Create: default to bitmap=internal for large arrays.

Here, "large" means components are 100G or more.  It is
usually beneficial to have write-intent bitmaps on such arrays.
They can be suppressed with --bitmap=none

Signed-off-by: NeilBrown <neilb@suse.de>

Build.c

@@ -154,6 +154,8 @@ int Build(char *mddev, struct mddev_dev *devlist,
 		goto abort;
 	}
 
+	if (s->bitmap_file && strcmp(s->bitmap_file, "none") == 0)
+		s->bitmap_file = NULL;
 	if (s->bitmap_file && s->level <= 0) {
 		pr_err("bitmaps not meaningful with level %s\n",
 			map_num(pers, s->level)?:"given");

Create.c

@@ -506,6 +506,17 @@ int Create(struct supertype *st, char *mddev,
 				pr_err("size set to %lluK\n", s->size);
 		}
 	}
+
+	if (!s->bitmap_file &&
+	    s->level >= 1 &&
+	    (s->write_behind || s->size > 100*1024*1024ULL)) {
+		if (c->verbose > 0)
+			pr_err("automatically enabling write-intent bitmap on large array\n");
+		s->bitmap_file = "internal";
+	}
+	if (s->bitmap_file && strcmp(s->bitmap_file, "none") == 0)
+		s->bitmap_file = NULL;
+
 	if (!have_container && s->level > 0 && ((maxsize-s->size)*100 > maxsize)) {
 		if (c->runstop != 1 || c->verbose >= 0)
 			pr_err("largest drive (%s) exceeds size (%lluK) by more than 1%%\n",

mdadm.8.in

@@ -686,6 +686,12 @@ slash ('/') if it is a real file (not 'internal' or 'none').
 Note: external bitmaps are only known to work on ext2 and ext3.
 Storing bitmap files on other filesystems may result in serious problems.
 
+When creating an array on devices which are 100G or larger,
+.I mdadm
+automatically adds an internal bitmap as it will usually be
+beneficial.  This can be suppressed with
+.B "\-\-bitmap=none".
+
 .TP
 .BR \-\-bitmap\-chunk=
 Set the chunksize of the bitmap.  Each bit corresponds to that many
@@ -1932,6 +1938,15 @@ setting.
 .\".B \-\-size
 .\"is given, the apparent size of the smallest drive given is used.
 
+If the array type supports a write-intent bitmap, and if the devices
+in the array exceed 100G is size, an internal write-intent bitmap
+will automatically be added unless some other option is explicitly
+requested with the
+.B \-\-bitmap
+option.  In any case space for a bitmap will be reserved so that one
+can be added layer with
+.BR "\-\-grow \-\-bitmap=internal" .
+
 If the metadata type supports it (currently only 1.x metadata), space
 will be allocated to store a bad block list.  This allows a modest
 number of bad blocks to be recorded, allowing the drive to remain in

mdadm.c

@@ -1079,12 +1079,6 @@ int main(int argc, char *argv[])
 		case O(BUILD,Bitmap):
 		case O(CREATE,'b'):
 		case O(CREATE,Bitmap): /* here we create the bitmap */
-			if (strcmp(optarg, "none") == 0) {
-				pr_err("'--bitmap none' only"
-					" supported for --grow\n");
-				exit(2);
-			}
-			/* FALL THROUGH */
 		case O(GROW,'b'):
 		case O(GROW,Bitmap):
 			if (strcmp(optarg, "internal")== 0 ||