[CentOS] nfs cannot see mount points on other machine
tkb at midwestinstruments.com
Fri Jun 27 16:00:45 UTC 2008
Kai Schaetzl wrote:
> Toby Bluhm wrote on Fri, 27 Jun 2008 08:54:56 -0400:
>> I don't understand your talk about "fake" directories. They are not
>> fake, they truly exist in the filesystem.
> They are nevertheless fake. Consider the following:
> - system A has mount points / and /home
> - system B nfs mounts / on A without nohide at /nfs/A
> Result is that you see *all* directories of A on B, including /home. There
> is no way to know that it doesn't exist on A, unless you compare the
> directories on both machines. There is no indication that you are not
> writing to A:/home when you write to /nfs/A/home. That is what I call
> "fake". It's definitely not "hidden". "hidden" comes from "hiding" = you
> don't see it. I consider this behavior *very* misleading.
>> 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.
> That may be so. I'm quite happy with this behavior as long as nfs doesn't
> pretend that something is there that isn't.
> I was asking "where" that faked directory actually exists as it is gone
> when I unmount. If I understand your explanation correctly if I write to
> /nfs/A/home I'm actually writing to A, but not to the /home filesystem (as
> I think) but to a home directory on the / filesystem. Is that correct?
Yes. Let's use some examples. If A:/ is /dev/sda1 and A:/home is
/dev/sda2, then in your above situation, writes to B:/nfs/A/home will be
written to /dev/sda1 on A and _not_ /dev/sda2 on A. When A exports /,
it's actually only exporting the filesystem on /dev/sda1.
> That makes clear why it is gone when I unmount. Further, if I unmount
> /home on A I should still get /home when I list / on A.
Yes - you will get the /home that's on /dev/sda1
> Just now that
> "faked" home on /. Correct?
> I understand that this directory *does* exist on A (just not where one
> would think) *after* nfs mounting. However, from the standpoint of machine
> B it is a fake. It is artificially being created because an ls on A shows
> it. The correct behavior would be to *not list* any other mount points in
> the nfs mount.
Again, it's not fake, it's not artificial. It's truly there on
/dev/sda1, the storage device. If you umount /home, rmdir /home, you
can't mount /dev/sda2 on /home any more can you? If you mkdir /home2,
you can put stuff in there until you run out of space on /dev/sda1. If
you mount /dev/sda2 on /home2, the stuff you wrote to /home2 will still
be on /dev/sda1 and will be hidden by the overlay of /dev/sda2. If you
umount A:/ from B:/nfs/A and then write to B:/nfs/A - because B:/nfs/A
will still exist as a real directory - it will be stored in /dev/sda1 on
B. Whether you mount NFS exports or local disk partitions, it works the
By your analogy, umounting /dev/sda2 /home should make /home disappear
from / because it's not mounted. Or mounting /dev/sda1 as / should just
make /dev/sda2 fall into /home without any entry in fstab.
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