> On Tuesday 16 December 2008 10:03, Mike Snitzer wrote:
>> FYI, the ondisk location for the MD superblock really depends on the
>> version (of the MD superblock) that the user elects to use.
>>
>> v0.90 puts the MD superblock at the end of the disk (v1.0 puts it at
>> the same place).
>> v1.1 is at the start of the disk, v1.2 is 4K from the start of the disk.
>
> Hi Mike,
>
> Thanks for the MD superblock executive summary. So by 1.1 MD was
> finally doing the right thing.
>
> We want to do the right thing also. I think that means choosing a
> reasonable default and providing a way to handle unreasonable
> situations, such as protecting oneself from obsolete versions of MD
> with design mistakes. By default, we will clear the first 4K and
> write our superblock in the second 4K. That will take care of
> everything modern, I hope. (Except ReiserFS?)
>
> We might clear a bit of the volume at the top, too, though from my
> digging below it looks for the last five years, somebody would have
> to go out of their way to place a MD superblock at the top of a
> volume instead of within the first 8K.
>
> To handle other situations, I propose a mkfs option, something like:
>
> tux3 mkfs --clear[=supers]
>
> There are other reasons than defending against rogue superblocks for
> fully clearing a volume at mkfs time. (Security anyone?) So it could
> could be a popular option. One flavor of this would attempt to clear
> out other known superblocks. By default, I would like to keep the
> clearing minimal, not because of the time required (though on slow
> media like certain kinds of flash it could be significant) but because
> of a natural reluctance to accomodate ancient mistakes that have little
> practical relevance today. The NTFS issue that started this thread is
> clearly a practical issue, somebody actually hit it. The situation
> with MD is a little murky and could use some clarifying. We have this
> comment:
>
>
http://lxr.linux.no/linux+v2.6.27/drivers/md/md.c#L1053
> 1053 /*
> 1054 * Calculate the position of the superblock in 512byte sectors.
> 1055 * It is always aligned to a 4K boundary and
> 1056 * depeding on minor_version, it can be:
> 1057 * 0: At least 8K, but less than 12K, from end of device
> 1058 * 1: At start of device
> 1059 * 2: 4K from start of device.
> 1060 */
>
> MD superblock type 1 arrived in linux 2.5.65 on 2003-03-16, and
> included the types 0, 1 and 2:
>
>
http://lxr.linux.no/linux-bk+v2.5.65/drivers/md/md.c#L782
>
> So for five years we have had more a sensible situation with MD. I
> don't think we will take an undue risk by clearing nothing at all at
> the top of the volume, and we will provide various flavors of mkfs
> option for those who do not agree. Is everybody satisifed? I didn't
> think so :-)
>
> Regards,
>
> Daniel
>
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