Re: [PATCH 01/24] Unionfs: Documentation
From: Shaya Potter
Date: Wed Jan 10 2007 - 18:31:22 EST
Josef Sipek wrote:
On Wed, Jan 10, 2007 at 05:12:15PM +0100, Jan Kara wrote:
I see :). To me it just sounds as if you want to do remount-read-only
for source filesystems, which is operation we support perfectly fine,
and after that create union mount. But I agree you cannot do quite that
since you need to have write access later from your union mount. So
maybe it's not so easy as I thought.
On the other hand, there was some effort to support read-only bind-mounts of
read-write filesystems (there were even some patches floating around but
I don't think they got merged) and that should be even closer to what
you'd need...
Since the RO flag is per-mount point, how do you guarantee that no one is
messing with the fs? (I haven't looked at the patches that do per mount
ro flag, but this would require some over-arching ro flag - in the
superblock most likely.)
I thought about it, wrote an email, then cancelled it as it won't work.
what I thought was that you could a limited unionfs case would be with X
layers read-only and the top layer read-write, and what you would do is
dynamically make read only bind mounts for the the X layers and since
you control the top layer hide it from the system.
However, read only bind mounts are great if you want a limit a process
to accessing the files read-only, as they won't have access to the other
vfs_mounts, but it does nothing for the other vfs_mounts that are using
that same file system. hence, does us no good.
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