> On Tuesday 16 December 2008 01:48, OGAWA Hirofumi wrote:
>> > Clear the existent magic first, then overwrite:
>> >
>> > Yes, more sensible. Does MD really want us to clear 64 KB at the top
>> > of the volume? That would seem rather sloppy of MD. Really, it
>> > should rely on the just the final sector of a volume to know if it
>> > is an MD volume.
>>
>> IIRC, MD RAID is using final 64kb. I think we should clear the top
>> 1024bytes and final 64kb. (And with it, to make sure, we would check if
>> volume size is bigger than 64kb, instead of clearing 64kb blindly.)
>
> Here is the full story about md superblock:
>
>
http://lxr.linux.no/linux+v2.6.27/include/linux/raid/md_p.h#L19
>
> As I read it, the MD superblock can be as much as 128K less a byte
> below the top of the volume. It only actually uses 4K, and places that
> 4K at a silly place for no apparent reason. This is really sloppy,
> gross, disgusting, all that. But it is also existing practice. On the
> other hand, if everybody has been thinking the md superblock is in the
> top 64K and it actually isn't, do we do any harm by not attempting to
> clear it? I think clearing out the space below our superblock (which
> we have obligingly left free in case somebody needs to put a partition
> block there) is a really good idea, but not so sure about the md stuff
> at the top.
