assume that it should 'just work'. But right now it is not working.
Our code is not yet ready to correctly work with move mounts. Since pivot_root
has semantics similar to move mounts, pivot_root is also not working. Also
chroot to a union mount point is also not working atm. We will be working
to get all these right.

Regards,
Bharata.
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