Illumos 6251 - add tunable to disable free_bpobj processing

6251 - add tunable to disable free_bpobj processing
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Simon Klinkert <simon.klinkert@gmail.com>
Reviewed by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed by: Albert Lee <trisk@omniti.com>
Reviewed by: Xin Li <delphij@freebsd.org>
Approved by: Garrett D'Amore <garrett@damore.org>

References:
  https://www.illumos.org/issues/6251
  illumos/illumos-gate@139510f

Porting notes:
- Added as module option declaration.
- Added to zfs-module-parameters.5 man page.

Ported-by: Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

Author: grwilson

man/man5/zfs-module-parameters.5

@@ -784,6 +784,17 @@ Start syncing out a transaction group if there is at least this much dirty data.
 Default value: \fB67,108,864\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_free_bpobj_enabled\fR (int)
+.ad
+.RS 12n
+Enable/disable the processing of the free_bpobj object.
+.sp
+Default value: \fB1\fR.
+.RE
+
 .sp
 .ne 2
 .na

module/zfs/dsl_scan.c

@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2011, 2014 by Delphix. All rights reserved.
+ * Copyright (c) 2011, 2015 by Delphix. All rights reserved.
  */
 
 #include <sys/dsl_scan.h>
@@ -76,6 +76,11 @@ ulong zfs_free_max_blocks = 100000;
 	((scn)->scn_phys.scn_func == POOL_SCAN_SCRUB || \
 	(scn)->scn_phys.scn_func == POOL_SCAN_RESILVER)
 
+/*
+ * Enable/disable the processing of the free_bpobj object.
+ */
+int zfs_free_bpobj_enabled = 1;
+
 /* the order has to match pool_scan_type */
 static scan_cb_t *scan_funcs[POOL_SCAN_FUNCS] = {
 	NULL,
@@ -1502,7 +1507,8 @@ dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
 	 * have to worry about traversing it.  It is also faster to free the
 	 * blocks than to scrub them.
 	 */
-	if (spa_version(dp->dp_spa) >= SPA_VERSION_DEADLISTS) {
+	if (zfs_free_bpobj_enabled &&
+	    spa_version(dp->dp_spa) >= SPA_VERSION_DEADLISTS) {
 		scn->scn_is_bptree = B_FALSE;
 		scn->scn_zio_root = zio_root(dp->dp_spa, NULL,
 		    NULL, ZIO_FLAG_MUSTSUCCEED);
@@ -1918,4 +1924,7 @@ MODULE_PARM_DESC(zfs_no_scrub_prefetch, "Set to disable scrub prefetching");
 
 module_param(zfs_free_max_blocks, ulong, 0644);
 MODULE_PARM_DESC(zfs_free_max_blocks, "Max number of blocks freed in one txg");
+
+module_param(zfs_free_bpobj_enabled, int, 0644);
+MODULE_PARM_DESC(zfs_free_bpobj_enabled, "Enable processing of the free_bpobj");
 #endif