7570 tunable to allow zvol SCSI unmap to return on commit of txn to ZIL

Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Steve Gonczi <steve.gonczi@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Saso Kiselkov <saso.kiselkov@nexenta.com>
Reviewed by: Yuri Pankov <yuri.pankov@nexenta.com>
Approved by: Robert Mustacchi <rm@joyent.com>

usr/src/uts/common/fs/zfs/zvol.c

@@ -24,7 +24,7 @@
  * Portions Copyright 2010 Robert Milkowski
  *
  * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
- * Copyright (c) 2012, 2014 by Delphix. All rights reserved.
+ * Copyright (c) 2012, 2016 by Delphix. All rights reserved.
  * Copyright (c) 2013, Joyent, Inc. All rights reserved.
  * Copyright (c) 2014 Integros [integros.com]
  */
@@ -148,6 +148,12 @@ int zvol_maxphys = DMU_MAX_ACCESS/2;
  */
 boolean_t zvol_unmap_enabled = B_TRUE;
 
+/*
+ * If true, unmaps requested as synchronous are executed synchronously,
+ * otherwise all unmaps are asynchronous.
+ */
+boolean_t zvol_unmap_sync_enabled = B_FALSE;
+
 extern int zfs_set_prop_nvlist(const char *, zprop_source_t,
     nvlist_t *, nvlist_t *);
 static int zvol_remove_zv(zvol_state_t *);
@@ -1811,26 +1817,21 @@ zvol_ioctl(dev_t dev, int cmd, intptr_t arg, int flag, cred_t *cr, int *rvalp)
 
 		zfs_range_unlock(rl);
 
-		if (error == 0) {
-			/*
-			 * If the write-cache is disabled or 'sync' property
-			 * is set to 'always' then treat this as a synchronous
-			 * operation (i.e. commit to zil).
-			 */
-			if (!(zv->zv_flags & ZVOL_WCE) ||
-			    (zv->zv_objset->os_sync == ZFS_SYNC_ALWAYS))
-				zil_commit(zv->zv_zilog, ZVOL_OBJ);
-
-			/*
-			 * If the caller really wants synchronous writes, and
-			 * can't wait for them, don't return until the write
-			 * is done.
-			 */
-			if (df.df_flags & DF_WAIT_SYNC) {
-				txg_wait_synced(
-				    dmu_objset_pool(zv->zv_objset), 0);
-			}
+		/*
+		 * If the write-cache is disabled, 'sync' property
+		 * is set to 'always', or if the caller is asking for
+		 * a synchronous free, commit this operation to the zil.
+		 * This will sync any previous uncommitted writes to the
+		 * zvol object.
+		 * Can be overridden by the zvol_unmap_sync_enabled tunable.
+		 */
+		if ((error == 0) && zvol_unmap_sync_enabled &&
+		    (!(zv->zv_flags & ZVOL_WCE) ||
+		    (zv->zv_objset->os_sync == ZFS_SYNC_ALWAYS) ||
+		    (df.df_flags & DF_WAIT_SYNC))) {
+			zil_commit(zv->zv_zilog, ZVOL_OBJ);
 		}
+
 		return (error);
 	}