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Date:      Tue, 2 Dec 2008 16:15:38 -0800
From:      David Wolfskill <david@catwhisker.org>
To:        hackers@freebsd.org
Subject:   NFS (& amd?) dysfunction descending a hierarchy
Message-ID:  <20081203001538.GC96383@bunrab.catwhisker.org>

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I seem to have a fairly- (though not deterministly so) reproducible
mode of failure with an NFS-mounted directory hierarchy:  An attempt to
traverse a "sufficiently large" hierarchy (e.g., via "tar zcpf" or "rm
-fr") will fail to "visit" some subdirectories, typically apparently
acting as if the subdirectories in question do not actually exist
(despite the names having been returned in the output of a previous
readdir()).

The file system is mounted read-write, courtesy of amd(8); none of
the files has any non-default flags; there are no ACLs involved;
and I owned the lot (that is, as "owning user" of the files).

An example of "sufficiently large" has been demonstrated to be a recent
copy of a FreeBSD ports tree.  (The problem was discovered using a
hierarchy that had some proprietary content; I tried a copy of the ports
tree to see if I could replicate the issue with something a FreeBSD
hacker would more likely have handy.  And avoid NDA issues.  :-})

Now, before I go further: I'm not pointing the finger at FreeBSD,
here (yet).  At minimum, there could be fault with FreeBSD (as the NFS
client); with amd(8); with the NetApp Filer (as the NFS server);
or the network -- or the configuration(s) of any of them.

But I just tried this, using the same NFS server, but a machine running
Solaris 8 as an NFS client, and was unable to re-create the problem.

And I found a way to avoid having the problem occur using a FreeBSD NFS
client:  whack amd(8)'s config so that the dismount_interval is 12 hours
instead of the default 2 minutes, thus effectivly preventing amd(8) from
its normal attempts to unmount file systems.  Please note that I don't
consider this a fix -- or even an acceptable circumvention, in the long
term.  Rather, it's a diagnostic change, in an attempt to better
understand the nature of the problem.

Here are step-by-step instructions to recreate the problem;
unfortunately, I believe I don't have the resources to test this
anywhere but at work, though I will try it at home, to the extent
that I can:

* Set up the environment.
  * The failing environment uses NetApp filers as NFS servers.  I don't
    know what kind or how recent the software is on them, but can
    find out.  (I exepct they're fairly well-maintained.)
  * Ensure that the NFS space available is at least 10 GB or more.
    I will refer to this as "~/NFS/", as I tend to create such symlinks
    to keep track of things.
  * I used a dual, quad-core machine running FreeBSD RELENG_7_1 as of
    yesterday morning as an NFS client.  It also had a recently-updated
    /usr/ports tree, which was a CVS working directory (so each "real"
    subdirectory also had a CVS subdirectory within it).
  * Set up amd(8) so that ~/NFS is mounted on demand when it's
    referenced, and only via amd(8).  Ensure that the dismount_interval
    has the default value of 120 seconds.
* Create a reference tarball.
  * cd /usr && tar zcpf ~/NFS/ports.tgz ports/
* Create the test directory hierarchy.
  * cd ~/NFS && tar zxpf ports.tgz
* Clear any cache.
  * Unmount ~/NFS, then re-mount it.  Or just reboot the NFS client
    machine.  Or arrange to have done all of the above set-up stuff
    from a differnet NFS client.
* Set up for information capture (optional).
  * Use ps(1) or your favorite alternative tool to determine the PID for
    amd(8).  Note that `cat /var/run/amd.pid` won't do the trick.  :-{
  * Run ktrace(1) to capture activity from amd(8) and its descendants,
    e.g.:

	sudo ktrace -dip ${amd_pid} -f ktrace_amd.out

  * Start a packet-capture for NFS traffic, e.g.:

	sudo tcpdump -s 0 -n -w nfs.bpf host ${nfs_server}

* Start the test.
  * Do this under ktrace(1), if you did the above optional step:

	rm -fr ~/NFS/ports; echo $?

    As soon as rm(1) issues a whine, you might as well interrupt it
    (^C).

* Stop the information capture, if you started it.
  * ^C for the tcpdump(1) process.
  * sudo ktrace -C


If the packet capture file is too big for the analysis program you
prefer to digest as a unit, see the net/tcpslice port for a bit of
relief.  (Wireshark seems to want to read an entire packet capture file
into main memory.)

I have performed the above, with the "information-gathering" step; I can
*probably* make that information available, but I'll need to check --
some organizations get paranoid about things like host names.  I don't
expect that my current employer is, but I don't know yet, so I won't
promise.

In the mean time, I should be able to extract somewhat-relevant
information from what I've collected, if that would be useful.  While I
wouldn't mind sharing the results, I strongly suspect that blow-by-blow
analysis wouldn't be ideal for this (or any other) mailing list; I would
be very happy to work with others to figure out what's gone wrong (or is
misconfigured) and get things working properly.

If someone(s) would be willing to help, I'd appreciate it very much.  If
(enough) folks would actually prefer that the details stay in the list
(or some other list), I'm willing to do that, too.

Thanks!

Peace,
david
--=20
David H. Wolfskill				david@catwhisker.org
Depriving a girl or boy of an opportunity for education is evil.

See http://www.catwhisker.org/~david/publickey.gpg for my public key.

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