[Beowulf] NFS Read Errors
Michael H. Frese
Michael.Frese at NumerEx.com
Mon Dec 3 16:39:45 PST 2007
We were having trouble restarting from our homegrown parallel
magnetohydrodynamic code's checkpoint files. The files could be
read, but funny things happened in the run afterward. Eventually we
figured out that the restarted parallel run differed from the serial
restarted run from the same checkpoint.
After much gnashing of teeth and rending of apparel, we found that
the checkpoint files were being read incorrectly across NFS. That
let us simplify our search for the problem. We first found that the
local md5 digest [openssl dgst -md5 (file...)] on an NFS cp'ed
version of the file was different from that produced on the original
file. What was interesting was that the copy either took forEVER --
like 10 minutes or 20 minutes for a 1 GB file -- when the final
result was bad or it took about a minute when the file was
perfect. I'm guessing that whatever error checking that gets done on
the packets was rejecting so many it finally got a bad packet it
couldn't tell was bad.
When we found that doing the md5 digest on a remote file produced a
different result than doing it on the processor on which the disk was
mounted, our tests got simpler. And shorter, still, after we found
that we could get fairly frequent failures with 10 MB files or
smaller. Clearly we had an NFS failure, probably associated with hardware.
This was all between two specific nodes of our small cluster. [Old
hardware generally: AMD Athlon 32-bit single (MSI KT4V) and dual
(Tyan...) chip motherboards both running Redhat 9 one with the
2.4.20-8 kernels, though one is the smp version; NetGear GA311 NICs;
and a NetGear GS108 8 port Copper 1 GB/s switch. The single
processor motherboards have 32-bit PCI slots so their network speeds
are limited to 300 kbps as shown by netpipe. All of the LEDs at the
ends of the cables show 1000Mb connections.]
Then we started checking other pairs. Some were fine. Some were bad
in the same way. So we replaced the switch, changing to a 16 port
NetGear GS216. That seemed to cure most of the problem. But we
continued to have problems copying a file on one particular single
processor machine from the others.
That's where we are now. The md5 digest run on that machine
consistently shows the same result, whereas the digest for that file
produced on a remote machine will be almost stochastic. In some
cases it will eventually settle in to the right answer, and then the
speed goes WAY up. I suppose that happens because the file request
can be served from the local machine's cache. But why doesn't it
happen after it received bad blocks?
Most, if not all of the original network cards in those machines went
bad and have been replaced in the last few years, so I decided to try
a brand new GA311. No joy there. It still gives out the wrong
info. I guess the motherboard PCI bus controller is hinky, but I'm
far from sure.
We are in the process of upgrading and thus replacing all the
machines we have of that configuration due to space limitations and
their age, but I'm still curious what the problem could be.
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