On Fri, 29 Jan 2010 14:54:10 +0000
"Trela, Maciej" [off-list ref] wrote:

+static void *raid0_takeover_raid10(mddev_t *mddev)
+{
+	mdk_rdev_t *rdev;
+
+	/* Check layout: 
+	 *  assuming that far_copies is 1 and disks number is even 
+	 *  also all mirrors must be already degraded
+	 */
+	if ((mddev->layout >> 8) != 1) {
+	    printk(KERN_ERR "error: Raid0 cannot takover layout: %x\n", mddev->layout);
+		return ERR_PTR(-EINVAL);
+	}
+	if (mddev->raid_disks & 1) {
+	    printk(KERN_ERR "error: Raid0 cannot takover Raid10 with odd disk number.\n");
+		return ERR_PTR(-EINVAL);
+	}
+	if (mddev->degraded != (mddev->raid_disks>>1)) {
+	    printk(KERN_ERR "error: All mirrors must be already degraded!\n");
+		return ERR_PTR(-EINVAL);
+	}
+	
+	/* Set new parameters */
+	mddev->new_level = 0;
+	mddev->raid_disks -= mddev->degraded;
+	mddev->delta_disks = -mddev->degraded;
+	mddev->degraded = 0;
+	/* make sure it will be not marked as dirty */
+	mddev->recovery_cp = MaxSector;
+
+	list_for_each_entry(rdev, &mddev->disks, same_set) {	
+		rdev->raid_disk /= 2;
+	}

You assume here that near_copies is 2, but you never check for that.

Also - and this applies to raid5 and raid10 - the devices may not be all the
same size.  raid5 and raid10 will just use the size of the smallest device,
but raid0 won't.  For for a seamless conversion, you need to handle the case
that the devices are not the same size.  Either disallow that case, or make
use of the extra space.

NeilBrown