Comment 0 for bug 1587686

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

This bug affects the xenial kernel built-in ZFS as well as the package zfs-dkms. (I'm unsure how to tag this to multiple packages)

Upstream bug report: https://github.com/zfsonlinux/zfs/issues/4129
"ztest can occasionally fail because zdb cannot locate the pool after several hours of run time. This appears to be caused be an empty cache file."

How to reproduce: run ztest repeatedly such as a command like this and it will eventually fail:
ztest -T 3600 && rm /tmp/z* && sleep 3 && ztest -T 3600 && rm /tmp/z* && sleep 3 && ztest -T 3600 && rm /tmp/z* && sleep 3 && ztest -T 3600 && rm /tmp/z* && sleep 3 && ztest -T 3600 && rm /tmp/z* && sleep 3 && ztest -T 3600 && rm /tmp/z* && sleep 3 && ztest -T 3600 && rm /tmp/z* && sleep 3 && ztest -T 3600 && rm /tmp/z* && sleep 3 && ztest -T 3600 && rm /tmp/z* && sleep 3 && ztest -T 3600 && rm /tmp/z* && sleep 3 && ztest -T 3600 && rm /tmp/z* && sleep 3 && ztest -T 3600 && rm /tmp/z*
(I have /tmp mounted on tmpfs with a 10G limit but I don't believe this is related in any way, and I've confirmed it's not running out of space)

Upstream fix: https://github.com/zfsonlinux/zfs/commit/151f84e2c32f690b92c424d8c55d2dfccaa76e51
Description:
"Commit efc412b updated spa_config_write() for Linux 4.2 kernels to
truncate and overwrite rather than rename the cache file. This is
the correct fix but it should have only been applied for the kernel
build. In user space rename(2) is needed because ztest depends on
the cache file."
Associated pull request for above commit: https://github.com/zfsonlinux/zfs/pull/4130

I'm not sure why this wasn't backported to release but it's in zfs master. I've Reproduced this bug on xenial kernels 4.4.0-22-generic, 4.4.0-23-generic, 4.4.0-22-lowlatency, and 4.4.0-23-lowlatency as well as various xenial master-next builds. After applying the above commit patch to kernel and building/installing kernel manually, ztest runs fine. I've also separately tested the commit patch on zfs-dkms package which also appears to fix the issue. Note however, there may still be some other outstanding ztest related issues upstream - especially when preempt and hires timers are used. I'm currently testing more heavily against lowlatency builds and master-next.

(I'm unsure how to associate this bug with multiple packages but zfs-dkms and linux-image-* packages both are affected).

P.S. Also of note is https://github.com/zfsonlinux/zfs/commit/60a4ea3f948f1596b92b666fc7dd21202544edbb "Fix inverted logic on none elevator comparison" - which interestingly was signed-off-by canonical but curiously not included in the xenial kernel or zfs-dkms packages. It was however, backported to 0.6.5-release upstream.