On Sun, Feb 26, 2012 at 04:44:00PM +0000, Hugo Mills wrote:

   OK, the real problem you're seeing is that when btrfs removes a
device from the filesystem, that device is not modified in any way.
This means that the old superblock is left behind on it, containing
the FS label information. What you need to do is, immediately after
removing a device from the FS, zero the first part of the partition
with dd and /dev/zero.

A correction here: if the device being removed is writable, the
superblock is cleared so it's not recognized as a part of any other fs:

int btrfs_rm_device(struct btrfs_root *root, char *device_path)
...
        /*
         * at this point, the device is zero sized.  We want to
         * remove it from the devices list and zero out the old super
         */
        if (clear_super) {
                /* make sure this device isn't detected as part of
                 * the FS anymore
                 */
                memset(&disk_super->magic, 0, sizeof(disk_super->magic));
                set_buffer_dirty(bh);
                sync_dirty_buffer(bh);
        }

Doing this manually means zeroing 4k block at all offsets up to
partition size:

Superblock 0 offset 65536
Superblock 1 offset 67108864
Superblock 2 offset 274877906944
Superblock 3 offset 1125899906842624
Superblock 4 offset 4611686018427387904


david