Re: Superblocks

[Neil Brown <neilb@suse.de>]

On Friday October 26, gcormier@gmail.com wrote:
> Can someone help me understand superblocks and MD a little bit?
> 
> I've got a raid5 array with 3 disks - sdb1, sdc1, sdd1.
> 
> --examine on these 3 drives shows correct information.
> 
> 
> However, if I also examine the raw disk devices, sdb and sdd, they
> also appear to have superblocks with some semi valid looking
> information. sdc has no superblock.

If a partition starts a multiple of 64K from the start of the device,
and ends with about 64K of the end of the device, then a superblock on
the partition will also look like a superblock on the whole device.
This is one of the shortcomings of v0.90 superblocks.  v1.0 doesn't
have this problem.

> 
> How can I clear these? If I unmount my raid, stop md0, it won't clear it.

mdadm --zero-superblock device name

is the best way to remove an unwanted superblock.  Ofcourse in the
above described case, removing the unwanted superblock will remove the
wanted one aswell.


> 
> [root@zeus ~]# mdadm --zero-superblock /dev/hdd
> mdadm: Couldn't open /dev/hdd for write - not zeroing

As I think someone else pointed out "/dev/hdd" is not "/dev/sdd".

NeilBrown