Grow: fix --layout=preserve to match man page.

I think there was some confusion about what --layout=preserve
actually means, but in any case it wasn't doing what the man
page says it should.
So add some case analysis and make sure it does the right thing,
or complains if it cannot.

Reported-by: Patrik Horník <patrik@dsl.sk>
Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 29 insertions, 1 deletions

diff --git a/Grow.c b/Grow.c
index 330e719..9350a07 100644
--- a/Grow.c
+++ b/Grow.c
@@ -1885,9 +1885,37 @@ size_change_error:
 				*h = 0;
 				info.new_layout = map_name(r6layout, l);
 			}
+		} else {
+			fprintf(stderr, Name
+				": %s is only meaningful when reshaping"
+				" a RAID6 array.\n", layout_str);
+			rv = 1;
+			goto release;
 		}
 	} else if (strcmp(layout_str, "preserve") == 0) {
-		info.new_layout = UnSet;
+		/* This means that a non-standard RAID6 layout
+		 * is OK.
+		 * In particular:
+		 * - When reshape a RAID6 (e.g. adding a device)
+		 *   which is in a non-standard layout, it is OK
+		 *   to preserve that layout.
+		 * - When converting a RAID5 to RAID6, leave it in
+		 *   the XXX-6 layout, don't re-layout.
+		 */
+		if (info.array.level == 6 && info.new_level == UnSet)
+			info.new_layout = info.array.layout;
+		else if (info.array.level == 5 && info.new_level == 6) {
+			char l[40];
+			strcpy(l, map_num(r5layout, info.array.layout));
+			strcat(l, "-6");
+			info.new_layout = map_name(r6layout, l);
+		} else {
+			fprintf(stderr, Name
+				": %s in only meaningful when reshaping"
+				" to RAID6\n", layout_str);
+			rv = 1;
+			goto release;
+		}
 	} else {
 		int l = info.new_level;
 		if (l == UnSet)