diff --git a/lib/libc/sys/copy_file_range.2.sav b/lib/libc/sys/copy_file_range.2
--- a/lib/libc/sys/copy_file_range.2.sav
+++ b/lib/libc/sys/copy_file_range.2
@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD: head/lib/libc/sys/copy_file_range.2 359465 2020-03-30 21:44:00Z jhb $
 .\"
-.Dd March 30, 2020
+.Dd January 2, 2021
 .Dt COPY_FILE_RANGE 2
 .Os
 .Sh NAME
@@ -117,6 +117,14 @@
 .Dv SEEK_DATA
 arguments and this system call for the
 data ranges found.
+.Pp
+For best performance, call
+.Fn copy_file_range
+with the largest
+.Fa len
+value possible.
+It is interruptible on most file systems,
+so there is no penalty for using very large len values, even SSIZE_MAX.
 .Pp
 .Sh RETURN VALUES
 If it succeeds, the call returns the number of bytes copied, which can be fewer