compress metadata in later sync passes

Starting in sync pass 5 (zfs_sync_pass_dont_compress), we disable
compression (including of metadata).  Ostensibly this helps the sync
passes to converge (i.e. for a sync pass to not need to allocate
anything because it is 100% overwrites).

However, in practice it increases the average number of sync passes,
because when we turn compression off, a lot of block's size will change
and thus we have to re-allocate (not overwrite) them.  It also increases
the number of 128KB allocations (e.g. for indirect blocks and spacemaps)
because these will not be compressed.  The 128K allocations are
especially detrimental to performance on highly fragmented systems,
which may have very few free segments of this size, and may need to load
new metaslabs to satisfy 128K allocations.

We should increase zfs_sync_pass_dont_compress.  In practice on a highly
fragmented system we see a few 5-pass txg's, a tiny number of 6-pass
txg's, and no txg's with more than 6 passes.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: George Wilson <george.wilson@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
External-issue: DLPX-63431
Closes #8892

man/man5/zfs-module-parameters.5

@@ -2444,9 +2444,21 @@ Default value: \fB25\fR.
 \fBzfs_sync_pass_dont_compress\fR (int)
 .ad
 .RS 12n
-Don't compress starting in this pass
+Starting in this sync pass, we disable compression (including of metadata). 
+With the default setting, in practice, we don't have this many sync passes,
+so this has no effect.
 .sp
-Default value: \fB5\fR.
+The original intent was that disabling compression would help the sync passes
+to converge. However, in practice disabling compression increases the average
+number of sync passes, because when we turn compression off, a lot of block's
+size will change and thus we have to re-allocate (not overwrite) them. It
+also increases the number of 128KB allocations (e.g. for indirect blocks and
+spacemaps) because these will not be compressed. The 128K allocations are
+especially detrimental to performance on highly fragmented systems, which may
+have very few free segments of this size, and may need to load new metaslabs
+to satisfy 128K allocations.
+.sp
+Default value: \fB8\fR.
 .RE
 
 .sp

module/zfs/zio.c

@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2011, 2018 by Delphix. All rights reserved.
+ * Copyright (c) 2011, 2019 by Delphix. All rights reserved.
  * Copyright (c) 2011 Nexenta Systems, Inc. All rights reserved.
  * Copyright (c) 2017, Intel Corporation.
  */
@@ -96,9 +96,23 @@ int zio_slow_io_ms = (30 * MILLISEC);
  *
  * The 'zfs_sync_pass_deferred_free' pass must be greater than 1 to ensure that
  * regular blocks are not deferred.
+ *
+ * Starting in sync pass 8 (zfs_sync_pass_dont_compress), we disable
+ * compression (including of metadata).  In practice, we don't have this
+ * many sync passes, so this has no effect.
+ *
+ * The original intent was that disabling compression would help the sync
+ * passes to converge. However, in practice disabling compression increases
+ * the average number of sync passes, because when we turn compression off, a
+ * lot of block's size will change and thus we have to re-allocate (not
+ * overwrite) them. It also increases the number of 128KB allocations (e.g.
+ * for indirect blocks and spacemaps) because these will not be compressed.
+ * The 128K allocations are especially detrimental to performance on highly
+ * fragmented systems, which may have very few free segments of this size,
+ * and may need to load new metaslabs to satisfy 128K allocations.
  */
 int zfs_sync_pass_deferred_free = 2; /* defer frees starting in this pass */
-int zfs_sync_pass_dont_compress = 5; /* don't compress starting in this pass */
+int zfs_sync_pass_dont_compress = 8; /* don't compress starting in this pass */
 int zfs_sync_pass_rewrite = 2; /* rewrite new bps starting in this pass */
 
 /*