Appreciate you looking into this, I was only able to test your builds
for about 5 hours on generic kernel version so far (doing some hardware
upgrades at the moment, but my current test system is torture-test
stable).

My test hardware was 2x (westmere) Intel Xeon E5620's (2 NUMA nodes)
with 12GB (2GBx6) ECC RDIMMs on each CPU (24GB total) on ubuntu-server
16.04. ztest was ran on default /tmp however I had /tmp mounted on tmpfs
with 10G limit, but from what I could tell it was not exceeding that
limit.

I believe this issue becomes more apparent in 4.4.11 and 4.4.12 (and
possibly 4.4.13 now) for some reason since those were failing for me
within a few hours with this "fix" applied, whereas latest stable I
compiled  with fix seemed okay. I think there's some race conditions of
some sort with newer kernels, especially since I saw different results
on the lowlatency kernel awhile back (on the same stable release).

I'll do some more testing if I have some time, and I want to test this
on some other distros as well but I think the fix might not work on
future kernel releases that integrate 4.4.11, 4.4.12, and 4.4.13 since
some of the patches may have changed some core functions which uncovered
ZFS bugs again.

It's still possible it somehow only effects my hardware/OS only. Unless
I was compiling the kernel strangely, I was doing a git clone from
master-next, checking out latest stable (detached head) and
applying/commiting the patch. My 4.4.11 and 4.4.12 builds were were
manually applied cleanly from upstream on top of xenial master-next
(neither were merged into master-next at the time), so that could also
have been a possible issue - there was a few redundant patches I skipped
that were already in master-next though.

However, the bug still stands on stock stable xenial kernel - and this
patch seems to fix it (at least on generic, still unsure about
lowlatency).

Compiling debian/ubuntu kernels from git is pretty complicated though with 
conflicting documentation. I was using this command after checking out and 
appluing patch:
fakeroot debian/rules clean
fakeroot debian/rules updateconfigs
fakeroot debian/rules binary-headers binary-generic binary-perarch
(or binary-lowlatency for lowlatency builds)
I'm not using cloud-tools packages.

Anyways I guess you can close this and it can be reopened if I have time
to attempt to reproduce the bug. it's not a critical patch but it's
queued for 0.6.5-release upstream so there's probably no harm including
it in ubuntu kernel.

Thanks

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https://bugs.launchpad.net/bugs/1587686

Title:
  ZFS: Running ztest repeatedly for long periods of time eventually
  results in "zdb: can't open 'ztest': No such file or directory"

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