Index: sbin/swapon/swapon.8
===================================================================
--- sbin/swapon/swapon.8
+++ sbin/swapon/swapon.8
@@ -203,6 +203,15 @@
 overridden.
 512 byte blocks are used by default.
 .El
+.Sh NOTE
+ZVOL and ZFS swap files are discouraged for swap space.
+ZFS requires large amount of memory to write to swap device or file
+when the system is at low memory under pressure trying to make some
+memory available.
+OOM killer may start killing as ZFS doesn't write to swap space fast
+enough or to allocate memory for ZFS to write to swap
+although large swap space is available.
+.El
 .Sh FILES
 .Bl -tag -width ".Pa /dev/{ada,da}?s?b" -compact
 .It Pa /dev/{ada,da}?s?b