On 08/30/2012 08:24 PM, Goffredo Baroncelli wrote:

And magically the filesystem is now composed by three disks. However 4
physical devices are show. This because the disk /dev/vdi superblock
says that the disk is still valid (after the "btrfs device del" the disk
is not touched any more)

I have to correct myself. When a device is removed its superblock is 
zero-ed (from btrfs_rm_device():

[...]
         /*
          * 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);
         }
[...]


clear_super is set to true when the device is writeable.



However making a test I found both the behaviours: sometime the removed 
disk disappears from the output of "btrfs fi show" and sometime not...

May be that there is a bug somewhere...