[Bug 163461] [vfs] vfs.zfs.arc_max/vfs.zfs.arc_meta_limit defaults aren't wise

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=163461

Shane <FreeBSD at ShaneWare.Biz> changed:

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                 CC|                            |FreeBSD at ShaneWare.Biz

--- Comment #3 from Shane <FreeBSD at ShaneWare.Biz> ---
Rather than start a new report I will jump into this old one.

Currently arc_max is set to the larger of (5/8 of kmem_size) or (kmem_size
minus 1G) - for a system with 8G of ram 5/8 is 5G with kmem size minus 1G about
6GB

While that sounds ok, we also have max_wired which seems to default to 5G (I
see that on an 8G and 16G setup) the issue results from arc being wired but not
included in the max_wired count, that means an 8G system allows 10G to get
wired! a 16GB system allows 20GB to be wired!

When a system has more than 70% of physical ram wired, it gets exteremely slow
and over 80% wired gives a very high chance of needing a hard reset.

At the minimum we need to consider max_wired in the arc_max calculation.

I do believe the arc_max default should be even smaller to be better suited to
more common setups that run processes that use ram, a sysadmin starting a file
server that wants to use most of the ram for cache should know how to set this
as desired, the other 80-90% of users shouldn't have to adjust a setting that
is suited to the smaller use case.

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