linux-kernel - blocks of zeros (NULLs) in NFS files in kernels >= 2.6.20

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Date:	Fri, 5 Sep 2008 12:19:40 -0700
From:	Aaron Straus <aaron@...finllc.com>
To:	neilb@....unsw.edu.au, nfs@...ts.sourceforge.net,
	trond.myklebust@....uio.no, linux-kernel@...r.kernel.org
Subject: blocks of zeros (NULLs) in NFS files in kernels >= 2.6.20

Hi all,

  We're hitting some bad behavior in NFS v3.  The situation is this:

   machine A - NFS server

   machine B - NFS client (writer)
   machine C - NFS client (reader)

   (all machines x86 SMP)

  machine A exports a directory on ext3 filesystem:

	/srv/home       192.168.0.0/24(rw,sync,no_subtree_check)

  machines B and C mount that directory normally

        mount A:/srv/home /mntpnt

  machine B opens a file and writes to it (think a log file)

  machine C stats that file, opens it and reads it (think tailing the
                                                    log file)

  The issue is that machine C will often see large blocks of NULLs
(zeros) in the file.  If you do the same read again just after you see
the block of NULLs you will see proper the data.

  Attached are two simple python programs that demonstrate the problem.

  To use them (they will write to a file called test-nfs in CWD):

 (on machine B in one window)

   python writer.py

 (on machine C in another window)

   python reader.py

  reader.py will die when it sees NULLs in the file.  Usually for us
this happens after about 60s (two timeouts I think).   The first NULL is
usually either at index 4000 or 8000 depending on the kernel.

  Now the version of the kernel the server is running doesn't seem to
matter.  The reader also doesn't seem to matter (though I didn't test
this completely).  The writer seems to be the issue:

  Writer_Version     Outcome:
  <= 2.6.19          OK
  >= 2.6.20	     BAD

  I've tested both vanilla kernel.org kernels and Ubuntu 8.04 kernels.

  I can try to bisect between 2.6.19 <-> 2.6.20. 

  Anyone else hitting this problem?  Any better ideas?

  					Thanks,
					=a=

-- 
===================
Aaron Straus
aaron@...finllc.com

View attachment "reader.py" of type "text/x-python" (844 bytes)

View attachment "writer.py" of type "text/x-python" (258 bytes)