[PLUG] How to umount NFS hung mouts

Steven A. Adams stevea at nwtechops.com
Thu Aug 14 11:52:02 UTC 2003


On Thu, 2003-08-14 at 04:26, Felix Lee wrote:
> But if the networks are reliable, what's your rationale for
> mounting soft?  There's no performance gain for mounting soft
> instead of hard.

Transitional servers and those that are not in my control where
stability is a question. In these cases the data that is contained on
the servers is easily recreated or restored and expectation is low.

> The historical example is that close() over NFS can return errors
> that used to be returned by write(), such as "sorry, disk is
> full", and several programs (like emacs) had to be changed to
> notice that.
> 
> That's not the same as the soft vs hard issue, but I've never
> looked at the potential differences between writes on soft mounts
> versus hard mounts, because nobody I know mounts writable
> filesystems soft, and everyone recommends against it, so it never
> seemed like something worth analyzing.  It's not just write() and
> close().  I'd also want to look at how programs use unlink() and
> rename() and so forth.  The "soft" decision happens at the SunRPC
> level, it affects every NFS operation and adds the possibility
> that any operation can fail with an EIO error.
> 

I'm sorry, I have a problem blaming nfs, or any other protocol, because
developers write applications that don't gracefully handle errors.

> Yes, I know, I'm a dinosaur.  Emacs is careful now to check the
> return value from close() when writing files.  I don't have much
> confidence that everyone else does too, and I don't have any
> confidence that people have thought much about the problem of
> doing reliable changes on soft-mounted filesystems.
> 
> Now that I think about it, soft-mounted filesystems may have
> security implications too.  One thing that comes to mind is
> xauth.  I just checked, xauth ignores the return value from
> close() or unlink(), though this doesn't seem like it's
> exploitable for anything other than boring DoS.

Implicating the filesystem protocol because an application is not
properly using that protocol? You have to admit, that's really
stretching things.

All in all, I agree with most of the arguments against using soft
mounts. I just can't agree that they should be avoided at all costs. I
still feel that the original post for this thread would be a good use
for soft mounts.






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