[ replacing cc: nfs@xxxxxx with linux-nfs@xxxxxxxxxxxxxxx, and neil's
old address with his current one ]
On Sep 5, 2008, at Sep 5, 2008, 3:19 PM, Aaron Straus wrote:
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
Up to which kernel? Recent ones may address this issue already.
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.
That's a good start.
Comparing a wire trace with strace output, starting with the writing
client, might also be illuminating. We prefer wireshark as it uses
good default trace settings, parses the wire bytes and displays them
coherently, and allows you to sort the frames in various useful ways.
--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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