[Tux3] Review incoming changes

[Daniel Phillips]

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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