[Bug 228557] 11.3-BETA3 excessive swapping (swap on zvol)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=228557

            Bug ID: 228557
           Summary: 11.3-BETA3 excessive swapping (swap on zvol)
           Product: Base System
           Version: 11.1-STABLE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: pascal.guitierrez at gmail.com

Since upgrading from 11.1-RELEASE p10 to 11.2-BETA3 on amd64 we are seeing
excessive swapping, even when the system is idle.

Swap is on zvol, with system constantly swapping in/out for no apparent reason.
There is no load on the system:

5:22PM  up 1 day, 7 mins, 1 user, load averages: 0.06, 0.20, 0.18

However the following lines from top confirm that excessive swapping is
occurring:
Swap: 4096M Total, 158M Used, 3938M Free, 3% Inuse, 448K In, 136K Out
Swap: 4096M Total, 156M Used, 3940M Free, 3% Inuse, 8K In, 44K Out
Swap: 4096M Total, 157M Used, 3939M Free, 3% Inuse, 332K In, 24K Out

etc....

System has 64GB of RAM, 8-core Xeon E5-2620 v4 CPU, all memory and ZFS tunables
are at defaults.
ZFS settings that were changed on swap zvol are:
volblocksize          4K
sync                  always
compression           off

The following process seems to constantly active:
pagedaemon{laundry: dom0}

Did not exhibit this behaviour on 11.1-RELEASE with exactly the same
configuration.

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