Improve zfs receive performance with lightweight write

The performance of `zfs receive` can be bottlenecked on the CPU consumed
by the `receive_writer` thread, especially when receiving streams with
small compressed block sizes.  Much of the CPU is spent creating and
destroying dbuf's and arc buf's, one for each `WRITE` record in the send
stream.

This commit introduces the concept of "lightweight writes", which allows
`zfs receive` to write to the DMU by providing an ABD, and instantiating
only a new type of `dbuf_dirty_record_t`.  The dbuf and arc buf for this
"dirty leaf block" are not instantiated.

Because there is no dbuf with the dirty data, this mechanism doesn't
support reading from "lightweight-dirty" blocks (they would see the
on-disk state rather than the dirty data).  Since the dedup-receive code
has been removed, `zfs receive` is write-only, so this works fine.

Because there are no arc bufs for the received data, the received data
is no longer cached in the ARC.

Testing a receive of a stream with average compressed block size of 4KB,
this commit improves performance by 50%, while also reducing CPU usage
by 50% of a CPU.  On a per-block basis, CPU consumed by receive_writer()
and dbuf_evict() is now 1/7th (14%) of what it was.

Baseline: 450MB/s, CPU in receive_writer() 40% + dbuf_evict() 35%
New: 670MB/s, CPU in receive_writer() 17% + dbuf_evict() 0%

The code is also restructured in a few ways:

Added a `dr_dnode` field to the dbuf_dirty_record_t.  This simplifies
some existing code that no longer needs `DB_DNODE_ENTER()` and related
routines.  The new field is needed by the lightweight-type dirty record.

To ensure that the `dr_dnode` field remains valid until the dirty record
is freed, we have to ensure that the `dnode_move()` doesn't relocate the
dnode_t.  To do this we keep a hold on the dnode until it's zio's have
completed.  This is already done by the user-accounting code
(`userquota_updates_task()`), this commit extends that so that it always
keeps the dnode hold until zio completion (see `dnode_rele_task()`).

`dn_dirty_txg` was previously zeroed when the dnode was synced.  This
was not necessary, since its meaning can be "when was this dnode last
dirtied".  This change simplifies the new `dnode_rele_task()` code.

Removed some dead code related to `DRR_WRITE_BYREF` (dedup receive).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: George Wilson <gwilson@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #11105

module/zfs/dsl_pool.c

@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2011, 2019 by Delphix. All rights reserved.
+ * Copyright (c) 2011, 2020 by Delphix. All rights reserved.
  * Copyright (c) 2013 Steven Hartland. All rights reserved.
  * Copyright (c) 2014 Spectra Logic Corporation, All rights reserved.
  * Copyright 2016 Nexenta Systems, Inc.  All rights reserved.
@@ -566,6 +566,11 @@ dsl_pool_sync_mos(dsl_pool_t *dp, dmu_tx_t *tx)
 	zio_t *zio = zio_root(dp->dp_spa, NULL, NULL, ZIO_FLAG_MUSTSUCCEED);
 	dmu_objset_sync(dp->dp_meta_objset, zio, tx);
 	VERIFY0(zio_wait(zio));
+	dmu_objset_sync_done(dp->dp_meta_objset, tx);
+	taskq_wait(dp->dp_sync_taskq);
+	multilist_destroy(dp->dp_meta_objset->os_synced_dnodes);
+	dp->dp_meta_objset->os_synced_dnodes = NULL;
+
 	dprintf_bp(&dp->dp_meta_rootbp, "meta objset rootbp is %s", "");
 	spa_set_rootblkptr(dp->dp_spa, &dp->dp_meta_rootbp);
 }
@@ -677,7 +682,7 @@ dsl_pool_sync(dsl_pool_t *dp, uint64_t txg)
 	 */
 	for (ds = list_head(&synced_datasets); ds != NULL;
 	    ds = list_next(&synced_datasets, ds)) {
-		dmu_objset_do_userquota_updates(ds->ds_objset, tx);
+		dmu_objset_sync_done(ds->ds_objset, tx);
 	}
 	taskq_wait(dp->dp_sync_taskq);