[CentOS] nfs cannot see mount points on other machine

Toby Bluhm tkb at midwestinstruments.com
Fri Jun 27 12:54:56 UTC 2008

Kai Schaetzl wrote:
> Jason Pyeron wrote on Thu, 26 Jun 2008 18:03:49 -0400:
>> Any ideas what the "dangerous" inode confusion is about? Is it relevant today?
> I have no idea. I think the proposed problem is that the client doesn't know 
> that it's traversing filesystems, so, the same inode number on filesystem / and 
> /b is each time the inode number on /. I have no idea if this can actually 
> happen or how this is worked out or if it is still a problem.
> But I think the way it is now by default is not a good solution. As I wrote you 
> can work and copy to these faked folders and they disappear and reappear with 
> mounting although they are actually somewhere on the local filesystem. It looks 
> like the mounting creates a local directory listing that is only available when 
> it's mounted. The way it works without nohide is really able to trick you to 
> think you are writing to the remote side, but you aren't. I think this is 
> dangerous. They should indeed have *hidden* those folders instead of faking 
> them. The talk about "hidden" is wrong in my eyes. They do not hide they pretend 
> things that are not there.
> >From that perspective I think using nohide is the better option. But I don't 
> know if that "inode problem" could really hit or not. I haven't seen it so far.
> I think what would be a bad idea is to cross-mount the nfs shares themselves, 
> but this is prevented unless you explicitely export them.

I don't understand your talk about "fake" directories. They are not 
fake, they truly exist in the filesystem. It's just that another 
filesystem is being overlaid in that dir. Some fs like dev, proc, sys 
exist only in memory and are created on the fly at boottime, others are 
just disk partitions. Nothing mysterious there.

That NFS only exports a single partition at a time is probably due to 
the duplicate inode problem - maybe other stuff - I don't know. At any 
rate, just export the additional fs and mount it where you like. Again, 
nothing mysterious and it has been done that way since NFS was invented.

Want to prove that your "fake" dirs are not fake? Boot the rescue CD and 
don't have it automatically find your Linux partitions. Mount what 
ever's your / partition on /tmp/sysimage. There's /tmp/sysimage/dev and 
it has an inode number. Let's mkdir 
/tmp/sysimage/dev/testdirontherootpartition. That has an inode number 
too. Boot back to the OS - /dev/testdirontherootpartition doesn't exist 
- it's been overlaid by the udev system. Boot back to the resue CD - 
/tmp/sysimage/dev/testdirontherootpartition is there - same inode as it 
was before.

Same goes for your /home dir when it's a separate partition - if /home 
is not mounted, anything you write there will be written to the root 
partition and will be hidden when /home is mounted.

Toby Bluhm
Alltech Medical Systems America, Inc.
30825 Aurora Road Suite 100
Solon Ohio 44139
440-424-2240 ext203

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