Default to zfs_dmu_offset_next_sync=1

Strict hole reporting was previously disabled by default as a
performance optimization.  However, this has lead to confusion
over the expected behavior and a variety of workarounds being
adopted by consumers of ZFS.  Change the default behavior to
always report holes and force the TXG sync.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#12746

man/man4/zfs.4

@@ -1601,12 +1601,12 @@ Allow no-operation writes.
 The occurrence of nopwrites will further depend on other pool properties
 .Pq i.a. the checksumming and compression algorithms .
 .
-.It Sy zfs_dmu_offset_next_sync Ns = Ns Sy 0 Ns | Ns 1 Pq int
+.It Sy zfs_dmu_offset_next_sync Ns = Ns Sy 1 Ns | Ns 0 Pq int
 Enable forcing TXG sync to find holes.
-When enabled forces ZFS to act like prior versions when
+When enabled forces ZFS to sync data when
 .Sy SEEK_HOLE No or Sy SEEK_DATA
-flags are used, which, when a dnode is dirty,
-causes TXGs to be synced so that this data can be found.
+flags are used allowing holes in a file to be accurately reported.
+When disabled holes will not be reported in recently dirtied files.
 .
 .It Sy zfs_pd_bytes_max Ns = Ns Sy 52428800 Ns B Po 50MB Pc Pq int
 The number of bytes which should be prefetched during a pool traversal, like

module/zfs/dmu.c

@@ -73,9 +73,13 @@ int zfs_nopwrite_enabled = 1;
 unsigned long zfs_per_txg_dirty_frees_percent = 5;
 
 /*
- * Enable/disable forcing txg sync when dirty in dmu_offset_next.
+ * Enable/disable forcing txg sync when dirty checking for holes with lseek().
+ * By default this is enabled to ensure accurate hole reporting, it can result
+ * in a significant performance penalty for lseek(SEEK_HOLE) heavy workloads.
+ * Disabling this option will result in holes never being reported in dirty
+ * files which is always safe.
  */
-int zfs_dmu_offset_next_sync = 0;
+int zfs_dmu_offset_next_sync = 1;
 
 /*
  * Limit the amount we can prefetch with one call to this amount.  This
@@ -2107,8 +2111,8 @@ dmu_offset_next(objset_t *os, uint64_t object, boolean_t hole, uint64_t *off)
 		 * If the zfs_dmu_offset_next_sync module option is enabled
 		 * then strict hole reporting has been requested.  Dirty
 		 * dnodes must be synced to disk to accurately report all
-		 * holes.  When disabled (the default) dirty dnodes are
-		 * reported to not have any holes which is always safe.
+		 * holes.  When disabled dirty dnodes are reported to not
+		 * have any holes which is always safe.
 		 *
 		 * When called by zfs_holey_common() the zp->z_rangelock
 		 * is held to prevent zfs_write() and mmap writeback from