[Bug 199189] SWAP on ZFS can crash server

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=199189

--- Comment #16 from Marcus Reid <marcus at blazingdot.com> ---
As for the machine in my last comment, I've tuned the problem away and I'd like
to share how:

vm.v_free_target=60000
vm.v_free_min=55000
vm.v_free_severe=50000
vm.v_free_reserved=32000
vfs.zfs.arc_free_target=180000

Now, free memory never gets down below about 50MB or so, and normally stays up
above 150MB even when under pressure and swapping.  I'm swapping to a zvol with
all of the swap tunables set.

The default values for these look more like:

vm.v_free_target: 42948
vm.v_free_min: 12741
vm.v_free_severe: 7706
vm.v_free_reserved: 2672
vfs.zfs.arc_free_target: 14014

Note that I increased vm.v_free_reserved quite a bit, but I don't know exactly
what that one does.

vm.v_free_reserved: Pages reserved for deadlock

The description seems to make it a prime candidate for tuning in this
situation, so even though nobody has mentioned it yet I raised it.

Things work great. I will post again if I manage to break it again.

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