nfsd.8: Document ways to minimize Copy operation times

For NFSv4.2, a Copy operation can take a long time to complete.
If there is a concurrent ExchangeID or DelegReturn operation
which requires the exclusive lock on all NFSv4 state, this can
result in a stall of the nfsd server.

This patch documents ways to avoid this problem.

This is a content change.

Reviewed by:	karels, wollman, pauamma_gundo.com (manpages)
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D44395

usr.sbin/nfsd/nfsd.8

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd March 22, 2023
+.Dd March 16, 2024
 .Dt NFSD 8
 .Os
 .Sh NAME
@@ -353,3 +353,25 @@ to be set to one on the MDS as a workaround.
 Linux 5.n kernels appear to have been patched such that this
 .Xr sysctl 8
 does not need to be set.
+.Pp
+For NFSv4.2, a Copy operation can take a long time to complete.
+If there is a concurrent ExchangeID or DelegReturn operation
+which requires the exclusive lock on all NFSv4 state, this can
+result in a
+.Dq stall
+of the
+.Nm
+server.
+If your storage is on ZFS without block cloning enabled,
+setting the
+.Xr sysctl 8
+.Va vfs.zfs.dmu_offset_next_sync
+to 0 can often avoid this problem.
+It is also possible to set the
+.Xr sysctl 8
+.Va vfs.nfsd.maxcopyrange
+to 10-100 megabytes to try and reduce Copy operation times.
+As a last resort, setting
+.Xr sysctl 8
+.Va vfs.nfsd.maxcopyrange
+to 0 disables the Copy operation.