NetBSD/amd64 7.0 domU freezes while running pbulk.sh (was Re: Raspberry Pi 2, nfs mount hangs after some time)

["Emile `iMil' Heitor" <imil%home.imil.net@localhost>]

On Mon, 2 Nov 2015, Emile `iMil' Heitor wrote:

> I'm trying to get rid of those hangs for weeks now, tried every mount flag
> combination without success, the system would freeze randomly, leaving the 
> whole
> OS unresponsive. There's no log, no kernel message, the domU actually 
> responds
> to network solicitations (ping, telnet 22...) but once it's frozen, it is
> impossible to run any command, it will just hang.
>
> The exact same setup is successfully running since Sept 2014 on
> NetBSD 6.1/amd64.
>
> Any idea how to get some valuable information to help tracking down this
> awful behaviour?

A bit of follow-up. I've been trying many workarounds during the past weeks, and
right now I'm not convinced it even is an NFS problem.
I've setup a tmpfs bulk build directory, and even that way, NetBSD 7.0 would
freeze randomly after a couple of minutes while processing `pbulk.sh'.
What I can say:

- the server is a fresh diskless NetBSD 7.0 domU (PXE/NFS)
- there's not a single information about the freeze, not even in the console
- I've only witnessed those freezes when calling `pbulk.sh' (couldn't get
  further anyway)
- cvs co pkgsrc does not freezes, I ran it many times without issues
- the domU stays up for days if no operation is made
- I started this domU on various dom0s to validate this was not a hardware
  problem, always had the same symptoms
- I tried a custom 7.0_STABLE kernel without success

If this issue _is_ NFS related, which I doubt now, it is then read-related, as
the build is done in tmpfs.

Again, as there's no log at all, what would help debugging this behaviour?

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Emile `iMil' Heitor * <imil@{home.imil.net,NetBSD.org,gcu.info}>
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