ddt: add support for prefetching tables into the ARC

This change adds a new `zpool prefetch -t ddt $pool` command which
causes a pool's DDT to be loaded into the ARC. The primary goal is to
remove the need to "warm" a pool's cache before deduplication stops
slowing write performance. It may also provide a way to reload portions
of a DDT if they have been flushed due to inactivity.

Sponsored-by: iXsystems, Inc.
Sponsored-by: Catalogics, Inc.
Sponsored-by: Klara, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Will Andrews <will.andrews@klarasystems.com>
Signed-off-by: Fred Weigel <fred.weigel@klarasystems.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Signed-off-by: Don Brady <don.brady@klarasystems.com>
Co-authored-by: Will Andrews <will.andrews@klarasystems.com>
Co-authored-by: Don Brady <don.brady@klarasystems.com>
Closes #15890

module/zfs/ddt_zap.c

@@ -147,6 +147,12 @@ ddt_zap_prefetch(objset_t *os, uint64_t object, const ddt_key_t *ddk)
 	(void) zap_prefetch_uint64(os, object, (uint64_t *)ddk, DDT_KEY_WORDS);
 }
 
+static void
+ddt_zap_prefetch_all(objset_t *os, uint64_t object)
+{
+	(void) zap_prefetch_object(os, object);
+}
+
 static int
 ddt_zap_update(objset_t *os, uint64_t object, const ddt_key_t *ddk,
     const ddt_phys_t *phys, size_t psize, dmu_tx_t *tx)
@@ -231,6 +237,7 @@ const ddt_ops_t ddt_zap_ops = {
 	ddt_zap_lookup,
 	ddt_zap_contains,
 	ddt_zap_prefetch,
+	ddt_zap_prefetch_all,
 	ddt_zap_update,
 	ddt_zap_remove,
 	ddt_zap_walk,