based on
  commit 6fb79233b050b4a3575f0e466ab04b5d301ac1de
  Author: Neil Brown <neilb@suse.de>
  Date:   Mon Apr 28 16:30:09 2008 +1000
  
      Allow creation of a RAID6 with a single missing device.
      
      This did not work before as we couldn't mark it clean as there would
      be some parity blocks out of sync, and raid6 will not assemble a
      dirty degraded array.
      So make such arrays doubly degraded (the last device becomes a spare)
      and clean.

---
 Create.c |   20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

--- Create.c.orig
+++ Create.c
@@ -63,6 +63,7 @@ int Create(struct supertype *st, char *m
 	int fail=0, warn=0;
 	struct stat stb;
 	int first_missing = subdevs * 2;
+	int second_missing = subdevs * 2;
 	int missing_disks = 0;
 	int insert_point = subdevs * 2; /* where to insert a missing drive */
 	void *super;
@@ -203,6 +204,8 @@ int Create(struct supertype *st, char *m
 		if (strcasecmp(dname, "missing")==0) {
 			if (first_missing > dnum)
 				first_missing = dnum;
+			if (second_missing > dnum && dnum > first_missing)
+				second_missing = dnum;
 			missing_disks ++;
 			continue;
 		}
@@ -341,6 +344,18 @@ int Create(struct supertype *st, char *m
 			break;
 		}
 	}
+	/* For raid6, if creating with 1 missing drive, make a good drive
+	 * into a spare, else the create will fail
+	 */
+	if (assume_clean == 0 && force == 0 && first_missing < raiddisks &&
+	    second_missing >= raiddisks && level == 6) {
+		insert_point = raiddisks - 1;
+		if (insert_point == first_missing)
+			insert_point--;
+		sparedisks ++;
+		array.active_disks--;
+		missing_disks++;
+	}
 
 	if (level <= 0 && first_missing != subdevs * 2) {
 		fprintf(stderr,
@@ -360,11 +375,12 @@ int Create(struct supertype *st, char *m
 	if (fstat(mdfd, &stb)==0)
 		array.md_minor = minor(stb.st_rdev);
 	array.not_persistent = 0;
-	/*** FIX: Need to do something about RAID-6 here ***/
+
 	if ( ( (level == 4 || level == 5) &&
 	       (insert_point < raiddisks || first_missing < raiddisks) )
 	     ||
-	     ( level == 6 && missing_disks == 2)
+	     ( level == 6 && (insert_point < raiddisks
+			      || second_missing < raiddisks))
 	     ||
 	     assume_clean
 		)